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5 DataLink Layer 5-1
Chapter 5 The Data Link LayerOur goals
understand principles behind data link layer services
error detection correctionsharing a broadcast channel multiple accesslink layer addressingreliable data transfer flow control done
instantiation and implementation of various link layer technologies
5 DataLink Layer 5-2
Link Layer IntroductionSome terminology
hosts and routers are nodescommunication channels that connect adjacent nodes along communication path are links
wired linkswireless linksLANs
layer-2 packet is a frameencapsulates datagram
ldquolinkrdquo
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-3
Link layer contextDatagram transferred by different link protocols over different links
eg Ethernet on first link frame relay on intermediate links 80211 on last link
Each link protocol provides different services
eg may or may not provide rdt over link
transportation analogytrip from Princeton to Lausanne
limo Princeton to JFKplane JFK to Genevatrain Geneva to Lausanne
tourist = datagramtransport segment = communication linktransportation mode = link layer protocoltravel agent = routing algorithm
5 DataLink Layer 5-4
Link Layer ServicesFraming link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressReliable delivery between adjacent nodes
we learned how to do this already (chapter 3)seldom used on low bit error link (fiber some twisted pair)wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-5
Link Layer Services (more)
Flow Controlpacing between adjacent sending and receiving nodes
Error Detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-6
Adapters Communicating
link layer implemented in ldquoadapterrdquo (aka NIC)
Ethernet card PCMCIA card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomousimplements link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-7
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-8
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
1
5 DataLink Layer 5-9
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into the checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-10
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 ATM HDLC)
5 DataLink Layer 5-11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC (hybrid fiber-coax used in cable TV)80211 wireless LAN
80211)
5 DataLink Layer 5-13
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-2
Link Layer IntroductionSome terminology
hosts and routers are nodescommunication channels that connect adjacent nodes along communication path are links
wired linkswireless linksLANs
layer-2 packet is a frameencapsulates datagram
ldquolinkrdquo
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-3
Link layer contextDatagram transferred by different link protocols over different links
eg Ethernet on first link frame relay on intermediate links 80211 on last link
Each link protocol provides different services
eg may or may not provide rdt over link
transportation analogytrip from Princeton to Lausanne
limo Princeton to JFKplane JFK to Genevatrain Geneva to Lausanne
tourist = datagramtransport segment = communication linktransportation mode = link layer protocoltravel agent = routing algorithm
5 DataLink Layer 5-4
Link Layer ServicesFraming link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressReliable delivery between adjacent nodes
we learned how to do this already (chapter 3)seldom used on low bit error link (fiber some twisted pair)wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-5
Link Layer Services (more)
Flow Controlpacing between adjacent sending and receiving nodes
Error Detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-6
Adapters Communicating
link layer implemented in ldquoadapterrdquo (aka NIC)
Ethernet card PCMCIA card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomousimplements link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-7
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-8
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
1
5 DataLink Layer 5-9
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into the checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-10
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 ATM HDLC)
5 DataLink Layer 5-11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC (hybrid fiber-coax used in cable TV)80211 wireless LAN
80211)
5 DataLink Layer 5-13
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-3
Link layer contextDatagram transferred by different link protocols over different links
eg Ethernet on first link frame relay on intermediate links 80211 on last link
Each link protocol provides different services
eg may or may not provide rdt over link
transportation analogytrip from Princeton to Lausanne
limo Princeton to JFKplane JFK to Genevatrain Geneva to Lausanne
tourist = datagramtransport segment = communication linktransportation mode = link layer protocoltravel agent = routing algorithm
5 DataLink Layer 5-4
Link Layer ServicesFraming link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressReliable delivery between adjacent nodes
we learned how to do this already (chapter 3)seldom used on low bit error link (fiber some twisted pair)wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-5
Link Layer Services (more)
Flow Controlpacing between adjacent sending and receiving nodes
Error Detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-6
Adapters Communicating
link layer implemented in ldquoadapterrdquo (aka NIC)
Ethernet card PCMCIA card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomousimplements link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-7
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-8
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
1
5 DataLink Layer 5-9
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into the checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-10
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 ATM HDLC)
5 DataLink Layer 5-11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC (hybrid fiber-coax used in cable TV)80211 wireless LAN
80211)
5 DataLink Layer 5-13
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-4
Link Layer ServicesFraming link access
encapsulate datagram into frame adding header trailerchannel access if shared mediumldquoMACrdquo addresses used in frame headers to identify source dest
bull different from IP addressReliable delivery between adjacent nodes
we learned how to do this already (chapter 3)seldom used on low bit error link (fiber some twisted pair)wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-5
Link Layer Services (more)
Flow Controlpacing between adjacent sending and receiving nodes
Error Detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-6
Adapters Communicating
link layer implemented in ldquoadapterrdquo (aka NIC)
Ethernet card PCMCIA card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomousimplements link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-7
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-8
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
1
5 DataLink Layer 5-9
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into the checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-10
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 ATM HDLC)
5 DataLink Layer 5-11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC (hybrid fiber-coax used in cable TV)80211 wireless LAN
80211)
5 DataLink Layer 5-13
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-5
Link Layer Services (more)
Flow Controlpacing between adjacent sending and receiving nodes
Error Detectionerrors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correctionreceiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplexwith half duplex nodes at both ends of link can transmit but not at same time
5 DataLink Layer 5-6
Adapters Communicating
link layer implemented in ldquoadapterrdquo (aka NIC)
Ethernet card PCMCIA card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomousimplements link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-7
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-8
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
1
5 DataLink Layer 5-9
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into the checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-10
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 ATM HDLC)
5 DataLink Layer 5-11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC (hybrid fiber-coax used in cable TV)80211 wireless LAN
80211)
5 DataLink Layer 5-13
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-6
Adapters Communicating
link layer implemented in ldquoadapterrdquo (aka NIC)
Ethernet card PCMCIA card 80211 card
sending sideencapsulates datagram in a frameadds error checking bits rdt flow control etc
receiving sidelooks for errors rdt flow control etcextracts datagram passes to rcving node
adapter is semi-autonomousimplements link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-7
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-8
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
1
5 DataLink Layer 5-9
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into the checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-10
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 ATM HDLC)
5 DataLink Layer 5-11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC (hybrid fiber-coax used in cable TV)80211 wireless LAN
80211)
5 DataLink Layer 5-13
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-7
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-8
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
1
5 DataLink Layer 5-9
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into the checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-10
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 ATM HDLC)
5 DataLink Layer 5-11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC (hybrid fiber-coax used in cable TV)80211 wireless LAN
80211)
5 DataLink Layer 5-13
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-8
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
1
5 DataLink Layer 5-9
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into the checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-10
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 ATM HDLC)
5 DataLink Layer 5-11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC (hybrid fiber-coax used in cable TV)80211 wireless LAN
80211)
5 DataLink Layer 5-13
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-9
Internet checksum
Sendertreat segment contents as sequence of 16-bit integerschecksum addition (1rsquos complement sum) of segment contentssender puts checksum value into the checksum field
Receivercompute checksum of received segmentcheck if computed checksum equals checksum field value
NO - error detectedYES - no error detected But maybe errors nonethelessMore later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-10
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 ATM HDLC)
5 DataLink Layer 5-11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC (hybrid fiber-coax used in cable TV)80211 wireless LAN
80211)
5 DataLink Layer 5-13
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-10
Checksumming Cyclic Redundancy Checkview data bits D as a binary numberchoose r+1 bit pattern (generator) Ggoal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero remainder error detectedcan detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 ATM HDLC)
5 DataLink Layer 5-11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC (hybrid fiber-coax used in cable TV)80211 wireless LAN
80211)
5 DataLink Layer 5-13
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently
if we divide D2r by G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC (hybrid fiber-coax used in cable TV)80211 wireless LAN
80211)
5 DataLink Layer 5-13
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo
point-to-pointPPP for dial-up accesspoint-to-point link between Ethernet switch and host
broadcast (shared wire or medium)traditional Ethernetupstream HFC (hybrid fiber-coax used in cable TV)80211 wireless LAN
80211)
5 DataLink Layer 5-13
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-13
Multiple Access protocolssingle shared broadcast channel two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same timemultiple access protocol
distributed algorithm that determines how nodes share channel ie determine when node can transmitcommunication about channel sharing must use channel itself
no out-of-band channel for coordination
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-14
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send at
average rate RM3 Fully decentralized
no special node to coordinate transmissionsno synchronization of clocks slots
4 Simple
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-15
MAC Protocols a taxonomyThree broad classes
Channel Partitioningdivide channel into smaller ldquopiecesrdquo (time slots frequency code)allocate piece to node for exclusive use
Random Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
ldquoTaking turnsrdquoNodes take turns but nodes with more to send can take longer turns
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-16
Channel Partitioning MAC protocols TDMA
TDMA time division multiple accessaccess to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkts slots 256 idle
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-17
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple accesschannel spectrum divided into frequency bandseach station assigned fixed frequency bandunused transmission time in frequency bands go idle example 6-station LAN 134 have pkts frequency bands 256 idle
freq
uenc
y ba
nds
time
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-18
Random Access Protocols
When node has packet to sendtransmit at full channel data rate Rno a priori coordination among nodes
two or more transmitting nodes rarr ldquocollisionrdquorandom access MAC protocol specifies
how to detect collisionshow to recover from collisions (eg via delayed retransmissions)
Examples of random access MAC protocolsslotted ALOHAALOHACSMA CSMACD CSMACA
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-19
Slotted ALOHA
Assumptionsall frames same sizetime is divided into equal size slots time to transmit 1 framenodes start to transmit frames only at beginning of slotsnodes are synchronizedif 2 or more nodes transmit in slot all nodes detect collision
Operationwhen node obtains fresh frame it transmits in next slotif no collision node can send new frame in next slotif collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-20
Slotted ALOHA
Prossingle active node can continuously transmit at full rate of channelhighly decentralized only slots in nodes need to be in syncsimple
Conscollisions wasting slotsidle slotsnodes may be able to detect collision in less than time to transmit packetclock synchronization
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-21
Slotted Aloha efficiency
Suppose N nodes with many frames to send each transmits in slot with probability pprob that node 1 has success in a slot= p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1
as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-22
Pure (unslotted) ALOHAunslotted Aloha simpler no synchronizationwhen frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-23
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0] P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n rarr infin
= 1(2e) = 18 Even worse
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-24
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame
If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-25
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission
collisionentire packet transmission time wastednoterole of distance amp propagation delay in determining collision probability
spatial layout of nodes
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-26
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short timecolliding transmissions aborted reducing channel wastage
collision detectioneasy in wired LANs measure signal strengths compare transmitted received signalsdifficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-27
CSMACD collision detection
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-28
ldquoTaking Turnsrdquo MAC protocols
channel partitioning MAC protocolsshare channel efficiently and fairly at high loadinefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocolsefficient at low load single node can fully utilize channelhigh load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-29
ldquoTaking Turnsrdquo MAC protocolsPolling
master node ldquoinvitesrdquo slave nodes to transmit in turnconcerns
polling overhead latencysingle point of failure (master)
Token passingcontrol token passed from one node to next sequentiallytoken messageconcerns
token overhead latencysingle point of failure (token)
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-30
Summary of MAC protocols
What do you do with a shared mediaChannel Partitioning by time frequency or code
bull Time Division Frequency DivisionRandom partitioning (dynamic)
bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire) hard
in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-31
MAC Addresses and ARP
32-bit IP address network-layer addressused to get datagram to destination IP subnet
MAC (or LAN or ldquophysicalrdquo or Ethernet) address
used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-32
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-33
LAN Address (more)
MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy
(a) MAC address like Social Security Number(b) IP address like postal address
MAC flat address allows easier portability can move LAN card from one LAN to another
IP hierarchical address NOT portabledepends on IP subnet to which node is attached
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-34
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP tableARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgtTTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-35
ARP protocol Same LAN (network)
A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP tableA broadcasts ARP query packet containing Bs IP address
Dest MAC address = FF-FF-FF-FF-FF-FFall machines on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address
frame sent to Arsquos MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state information that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquonodes create their ARP tables without intervention from net administrator
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-36
Routing to another LANwalkthrough send datagram from A to B via R
assume A knowrsquos Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-37
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagramArsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos destined to BR uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-38
Ethernetldquodominantrdquo wired LAN technology
cheap $20 for 100Mbsfirst widely used LAN technologySimpler cheaper than token LANs and ATMKept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-39
Star topologyBus topology popular through mid 90sNow star topology prevailsConnection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-40
Ethernet Frame StructureSending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronize receiver sender clock rates
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-41
Ethernet Frame Structure (more)
Addresses 6 bytesif adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocolotherwise adapter discards frame
Type 2 bytes indicates the higher (network) layer protocol (commonly IP but may also be ARP Novell IPX and AppleTalk etc)CRC 4 bytes checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-42
Unreliable connectionless service
Connectionless No handshaking between sending and receiving adapter Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter
stream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-43
Ethernet uses CSMACD
No slotsadapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-44
Ethernet CSMACD algorithm1 Adapter receives
datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the mthcollision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-45
Frame Size limitations for EthernetMinimum Frame size (Fmin)For proper collision detection
Fmin = Min frame sizeR = Ethernetrsquos transmission rate
eg 10 Mbsdmax = max Ethernet segment
lengthS = Propagation speed (2x108
ms)FminR ge 2dmaxS
Maximum Frame Size (Fmax)For fairness among competing nodes
Fmin=64 Bytes Fmax=1500 Bytes
A Bd
2dS
tim
e
Arsquos frame
Brsquos frame
Arsquos frame in yellowBrsquos frame in green
For proper collision detection Arsquos frame should last at least until Brsquos frame reaches A
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-46
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Random retransmission delayK 512 bit transmission times where K is randomly selected bit time is 01 microsec for 10 Mbps and 001 microsec for 100 Mbps Ethernet
Exponential BackoffGoal adapt retransmission attempts to estimated current load
heavy load random wait will be longer
first collision choose K from 01 delay is K 512 bit transmission timesafter second collision choose K from 0123hellipafter ten collisions choose K from 01234hellip1023for max value of K=1023 wait time is about 50 msec for 10 Mbps 5 msec for 100 Mbps Ethernet
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-47
CSMACD efficiency
tprop = max prop between 2 nodes in LANttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0Goes to 1 as ttrans goes to infinityMuch better than ALOHA but still decentralized simple and cheap
1efficiency1 5 prop transt t
asymp+
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-48
10BaseT and 100BaseT10100 Mbps ratesT stands for Twisted PairBase stands for Baseband (unmodulated)Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other linksat the same rateno frame bufferingno CSMACD at hub adapters detect collisionsprovides net management functionality
twisted pair
hub
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-50
Gbit Ethernet
uses standard Ethernet frame formatallows for point-to-point links and shared broadcast channelsin shared mode CSMACD is used short distances between nodes required for efficiencyFull-Duplex at 1 Gbps for point-to-point links10 Gbps now
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-51
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between nodesBut individual segment collision domains become one large collision domainCanrsquot interconnect 10BaseT amp 100BaseT
hub hub hub
hub
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-52
SwitchLink layer device
stores and forwards Ethernet framesexamines frame header and selectivelyforwards frame based on MAC dest addresswhen frame is to be forwarded on segment uses CSMACD to access segment
transparenthosts are unaware of presence of switches
plug-and-play self-learningswitches do not need to be configured
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-53
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub hubhub
switch1
2 3
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-54
Self learning
A switch has a switch tableentry in switch table
(MAC Address Interface Time Stamp)stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segmentrecords senderlocation pair in switch table
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-55
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif entry found for destination
thenif dest on segment from which frame arrived
then drop the frameelse forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-56
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEG
1123
12 3
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-57
Switch exampleSuppose C sends frame to D
Switch receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table switch forwards frame into interfaces 2 and 3
frame received by D
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-58
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGC
11231
12 3
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-59
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from Dnotes in bridge table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub hub hub
switch
B CD G H
A
EF
I
address interfaceABEGCD
112312
12 3
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-60
Switch traffic isolationswitch installation breaks subnet into LAN segmentsswitch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-61
Switches dedicated accessSwitch with many interfacesHosts have direct connection to switchNo collisions full duplex
Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions
combinations of shareddedicated 101001000 Mbps interfaces possible
switch
A
Arsquo
B
C
Crsquo
Brsquo
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-62
Institutional network
hub hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-63
Switches vs Routersboth store-and-forward devices
routers network layer devices (examine network layer headers)switches are link layer devices
routers maintain routing tables implement routing algorithmsswitches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-64
Summary comparison
hubs routers switches
traffic isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-65
VLANs motivation
What happens ifCS user moves office to EE but wants connect to CS switchsingle broadcast domain
all layer-2 broadcast traffic (ARP DHCP) crosses entire LAN (securityprivacy efficiency issues)
each lowest level switch has only few ports in use
Computer Science Electrical
EngineeringComputerEngineering
Whatrsquos wrong with this picture
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-66
VLANs Port-based VLAN switch ports grouped (by switch management software) so that single physical switch helliphellip
Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure
Virtual Local Area Network
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
Electrical Engineering(VLAN ports 1-8)
hellip
1
82
7 9
1610
15
hellip
Computer Science(VLAN ports 9-16)
hellip operates as multiple virtual switches
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-67
Port-based VLAN
1
8
9
16102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints rather than switch port
dynamic membershipports can be dynamically assigned among VLANs
router
forwarding between VLANSdone via routing (just as with separate switches)
in practice vendors sell combined switches plus routers
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-68
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches canrsquot be vanilla 8021 frames (must carry VLAN ID info)8021q protocol addsremoves additional header fields for frames forwarded between trunk ports
1
8
9
102
7
hellip
Electrical Engineering(VLAN ports 1-8)
Computer Science(VLAN ports 9-15)
15
hellip
2
73
Ports 235 belong to EE VLANPorts 4678 belong to CS VLAN
5
4 6 816
1
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-69
Type
2-byte Tag Protocol Identifier(value 81-00)
Tag Control Information (12 bit VLAN ID field 3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-70
Point to Point Data Link Controlone sender one receiver one link easier than broadcast link
no Media Access Controlno need for explicit MAC addressingeg dialup link ISDN line
popular point-to-point DLC protocolsPPP (point-to-point protocol)HDLC High level data link control (Data link used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-71
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame
carry network layer data of any network layer protocol (not just IP) at same timeability to demultiplex upwards
bit transparency must carry any bit pattern in the data fielderror detection (no correction)connection liveness detect signal link failure to network layernetwork layer address negotiation endpoint can learnconfigure each otherrsquos network address
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-72
PPP non-requirements
no error correctionrecoveryno flow controlout of order delivery OK no need to support multipoint links (eg polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-73
PPP Data Frame
Flag delimiter (framing)Address does nothing (only one option)Control does nothing in the future possible multiple control fieldsProtocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-74
PPP Data Frame
info upper layer data being carriedcheck cyclic redundancy check for error detection
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-75
Byte Stuffingldquodata transparencyrdquo requirement data field must
be allowed to include flag pattern lt01111110gtQ is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byteReceiver
two 01111110 bytes in a row discard first byte continue data receptionsingle 01111110 flag byte
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-76
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-77
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers mustconfigure PPP link (max frame length authentication)learnconfigure networklayer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-78
ATM and MPLS
ATM MPLS separate networks in their own right
different service models addressing routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
ATM MPLS of technical interest in their own right
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-79
Asynchronous Transfer Mode ATM1990rsquos00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architectureGoal integrated end-end transport of carry voice video data
meeting timingQoS requirements of voice video (versus Internet best-effort model)ldquonext generationrdquo telephony technical roots in telephone worldpacket-switching (fixed length packets called ldquocellsrdquo) using virtual circuits
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-80
Multiprotocol label switching (MPLS)
initial goal speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approachbut IP datagram still keeps IP address
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-81
MPLS capable routers
aka label-switched routerforwards packets to outgoing interface based only on label value (donrsquot inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwardingRSVP-TEforwarding possible along paths that IP alone would not allow (eg source-specific routing) use MPLS for traffic engineering
must co-exist with IP-only routers
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-82
R1R2
DR3R4
R50
100
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 112 9 D 0
in out outlabel label dest interface
10 A 012 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-83
Synthesis a day in the life of a web request
journey down protocol stack completeapplication transport network link
putting-it-all-together synthesisgoal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www pagescenario student attaches laptop to campus network requestsreceives wwwgooglecom
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-84
A day in the life scenario
Comcast network 68800013
Googlersquos network 64233160019 64233169105
web server
DNS server
school network 68802024
browser
web page
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-85
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP encapsulated in IP encapsulated in 8021EthernetEthernet frame broadcast(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-86
A day in the lifehellip connecting to the Internet
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS server IP address of its first-hop router
DHCP client receives DHCP ACK reply
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-87
A day in the lifehellip ARP (before DNS before HTTP)before sending HTTP request need IP address of wwwgooglecomDNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP
ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfaceclient now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
EthPhy
ARP
ARP
ARP reply
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-88
A day in the lifehellip using DNS
DNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcastnetwork routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS serverdemuxrsquoed to DNS serverDNS server replies to client with IP address of wwwgooglecom
Comcast network 68800013
DNS serverDNSUDP
IPEthPhy
DNS
DNS
DNS
DNS
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-89
A day in the lifehellip TCP connection carrying HTTP
HTTPTCPIP
EthPhy
HTTP
to send HTTP request client first opens TCP socket to web serverTCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established64233169105web server
SYN
SYN
SYN
SYN
TCPIP
EthPhy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-90
A day in the lifehellip HTTP requestreply
HTTPTCPIP
EthPhy
HTTP
HTTP request sent into TCP socketIP datagram containing HTTP request routed to wwwgooglecom
IP datgram containing HTTP reply routed back to client
64233169105web server
HTTPTCPIP
EthPhy
web server responds with HTTP reply (containing web page)
HTTP
HTTP
HTTPHTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally ()displayed
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-91
Chapter 6 Wireless and Mobile Networks
Background wireless (mobile) phone subscribers now exceeds wired phone subscriberscomputer nets laptops palmtops PDAs Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
wireless communication over wireless linkmobility handling the mobile user who changes point of attachment to network
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-92
Elements of a wireless network
network infrastructure
wireless hostslaptop PDA IP phonerun applicationsmay be stationary (non-mobile) or mobile
wireless does notalways mean mobility
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-93
Elements of a wireless network
network infrastructure
base stationtypically connected to wired networkrelay - responsible for sending packets between wired network and wireless host(s) in its ldquoareardquo
eg cell towers 80211 access points
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-94
Elements of a wireless network
network infrastructure
wireless linktypically used to connect mobile(s) to base stationalso used as backbone link multiple access protocol coordinates link access various data rates transmission distance
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-95
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m ndash 4 Km
Long-rangeoutdoor
5Km ndash 20 Km
056
384
1
4
5-11
54
IS-95 CDMA GSM 2G
UMTSWCDMA CDMA2000 3G
80215
80211b
80211ag
UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced
80216 (WiMAX)
80211ag point-to-point
200 80211n
Dat
a ra
te (M
bps) data
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-96
Elements of a wireless network
network infrastructure
infrastructure modebase station connects mobiles into wired networkhandoff mobile changes base station providing connection into wired network
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-97
Elements of a wireless networkad hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network route among themselves
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-98
Wireless network taxonomy
single hop multiple hops
infrastructure(eg APs)
noinfrastructure
host connects to base station (WiFiWiMAX cellular) which connects to
larger Internet
no base station noconnection to larger Internet (Bluetooth
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet mesh net
no base station noconnection to larger
Internet May have torelay to reach other a given wireless node
MANET VANET
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-99
Wireless Link Characteristics (1)Differences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as wellmultipath propagation radio signal reflects off objects arriving at destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-100
Wireless Link Characteristics (2)SNR signal-to-noise ratio
larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)
SNR versus BER tradeoffsgiven physical layerincrease power -gt increase SNR-gtdecrease BERgiven SNR choose physical layer that meets BER requirement giving highest thruput
bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-101
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problemB A hear each otherB C hear each otherA C can not hear each other
means A C unaware of their interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal attenuationB A hear each otherB C hear each otherA C can not hear each other interfering at B
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-102
IEEE 80211 Wireless LAN80211b
24-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
80211a5-6 GHz rangeup to 54 Mbps
80211g24-5 GHz rangeup to 54 Mbps
80211n multiple antennae24-5 GHz rangeup to 200 Mbps
all use CSMACA for multiple accessall have base-station and ad-hoc network versions
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-103
80211 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS)(aka ldquocellrdquo) in infrastructure mode contains
wireless hostsaccess point (AP) base stationad hoc mode hosts only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-104
80211 Channels association
80211b 24GHz-2485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible channel can be same as that chosen by neighboring AP
host must associate with an APscans channels listening for beacon framescontaining APrsquos name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in APrsquos subnet
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-105
80211 passiveactive scanning
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning (1) probe request frame broadcast
from H1(2) probe response frame sent from
APs(3) association request frame sent
H1 to selected AP (4) association response frame sent
H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
12 3
1
Passive Scanning(1) beacon frames sent from APs(2) association request frame sent H1
to selected AP (3) association response frame sent
H1 to selected AP
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-106
IEEE 80211 multiple accessavoid collisions 2+ nodes transmitting at same time80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node80211 no collision detection
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)canrsquot sense all collisions in any case hidden terminal fadinggoal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-107
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 280211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-108
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framessender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but theyrsquore short)BS broadcasts clear-to-send CTS in response to RTSCTS heard by all nodes
sender transmits data frameother stations defer transmissions
avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-109
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-110
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
80211 frame addressing
Address 2 MAC addressof wireless host or AP transmitting this frame
Address 1 MAC addressof wireless host or AP to receive this frame
Address 3 MAC addressof router interface to which AP is attached
Address 4 used only in ad hoc mode
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-111
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3
80211 frame
R1 MAC addr H1 MAC addr dest address source address
8023 frame
80211 frame addressing
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-112
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Type FromAPSubtype To
APMore frag WEPMore
dataPower
mgtRetry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
80211 frame moreduration of reserved transmission time (RTSCTS)
frame seq (for RDT)
frame type(RTS CTS ACK data)
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-113
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
80211 mobility within same subnet
routerH1 remains in same IP subnet IP address can remain sameswitch which AP is associated with H1
self-learning (Ch 5) switch will see frame from H1 and ldquorememberrdquo which switch port can be used to reach H1
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-114
80211 advanced capabilities
Rate Adaptationbase station mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-115
80211 advanced capabilitiesPower Management
node-to-AP ldquoI am going to sleep until next beacon framerdquo
AP knows not to transmit frames to this nodenode wakes up before next beacon frame
beacon frame contains list of mobiles with AP-to-mobile frames waiting to be sent
node will stay awake if AP-to-mobile frames to be sent otherwise sleep again until next beacon frame
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-116
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
80215 personal area network
less than 10 m diameterreplacement for cables (mouse keyboard headphones)ad hoc no infrastructuremasterslaves
slaves request permission to send (to master)master grants requests
80215 evolved from Bluetooth specification
24-25 GHz radio bandup to 721 kbps
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-117
80216 WiMAXlike 80211 amp cellular base station model
transmissions tofrom base station by hosts with omnidirectionalantennabase station-to-base station backhaul with point-to-point antenna
unlike 80211range ~ 6 miles (ldquocity rather than coffee shoprdquo)~14 Mbps
point-to-multipoint
point-to-point
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-118
80216 WiMAX downlink uplink scheduling
transmission framedown-link subframe base station to node uplink subframe node to base station
prea
m
DL-MAP
UL-MAP
DLburst 1 SS 1DL
burst 2DL
burst nInitialmaint
requestconn
downlink subframe
SS 2 SS k
uplink subframe
hellip
hellip
hellip
hellip
base station tells nodes who will get to receive (DL map) and who will get to send (UL map) and when
WiMAX standard provide mechanism for scheduling but not scheduling algorithm
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-119
Mobile Switching
Center
Public telephonenetwork andInternet
Mobile Switching
Center
Components of cellular network architecture
connects cells to wide area netmanages call setuphandles mobility
MSC
covers geographical region
base station (BS) analogous to 80211 AP
mobile users attach to network through BS
air-interfacephysical and link layer protocol between mobile and BS
cell
wired network
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-120
Cellular networks the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMATDMAdivide spectrum in frequency channels divide each channel into time slotsCDMA code division multiple access
frequencybands
time slots
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-121
Cellular standards brief survey
2G systems voice channelsIS-136 TDMA combined FDMATDMA (north america)GSM (global system for mobile communications) combined FDMATDMA
most widely deployedIS-95 CDMA code division multiple access
IS-136 GSM IS-95GPRS EDGECDMA-2000
UMTS
TDMAFDMADonrsquot drown in a bowlof alphabet soup use thisfor reference only
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-122
Cellular standards brief survey
25 G systems voice and data channelsfor those who canrsquot wait for 3G service 2G extensionsgeneral packet radio service (GPRS)
evolved from GSM data sent on multiple channels (if available)
enhanced data rates for global evolution (EDGE)also evolved from GSM using enhanced modulation data rates up to 384K
CDMA-2000 (phase 1)data rates up to 144Kevolved from IS-95
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-123
Cellular standards brief survey
3G systems voicedataUniversal Mobile Telecommunications Service (UMTS)
data service High Speed UplinkDownlink packet Access (HSDPAHSUPA) 3 Mbps
CDMA-2000 CDMA in TDMA slotsdata service 1xEvolution Data Optimized (1xEVDO) up to 14 Mbps
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-124
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSCPublic telephonenetwork
GatewayMSC
G
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel
5 DataLink Layer 5-125
25G (voice+data) network architecture
BSCMSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight new cellular datanetwork operates in parallel(except at edge) with existing cellular voice network
voice network unchanged in coredata network operates in parallel