Chapter 5 Chapter 5: The Data Link Layer Link Layer and ...network2006/slide/Ch05.pdfreliable data transfer, flow control: done! ... data-link layer has responsibility of transferring

5 DataLink Layer 5-1

Chapter 5Link Layer and LANs

Computer Networking A Top Down Approach Featuring the Internet 3rd edition Jim Kurose Keith RossAddison-Wesley July 2004


Chapter 5 The Data Link Layer

Our goalsr understand principles behind data link layer

servicesm error detection correction

m sharing a broadcast channel multiple access

m link layer addressing

m reliable data transfer flow control done

r instantiation and implementation of various link layer technologies


Link Layer

r 51 Introduction and services

r 52 Error detection and correction

r 53Multiple access protocols

r 54 Link-Layer Addressing

r 55 Ethernet

r 56 Hubs and switches

r 57 PPP

r 58 Link Virtualization ATM and MPLS


Link Layer IntroductionSome terminologyr hosts and routers are nodes

r communication channels that connect adjacent nodes along communication path are links

m wired links

m wireless links

m LANs

r 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


Link layer context

r Datagram transferred by different link protocols over different links

m eg Ethernet on first link frame relay on intermediate links 80211 on last link

r Each link protocol provides different services

m eg may or may not provide rdt over link

transportation analogyr trip from Princeton to

Lausanne

m limo Princeton to JFK

m plane JFK to Geneva

m train Geneva to Lausanne

r tourist = datagram

r transport segment = communication link

r transportation mode = link layer protocol

r travel agent = routing algorithm


Link Layer Services

r Framing link accessm encapsulate datagram into frame adding header trailer

m channel access if shared medium

m ldquoMACrdquo addresses used in frame headers to identify source dest

bull different from IP address

r Reliable delivery between adjacent nodesm we learned how to do this already (chapter 3)

m seldom used on low bit error link (fiber some twisted pair)

m wireless links high error rates

bull Q why both link-level and end-end reliability


Link Layer Services (more)

r Flow Controlm pacing between adjacent sending and receiving nodes

r Error Detectionm errors caused by signal attenuation noise

m receiver detects presence of errors

bull signals sender for retransmission or drops frame

r Error Correctionm receiver identifies and corrects bit error(s) without

resorting to retransmission

r Half-duplex and full-duplexm with half duplex nodes at both ends of link can transmit

but not at same time5 DataLink Layer 5-8

Adaptors Communicating

r link layer implemented in ldquoadaptorrdquo (aka NIC)

m Ethernet card PCMCI card 80211 card

r sending sidem encapsulates datagram in

a frame

m adds error checking bits rdt flow control etc

r receiving sidem looks for errors rdt flow

control etc

m extracts datagram passes to rcving node

sendingnode

frame

rcvingnode

datagram

frame

adapter adapter

link layer protocol


Link Layer





r 55 Ethernet


r 57 PPP

r 58 Link Virtualization ATM


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


Parity Checking

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0


Link Layer





r 55 Ethernet


r 57 PPP



Multiple Access Links and Protocols

Two types of ldquolinksrdquor point-to-point

m PPP for dial-up access

m point-to-point link between Ethernet switch and host

r broadcast (shared wire or medium)m traditional Ethernet

m upstream HFC

m 80211 wireless LAN


Multiple Access protocols

r single shared broadcast channel

r two or more simultaneous transmissions by nodes interference

m collision if node receives two or more signals at the same time

multiple access protocolr distributed algorithm that determines how nodes

share channel ie determine when node can transmit

r communication about channel sharing must use channel itself

m no out-of-band channel for coordination


Ideal Mulitple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralizedm no special node to coordinate transmissions

m no synchronization of clocks slots

4 Simple


MAC Protocols a taxonomy

Three broad classes

r Channel Partitioningm divide channel into smaller ldquopiecesrdquo (time slots

frequency code)

m allocate piece to node for exclusive use

r Random Accessm channel not divided allow collisions

m ldquorecoverrdquo from collisions

r ldquoTaking turnsrdquom Nodes take turns but nodes with more to send can take

longer turns


Channel Partitioning MAC protocols TDMA

TDMA time division multiple accessr access to channel in rounds

r each station gets fixed length slot (length = pkt trans time) in each round

r unused slots go idle

r example 6-station LAN 134 have pkt slots 256 idle


Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple accessr channel spectrum divided into frequency bands

r each station assigned fixed frequency band

r unused transmission time in frequency bands go idle

r example 6-station LAN 134 have pkt frequency bands 256 idle

freq

uenc

y b

and

s

time


Random Access Protocols

r When node has packet to sendm transmit at full channel data rate R

m no a priori coordination among nodes

r two or more transmitting nodes ldquocollisionrdquo

r random access MAC protocol specifies m how to detect collisions

m how to recover from collisions (eg via delayed retransmissions)

r Examples of random access MAC protocolsm slotted ALOHA

m ALOHA

m CSMA CSMACD CSMACA


Slotted ALOHA

Assumptions

r all frames same size

r time is divided into equal size slots time to transmit 1 frame

r nodes start to transmit frames only at beginning of slots

r nodes are synchronized

r if 2 or more nodes transmit in slot all nodes detect collision

Operation

r when node obtains fresh frame it transmits in next slot

r no collision node can send new frame in next slot

r if collision node retransmits frame in each subsequent slot with prob p until success


Slotted ALOHA

Pros

r single active node can continuously transmit at full rate of channel

r highly decentralized only slots in nodes need to be in sync

r simple

Consr collisions wasting slotsr idle slotsr nodes may be able to

detect collision in less than time to transmit packet

r clock synchronization


Slotted Aloha efficiency

r Suppose N nodes with many frames to send each transmits in slot with probability p

r prob that node 1 has success in a slot= p(1-p)N-1

r prob that any node has a success = Np(1-p)N-1

r For max efficiency with N nodes find p that maximizes Np(1-p)N-1

r 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


Pure (unslotted) ALOHA

r unslotted Aloha simpler no synchronization

r when frame first arrivesm transmit immediately

r collision probability increasesm frame sent at t0 collides with other frames sent in [t0-1t0+1]


Pure Aloha efficiency

P(success by given node) = P(node transmits)

P(no other node transmits in [p0-1p0]


= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse


CSMA (Carrier Sense Multiple Access)

CSMA listen before transmit

If channel sensed idle transmit entire frame

r If channel sensed busy defer transmission

r Human analogy donrsquot interrupt others


CSMA collisions

collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmission

collisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability


CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMAm collisions detected within short time

m colliding transmissions aborted reducing channel wastage

r collision detectionm easy in wired LANs measure signal strengths

compare transmitted received signals

m difficult in wireless LANs receiver shut off while transmitting

r human analogy the polite conversationalist


CSMACD collision detection


ldquoTaking Turnsrdquo MAC protocols

channel partitioning MAC protocols

m share channel efficiently and fairly at high load

m inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols

m efficient at low load single node can fully utilize channel

m high load collision overhead

ldquotaking turnsrdquo protocols

look for best of both worlds



Polling

r master node ldquoinvitesrdquo slave nodes to transmit in turn

r concernsm polling overhead

m latency

m single point of failure (master)

Token passing

r control token passed from one node to next sequentially

r token message

r concernsm token overhead

m latency

m single point of failure (token)


Summary of MAC protocols

r What do you do with a shared mediam Channel Partitioning by time frequency or code

bull Time Division Frequency Division

m Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACD

bull carrier sensing easy in some technologies (wire) hard in others (wireless)

bull CSMACD used in Ethernet

bull CSMACA used in 80211

m Taking Turnsbull polling from a central site token passing


LAN technologies

Data link layer so farm services error detectioncorrection multiple

access

Next LAN technologiesm addressing

m Ethernet

m hubs switches

m PPP


Link Layer





r 55 Ethernet


r 57 PPP



MAC Addresses and ARP

r 32-bit IP address m network-layer address

m used to get datagram to destination IP subnet

r MAC (or LAN or physical or Ethernet) addressm used to get datagram from one interface to

another physically-connected interface (same network)

m 48 bit MAC address (for most LANs) burned in the adapter ROM


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)


LAN Address (more)

r MAC address allocation administered by IEEE

r manufacturer buys portion of MAC address space (to assure uniqueness)

r Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address

r MAC flat address portability m can move LAN card from one LAN to another

r IP hierarchical address NOT portablem depends on IP subnet to which node is attached


ARP Address Resolution Protocol

r Each IP node (Host Router) on LAN has ARP table

r ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt

m TTL (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


ARP protocol Same LAN (network)

r A wants to send datagram to B and Brsquos MAC address not in Arsquos ARP table

r A broadcasts ARP query packet containing Bs IP address

m Dest MAC address = FF-FF-FF-FF-FF-FF

m all machines on LAN receive ARP query

r B receives ARP packet replies to A with its (Bs) MAC address

m frame sent to Arsquos MAC address (unicast)

r A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

m soft state information that times out (goes away) unless refreshed

r ARP is ldquoplug-and-playrdquom nodes create their ARP

tables without intervention from net administrator


Routing to another LAN

walkthrough send datagram from A to B via R

assume A knows B IP address

r Two ARP tables in router R one for each IP network (LAN)

A

RB


r A creates datagram with source A destination B

r A uses ARP to get Rrsquos MAC address for 111111111110

r A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

r Arsquos adapter sends frame

r Rrsquos adapter receives frame

r R removes IP datagram from Ethernet frame sees its destined to B

r R uses ARP to get Brsquos MAC address

r R creates frame containing A-to-B IP datagram sends to B

A

RB


Link Layer





r 55 Ethernet


r 57 PPP



Ethernet

ldquodominantrdquo wired LAN technology

r cheap $20 for 100Mbps

r first widely used LAN technology

r Simpler cheaper than token LANs and ATM

r Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch


Star topology

r Bus topology popular through mid 90s

r Now star topology prevails

r Connection choices hub or switch (more later)

hub orswitch


Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

r 7 bytes with pattern 10101010 followed by one byte with pattern 10101011

r used to synchronize receiver sender clock rates


Ethernet Frame Structure (more)r Addresses 6 bytes

m if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

m otherwise adapter discards frame

r Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)

r CRC checked at receiver if error is detected the frame is simply dropped


Unreliable connectionless service

r Connectionless No handshaking between sending and receiving adapter

r Unreliable receiving adapter doesnrsquot send acks or nacks to sending adapter

m stream of datagrams passed to network layer can have gaps

m gaps will be filled if app is using TCP

m otherwise app will see the gaps


Ethernet uses CSMACD

r No slots

r adapter doesnrsquot transmit if it senses that some other adapter is transmitting that is carrier sense

r transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

r Before attempting a retransmission adapter waits a random time that is random access


Ethernet CSMACD algorithm

1 Adaptor 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 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


Ethernetrsquos CSMACD (more)

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff

r Goal adapt retransmission attempts to estimated current load

m heavy load random wait will be longer

r first collision choose K from 01 delay is K 512 bit transmission times

r after second collision choose K from 0123hellip

r after ten collisions choose K from 01234hellip1023


CSMACD efficiency

r Tprop = max prop between 2 nodes in LAN

r ttrans = time to transmit max-size frame

r Efficiency goes to 1 as tprop goes to 0

r Goes to 1 as ttrans goes to infinity

r Much better than ALOHA but still decentralized simple and cheap

transproptt 51

1efficiency

+

=


10BaseT and 100BaseTr 10100 Mbps rate latter called ldquofast ethernetrdquo

r T stands for Twisted Pair

r Nodes connect to a hub ldquostar topologyrdquo 100 m max distance between nodes and hub

twisted pair

hub


HubsHubs are essentially physical-layer repeaters

m bits coming from one link go out all other links

m at the same rate

m no frame buffering

m no CSMACD at hub adapters detect collisions

m provides net management functionality

twisted pair

hub


Gbit Ethernet

r uses standard Ethernet frame formatm Backward compatible with 10BaseT and 100BaseT

technologies

r allows for point-to-point links and shared broadcast channels

r in shared mode CSMACD is used short distances between nodes required for efficiency

r uses hubs called here ldquoBuffered Distributorsrdquo

r Full-Duplex at 1 Gbps for point-to-point links

r 10 Gbps now


Link Layer





r 55 Ethernet

r 56 Interconnections Hubs and switches

r 57 PPP



Interconnecting with hubsr Backbone hub interconnects LAN segments

r Extends max distance between nodes

r But individual segment collision domains become one large collision domain

r Canrsquot interconnect 10BaseT amp 100BaseT

hub hubhub

hub


Switch

r Link layer device

m stores and forwards Ethernet frames

m examines frame header and selectivelyforwards frame based on MAC dest address

m when frame is to be forwarded on segment uses CSMACD to access segment

r transparent

m hosts are unaware of presence of switches

r plug-and-play self-learning

m switches do not need to be configured


Forwarding

bull How to determine onto which LAN segment to forward framebull Looks like a routing problem

hub hubhub

switch1

2 3


Self learning

r A switch has a switch table

r entry in switch table

m (MAC Address Interface Time Stamp)

m stale entries in table dropped (TTL can be 60 min)

r switch learns which hosts can be reached through which interfaces

m when frame received switch ldquolearnsrdquo location of sender incoming LAN segment

m records senderlocation pair in switch table


FilteringForwarding

When switch receives a frame

index switch table using MAC dest address

if entry found for destinationthen

if dest on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else floodforward on all but the interface on which the frame arrived


Switch example

Suppose C sends frame to D

r Switch receives frame from from Cm notes in table that C is on interface 1

m because D is not in table switch forwards frame into interfaces 2 and 3

r frame received by D

hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example

Suppose D replies back with frame to C

r Switch receives frame from from Dm notes in bridge table that D is on interface 2

m because C is in table switch forwards frame only to interface 1

r frame received by C

hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231


Switch traffic isolationr switch installation breaks subnet into LAN

segments

r switch filters packetsm same-LAN-segment frames not usually

forwarded onto other LAN segmentsm segments become separate collision domains

hub hub hub

switch

collision domain collision domain

collision domain


Switches dedicated access

r Switch with many interfaces

r Hosts have direct connection to switch

r No collisions full duplex

Switching A-to-Arsquo and B-to-Brsquosimultaneously no collisions

switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches

r cut-through switching frame forwarded from input to output port without first collecting entire frame

m slight reduction in latency

r combinations of shareddedicated 101001000 Mbps interfaces


Institutional network

hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server


Switches vs Routersr both store-and-forward devices

m routers network layer devices (examine network layer headers)

m switches are link layer devices

r routers maintain routing tables implement routing algorithms

r switches maintain switch tables implement filtering learning algorithms


Summary comparison

hubs routers switches

traffic isolation

no yes yes

plug amp play yes no yes

optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP



Point to Point Data Link Control

r one sender one receiver one link easier than broadcast link

m no Media Access Control

m no need for explicit MAC addressing

m eg dialup link ISDN line

r popular point-to-point DLC protocols

m PPP (point-to-point protocol)

m HDLC High-level data link control


PPP Design Requirements [RFC 1557]

r packet framing encapsulation of network-layer datagram in data link frame

m carry network layer data of any network layer protocol (not just IP) at same time

m ability to demultiplex upwards

r bit transparency must carry any bit pattern in the data field

r error detection (no correction)

r connection liveness detect signal link failure to network layer

r network layer address negotiation endpoint can learnconfigure each otherrsquos network address


PPP non-requirements

r no error correctionrecovery

r no flow control

r out of order delivery OK

r no need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers


PPP Data Frame

r Flag delimiter (framing)

r Address does nothing (only one option)

r Control does nothing in the future possible multiple control fields

r Protocol upper layer protocol to which frame delivered (eg PPP-LCP IP IPCP etc)


PPP Data Frame

r info upper layer data being carried

r check cyclic redundancy check for error detection


Byte Stuffingr ldquodata transparencyrdquo requirement data field must

be allowed to include flag pattern lt01111110gt

m Q is received lt01111110gt data or flag

r Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

r Receiver

m two 01111110 bytes in a row discard first byte continue data reception

m single 01111110 flag byte


Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data


PPP Data Control ProtocolBefore exchanging network-

layer data data link peers must

r configure PPP link (max frame length authentication)

r learnconfigure network

layer information

m for IP carry IP Control Protocol (IPCP) msgs(protocol field 8021) to configurelearn IP address


Link layer context

r Datagram transferred by different link protocols over different links

m eg Ethernet on first link frame relay on intermediate links 80211 on last link

r Each link protocol provides different services

m eg may or may not provide rdt over link

transportation analogyr trip from Princeton to

Lausanne

m limo Princeton to JFK

m plane JFK to Geneva

m train Geneva to Lausanne

r tourist = datagram

r transport segment = communication link

r transportation mode = link layer protocol

r travel agent = routing algorithm


Link Layer Services

r Framing link accessm encapsulate datagram into frame adding header trailer

m channel access if shared medium

m ldquoMACrdquo addresses used in frame headers to identify source dest

bull different from IP address

r Reliable delivery between adjacent nodesm we learned how to do this already (chapter 3)

m seldom used on low bit error link (fiber some twisted pair)

m wireless links high error rates

bull Q why both link-level and end-end reliability


Link Layer Services (more)

r Flow Controlm pacing between adjacent sending and receiving nodes

r Error Detectionm errors caused by signal attenuation noise

m receiver detects presence of errors

bull signals sender for retransmission or drops frame

r Error Correctionm receiver identifies and corrects bit error(s) without

resorting to retransmission

r Half-duplex and full-duplexm with half duplex nodes at both ends of link can transmit

but not at same time5 DataLink Layer 5-8

Adaptors Communicating

r link layer implemented in ldquoadaptorrdquo (aka NIC)

m Ethernet card PCMCI card 80211 card

r sending sidem encapsulates datagram in

a frame

m adds error checking bits rdt flow control etc

r receiving sidem looks for errors rdt flow

control etc

m extracts datagram passes to rcving node

sendingnode

frame

rcvingnode

datagram

frame

adapter adapter

link layer protocol


Link Layer





r 55 Ethernet


r 57 PPP






Parity Checking



0 0


Link Layer





r 55 Ethernet


r 57 PPP








m upstream HFC


















4 Simple



Three broad classes


frequency code)





longer turns













freq

uenc

y b

and

s

time









m ALOHA



Slotted ALOHA

Assumptions






Operation





Slotted ALOHA

Pros



r simple
























= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)










CSMA collisions



























Polling



m latency


Token passing


r token message


m latency












LAN technologies


access


m Ethernet

m hubs switches

m PPP


Link Layer





r 55 Ethernet


r 57 PPP












= 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 Address (more)



r Analogy












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


















A

RB










A

RB


Link Layer





r 55 Ethernet


r 57 PPP



Ethernet








Star topology




hub orswitch




Preamble


















r No slots















Exponential Backoff







CSMACD efficiency






transproptt 51

1efficiency

+

=





twisted pair

hub




m at the same rate




twisted pair

hub


Gbit Ethernet


technologies





r 10 Gbps now


Link Layer





r 55 Ethernet


r 57 PPP







hub hubhub

hub


Switch

r Link layer device




r transparent





Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information



Link Layer





r 55 Ethernet


r 57 PPP






Parity Checking



0 0


Link Layer





r 55 Ethernet


r 57 PPP








m upstream HFC


















4 Simple



Three broad classes


frequency code)





longer turns













freq

uenc

y b

and

s

time









m ALOHA



Slotted ALOHA

Assumptions






Operation





Slotted ALOHA

Pros



r simple
























= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)










CSMA collisions



























Polling



m latency


Token passing


r token message


m latency












LAN technologies


access


m Ethernet

m hubs switches

m PPP


Link Layer





r 55 Ethernet


r 57 PPP












= 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 Address (more)



r Analogy












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


















A

RB










A

RB


Link Layer





r 55 Ethernet


r 57 PPP



Ethernet








Star topology




hub orswitch




Preamble


















r No slots















Exponential Backoff







CSMACD efficiency






transproptt 51

1efficiency

+

=





twisted pair

hub




m at the same rate




twisted pair

hub


Gbit Ethernet


technologies





r 10 Gbps now


Link Layer





r 55 Ethernet


r 57 PPP







hub hubhub

hub


Switch

r Link layer device




r transparent





Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information








m upstream HFC


















4 Simple



Three broad classes


frequency code)





longer turns













freq

uenc

y b

and

s

time









m ALOHA



Slotted ALOHA

Assumptions






Operation





Slotted ALOHA

Pros



r simple
























= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)










CSMA collisions



























Polling



m latency


Token passing


r token message


m latency












LAN technologies


access


m Ethernet

m hubs switches

m PPP


Link Layer





r 55 Ethernet


r 57 PPP












= 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 Address (more)



r Analogy












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


















A

RB










A

RB


Link Layer





r 55 Ethernet


r 57 PPP



Ethernet








Star topology




hub orswitch




Preamble


















r No slots















Exponential Backoff







CSMACD efficiency






transproptt 51

1efficiency

+

=





twisted pair

hub




m at the same rate




twisted pair

hub


Gbit Ethernet


technologies





r 10 Gbps now


Link Layer





r 55 Ethernet


r 57 PPP







hub hubhub

hub


Switch

r Link layer device




r transparent





Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information














freq

uenc

y b

and

s

time









m ALOHA



Slotted ALOHA

Assumptions






Operation





Slotted ALOHA

Pros



r simple
























= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)










CSMA collisions



























Polling



m latency


Token passing


r token message


m latency












LAN technologies


access


m Ethernet

m hubs switches

m PPP


Link Layer





r 55 Ethernet


r 57 PPP












= 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 Address (more)



r Analogy












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


















A

RB










A

RB


Link Layer





r 55 Ethernet


r 57 PPP



Ethernet








Star topology




hub orswitch




Preamble


















r No slots















Exponential Backoff







CSMACD efficiency






transproptt 51

1efficiency

+

=





twisted pair

hub




m at the same rate




twisted pair

hub


Gbit Ethernet


technologies





r 10 Gbps now


Link Layer





r 55 Ethernet


r 57 PPP







hub hubhub

hub


Switch

r Link layer device




r transparent





Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information



Slotted ALOHA

Pros



r simple
























= p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)










CSMA collisions



























Polling



m latency


Token passing


r token message


m latency












LAN technologies


access


m Ethernet

m hubs switches

m PPP


Link Layer





r 55 Ethernet


r 57 PPP












= 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 Address (more)



r Analogy












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


















A

RB










A

RB


Link Layer





r 55 Ethernet


r 57 PPP



Ethernet








Star topology




hub orswitch




Preamble


















r No slots















Exponential Backoff







CSMACD efficiency






transproptt 51

1efficiency

+

=





twisted pair

hub




m at the same rate




twisted pair

hub


Gbit Ethernet


technologies





r 10 Gbps now


Link Layer





r 55 Ethernet


r 57 PPP







hub hubhub

hub


Switch

r Link layer device




r transparent





Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information









CSMA collisions



























Polling



m latency


Token passing


r token message


m latency












LAN technologies


access


m Ethernet

m hubs switches

m PPP


Link Layer





r 55 Ethernet


r 57 PPP












= 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 Address (more)



r Analogy












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


















A

RB










A

RB


Link Layer





r 55 Ethernet


r 57 PPP



Ethernet








Star topology




hub orswitch




Preamble


















r No slots















Exponential Backoff







CSMACD efficiency






transproptt 51

1efficiency

+

=





twisted pair

hub




m at the same rate




twisted pair

hub


Gbit Ethernet


technologies





r 10 Gbps now


Link Layer





r 55 Ethernet


r 57 PPP







hub hubhub

hub


Switch

r Link layer device




r transparent





Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information














Polling



m latency


Token passing


r token message


m latency












LAN technologies


access


m Ethernet

m hubs switches

m PPP


Link Layer





r 55 Ethernet


r 57 PPP












= 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 Address (more)



r Analogy












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


















A

RB










A

RB


Link Layer





r 55 Ethernet


r 57 PPP



Ethernet








Star topology




hub orswitch




Preamble


















r No slots















Exponential Backoff







CSMACD efficiency






transproptt 51

1efficiency

+

=





twisted pair

hub




m at the same rate




twisted pair

hub


Gbit Ethernet


technologies





r 10 Gbps now


Link Layer





r 55 Ethernet


r 57 PPP







hub hubhub

hub


Switch

r Link layer device




r transparent





Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information



Link Layer





r 55 Ethernet


r 57 PPP












= 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 Address (more)



r Analogy












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


















A

RB










A

RB


Link Layer





r 55 Ethernet


r 57 PPP



Ethernet








Star topology




hub orswitch




Preamble


















r No slots















Exponential Backoff







CSMACD efficiency






transproptt 51

1efficiency

+

=





twisted pair

hub




m at the same rate




twisted pair

hub


Gbit Ethernet


technologies





r 10 Gbps now


Link Layer





r 55 Ethernet


r 57 PPP







hub hubhub

hub


Switch

r Link layer device




r transparent





Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information









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


















A

RB










A

RB


Link Layer





r 55 Ethernet


r 57 PPP



Ethernet








Star topology




hub orswitch




Preamble


















r No slots















Exponential Backoff







CSMACD efficiency






transproptt 51

1efficiency

+

=





twisted pair

hub




m at the same rate




twisted pair

hub


Gbit Ethernet


technologies





r 10 Gbps now


Link Layer





r 55 Ethernet


r 57 PPP







hub hubhub

hub


Switch

r Link layer device




r transparent





Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information



Link Layer





r 55 Ethernet


r 57 PPP



Ethernet








Star topology




hub orswitch




Preamble


















r No slots















Exponential Backoff







CSMACD efficiency






transproptt 51

1efficiency

+

=





twisted pair

hub




m at the same rate




twisted pair

hub


Gbit Ethernet


technologies





r 10 Gbps now


Link Layer





r 55 Ethernet


r 57 PPP







hub hubhub

hub


Switch

r Link layer device




r transparent





Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information

















r No slots















Exponential Backoff







CSMACD efficiency






transproptt 51

1efficiency

+

=





twisted pair

hub




m at the same rate




twisted pair

hub


Gbit Ethernet


technologies





r 10 Gbps now


Link Layer





r 55 Ethernet


r 57 PPP







hub hubhub

hub


Switch

r Link layer device




r transparent





Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information






Exponential Backoff







CSMACD efficiency






transproptt 51

1efficiency

+

=





twisted pair

hub




m at the same rate




twisted pair

hub


Gbit Ethernet


technologies





r 10 Gbps now


Link Layer





r 55 Ethernet


r 57 PPP







hub hubhub

hub


Switch

r Link layer device




r transparent





Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information



Gbit Ethernet


technologies





r 10 Gbps now


Link Layer





r 55 Ethernet


r 57 PPP







hub hubhub

hub


Switch

r Link layer device




r transparent





Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information



Forwarding


hub hubhub

switch1

2 3


Self learning









FilteringForwarding








Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3


Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information



Switch example





hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231



segments



hub hub hub

switch


collision domain







switch

A

Arsquo

B

Brsquo

C

Crsquo


More on Switches






hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information




hub hubhub

switch

to externalnetwork

router

IP subnet

mail server

web server








Summary comparison


traffic isolation

no yes yes


optimal routing

no yes no

cut through

yes no yes


Link Layer





r 55 Ethernet


r 57 PPP























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information























r no flow control





PPP Data Frame






PPP Data Frame








r Receiver




Byte Stuffing








layer information



PPP Data Frame








r Receiver




Byte Stuffing








layer information


Documents

Chapter 5 Chapter 5: The Data Link Layer Link Layer and ...network2006/slide/Ch05.pdfreliable data transfer, flow control: done! ... data-link layer has responsibility of transferring