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1
ITC242 ndash Introduction to Data Communications
Chapter 10
2
Last Week
Internet Operation
bull Describe the characteristics of an Internet Address
bull Describe the different classes of IP addresses
bull Explain the purpose of subnet masks
3
Last Week
LAN architecture and protocols
bull Define the various types of Local Area Networks (LANs)
bull Discuss the different types of transmission media commonly used in LANs
4
Topic 10 - Ethernet
bull Learning Objectives
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
5
The Link Layer
Application
Physical
Link
Network
Transport
Session
Presentation
LAN-LINK
IP
TCP UDP
The 7-layer OSI Model The 4-layer Internet Model
Teln
et
FTP
SM
TP
HTTP
NN
TP
TFTP
6
Link Layer IntroductionSome terminologybull hosts and routers are nodesbull communication channels that
connect adjacent nodes along communication path are linksndash wired linksndash wireless linksndash LANs
bull layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
7
Link layer contextbull datagram transferred by
different link protocols over different linksndash eg Ethernet on first link
frame relay on intermediate links 80211 on last link
bull each link protocol provides different servicesndash eg may or may not
provide reliable data transmission over link
transportation analogybull trip from Melbourne to Brisbane
ndash car Melbourne to Alburyndash train Albury to Sydneyndash plane Sydney to Bridbane
bull tourist = datagrambull transport segment =
communication linkbull transportation mode = link layer
protocol
bull travel agent = routing algorithm
8
Link Layer Servicesbull framing link access
ndash encapsulate datagram into frame adding header trailerndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
bull reliable delivery between adjacent nodesndash we learned how to do this alreadyndash seldom used on low bit-error link (fiber some twisted
pair)ndash wireless links high error rates
bull Q why both link-level and end-end reliability
9
Link Layer Services (more)bull flow control
ndash pacing between adjacent sending and receiving nodesbull error detection
ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame bull error correction
ndash receiver identifies and corrects bit error(s) without resorting to retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but
not at same time
10
Where is the link layer implemented
bull in each and every hostbull link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI
card 80211 cardndash implements link
physical layerbull attaches into hostrsquos
system busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
11
Adaptors Communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes to
upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point
ndash PPP ( point-to-point protocol) for dial-up accessndash point-to-point link between Ethernet switch and host
bull broadcast (shared wire or medium) multiple sending and receiving nodes all connected to the same single shared broadcast channel Any one node transmits a frame the channel broadcasts the frame and each of other nodes receives a copyndash old-fashioned Ethernetndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
13
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by nodes
interference ndash collision if node receives two or more signals at the same
time
multiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
2
Last Week
Internet Operation
bull Describe the characteristics of an Internet Address
bull Describe the different classes of IP addresses
bull Explain the purpose of subnet masks
3
Last Week
LAN architecture and protocols
bull Define the various types of Local Area Networks (LANs)
bull Discuss the different types of transmission media commonly used in LANs
4
Topic 10 - Ethernet
bull Learning Objectives
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
5
The Link Layer
Application
Physical
Link
Network
Transport
Session
Presentation
LAN-LINK
IP
TCP UDP
The 7-layer OSI Model The 4-layer Internet Model
Teln
et
FTP
SM
TP
HTTP
NN
TP
TFTP
6
Link Layer IntroductionSome terminologybull hosts and routers are nodesbull communication channels that
connect adjacent nodes along communication path are linksndash wired linksndash wireless linksndash LANs
bull layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
7
Link layer contextbull datagram transferred by
different link protocols over different linksndash eg Ethernet on first link
frame relay on intermediate links 80211 on last link
bull each link protocol provides different servicesndash eg may or may not
provide reliable data transmission over link
transportation analogybull trip from Melbourne to Brisbane
ndash car Melbourne to Alburyndash train Albury to Sydneyndash plane Sydney to Bridbane
bull tourist = datagrambull transport segment =
communication linkbull transportation mode = link layer
protocol
bull travel agent = routing algorithm
8
Link Layer Servicesbull framing link access
ndash encapsulate datagram into frame adding header trailerndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
bull reliable delivery between adjacent nodesndash we learned how to do this alreadyndash seldom used on low bit-error link (fiber some twisted
pair)ndash wireless links high error rates
bull Q why both link-level and end-end reliability
9
Link Layer Services (more)bull flow control
ndash pacing between adjacent sending and receiving nodesbull error detection
ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame bull error correction
ndash receiver identifies and corrects bit error(s) without resorting to retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but
not at same time
10
Where is the link layer implemented
bull in each and every hostbull link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI
card 80211 cardndash implements link
physical layerbull attaches into hostrsquos
system busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
11
Adaptors Communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes to
upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point
ndash PPP ( point-to-point protocol) for dial-up accessndash point-to-point link between Ethernet switch and host
bull broadcast (shared wire or medium) multiple sending and receiving nodes all connected to the same single shared broadcast channel Any one node transmits a frame the channel broadcasts the frame and each of other nodes receives a copyndash old-fashioned Ethernetndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
13
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by nodes
interference ndash collision if node receives two or more signals at the same
time
multiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
3
Last Week
LAN architecture and protocols
bull Define the various types of Local Area Networks (LANs)
bull Discuss the different types of transmission media commonly used in LANs
4
Topic 10 - Ethernet
bull Learning Objectives
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
5
The Link Layer
Application
Physical
Link
Network
Transport
Session
Presentation
LAN-LINK
IP
TCP UDP
The 7-layer OSI Model The 4-layer Internet Model
Teln
et
FTP
SM
TP
HTTP
NN
TP
TFTP
6
Link Layer IntroductionSome terminologybull hosts and routers are nodesbull communication channels that
connect adjacent nodes along communication path are linksndash wired linksndash wireless linksndash LANs
bull layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
7
Link layer contextbull datagram transferred by
different link protocols over different linksndash eg Ethernet on first link
frame relay on intermediate links 80211 on last link
bull each link protocol provides different servicesndash eg may or may not
provide reliable data transmission over link
transportation analogybull trip from Melbourne to Brisbane
ndash car Melbourne to Alburyndash train Albury to Sydneyndash plane Sydney to Bridbane
bull tourist = datagrambull transport segment =
communication linkbull transportation mode = link layer
protocol
bull travel agent = routing algorithm
8
Link Layer Servicesbull framing link access
ndash encapsulate datagram into frame adding header trailerndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
bull reliable delivery between adjacent nodesndash we learned how to do this alreadyndash seldom used on low bit-error link (fiber some twisted
pair)ndash wireless links high error rates
bull Q why both link-level and end-end reliability
9
Link Layer Services (more)bull flow control
ndash pacing between adjacent sending and receiving nodesbull error detection
ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame bull error correction
ndash receiver identifies and corrects bit error(s) without resorting to retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but
not at same time
10
Where is the link layer implemented
bull in each and every hostbull link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI
card 80211 cardndash implements link
physical layerbull attaches into hostrsquos
system busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
11
Adaptors Communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes to
upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point
ndash PPP ( point-to-point protocol) for dial-up accessndash point-to-point link between Ethernet switch and host
bull broadcast (shared wire or medium) multiple sending and receiving nodes all connected to the same single shared broadcast channel Any one node transmits a frame the channel broadcasts the frame and each of other nodes receives a copyndash old-fashioned Ethernetndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
13
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by nodes
interference ndash collision if node receives two or more signals at the same
time
multiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
4
Topic 10 - Ethernet
bull Learning Objectives
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
5
The Link Layer
Application
Physical
Link
Network
Transport
Session
Presentation
LAN-LINK
IP
TCP UDP
The 7-layer OSI Model The 4-layer Internet Model
Teln
et
FTP
SM
TP
HTTP
NN
TP
TFTP
6
Link Layer IntroductionSome terminologybull hosts and routers are nodesbull communication channels that
connect adjacent nodes along communication path are linksndash wired linksndash wireless linksndash LANs
bull layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
7
Link layer contextbull datagram transferred by
different link protocols over different linksndash eg Ethernet on first link
frame relay on intermediate links 80211 on last link
bull each link protocol provides different servicesndash eg may or may not
provide reliable data transmission over link
transportation analogybull trip from Melbourne to Brisbane
ndash car Melbourne to Alburyndash train Albury to Sydneyndash plane Sydney to Bridbane
bull tourist = datagrambull transport segment =
communication linkbull transportation mode = link layer
protocol
bull travel agent = routing algorithm
8
Link Layer Servicesbull framing link access
ndash encapsulate datagram into frame adding header trailerndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
bull reliable delivery between adjacent nodesndash we learned how to do this alreadyndash seldom used on low bit-error link (fiber some twisted
pair)ndash wireless links high error rates
bull Q why both link-level and end-end reliability
9
Link Layer Services (more)bull flow control
ndash pacing between adjacent sending and receiving nodesbull error detection
ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame bull error correction
ndash receiver identifies and corrects bit error(s) without resorting to retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but
not at same time
10
Where is the link layer implemented
bull in each and every hostbull link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI
card 80211 cardndash implements link
physical layerbull attaches into hostrsquos
system busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
11
Adaptors Communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes to
upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point
ndash PPP ( point-to-point protocol) for dial-up accessndash point-to-point link between Ethernet switch and host
bull broadcast (shared wire or medium) multiple sending and receiving nodes all connected to the same single shared broadcast channel Any one node transmits a frame the channel broadcasts the frame and each of other nodes receives a copyndash old-fashioned Ethernetndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
13
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by nodes
interference ndash collision if node receives two or more signals at the same
time
multiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
5
The Link Layer
Application
Physical
Link
Network
Transport
Session
Presentation
LAN-LINK
IP
TCP UDP
The 7-layer OSI Model The 4-layer Internet Model
Teln
et
FTP
SM
TP
HTTP
NN
TP
TFTP
6
Link Layer IntroductionSome terminologybull hosts and routers are nodesbull communication channels that
connect adjacent nodes along communication path are linksndash wired linksndash wireless linksndash LANs
bull layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
7
Link layer contextbull datagram transferred by
different link protocols over different linksndash eg Ethernet on first link
frame relay on intermediate links 80211 on last link
bull each link protocol provides different servicesndash eg may or may not
provide reliable data transmission over link
transportation analogybull trip from Melbourne to Brisbane
ndash car Melbourne to Alburyndash train Albury to Sydneyndash plane Sydney to Bridbane
bull tourist = datagrambull transport segment =
communication linkbull transportation mode = link layer
protocol
bull travel agent = routing algorithm
8
Link Layer Servicesbull framing link access
ndash encapsulate datagram into frame adding header trailerndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
bull reliable delivery between adjacent nodesndash we learned how to do this alreadyndash seldom used on low bit-error link (fiber some twisted
pair)ndash wireless links high error rates
bull Q why both link-level and end-end reliability
9
Link Layer Services (more)bull flow control
ndash pacing between adjacent sending and receiving nodesbull error detection
ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame bull error correction
ndash receiver identifies and corrects bit error(s) without resorting to retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but
not at same time
10
Where is the link layer implemented
bull in each and every hostbull link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI
card 80211 cardndash implements link
physical layerbull attaches into hostrsquos
system busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
11
Adaptors Communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes to
upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point
ndash PPP ( point-to-point protocol) for dial-up accessndash point-to-point link between Ethernet switch and host
bull broadcast (shared wire or medium) multiple sending and receiving nodes all connected to the same single shared broadcast channel Any one node transmits a frame the channel broadcasts the frame and each of other nodes receives a copyndash old-fashioned Ethernetndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
13
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by nodes
interference ndash collision if node receives two or more signals at the same
time
multiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
6
Link Layer IntroductionSome terminologybull hosts and routers are nodesbull communication channels that
connect adjacent nodes along communication path are linksndash wired linksndash wireless linksndash LANs
bull layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
7
Link layer contextbull datagram transferred by
different link protocols over different linksndash eg Ethernet on first link
frame relay on intermediate links 80211 on last link
bull each link protocol provides different servicesndash eg may or may not
provide reliable data transmission over link
transportation analogybull trip from Melbourne to Brisbane
ndash car Melbourne to Alburyndash train Albury to Sydneyndash plane Sydney to Bridbane
bull tourist = datagrambull transport segment =
communication linkbull transportation mode = link layer
protocol
bull travel agent = routing algorithm
8
Link Layer Servicesbull framing link access
ndash encapsulate datagram into frame adding header trailerndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
bull reliable delivery between adjacent nodesndash we learned how to do this alreadyndash seldom used on low bit-error link (fiber some twisted
pair)ndash wireless links high error rates
bull Q why both link-level and end-end reliability
9
Link Layer Services (more)bull flow control
ndash pacing between adjacent sending and receiving nodesbull error detection
ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame bull error correction
ndash receiver identifies and corrects bit error(s) without resorting to retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but
not at same time
10
Where is the link layer implemented
bull in each and every hostbull link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI
card 80211 cardndash implements link
physical layerbull attaches into hostrsquos
system busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
11
Adaptors Communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes to
upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point
ndash PPP ( point-to-point protocol) for dial-up accessndash point-to-point link between Ethernet switch and host
bull broadcast (shared wire or medium) multiple sending and receiving nodes all connected to the same single shared broadcast channel Any one node transmits a frame the channel broadcasts the frame and each of other nodes receives a copyndash old-fashioned Ethernetndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
13
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by nodes
interference ndash collision if node receives two or more signals at the same
time
multiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
7
Link layer contextbull datagram transferred by
different link protocols over different linksndash eg Ethernet on first link
frame relay on intermediate links 80211 on last link
bull each link protocol provides different servicesndash eg may or may not
provide reliable data transmission over link
transportation analogybull trip from Melbourne to Brisbane
ndash car Melbourne to Alburyndash train Albury to Sydneyndash plane Sydney to Bridbane
bull tourist = datagrambull transport segment =
communication linkbull transportation mode = link layer
protocol
bull travel agent = routing algorithm
8
Link Layer Servicesbull framing link access
ndash encapsulate datagram into frame adding header trailerndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
bull reliable delivery between adjacent nodesndash we learned how to do this alreadyndash seldom used on low bit-error link (fiber some twisted
pair)ndash wireless links high error rates
bull Q why both link-level and end-end reliability
9
Link Layer Services (more)bull flow control
ndash pacing between adjacent sending and receiving nodesbull error detection
ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame bull error correction
ndash receiver identifies and corrects bit error(s) without resorting to retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but
not at same time
10
Where is the link layer implemented
bull in each and every hostbull link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI
card 80211 cardndash implements link
physical layerbull attaches into hostrsquos
system busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
11
Adaptors Communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes to
upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point
ndash PPP ( point-to-point protocol) for dial-up accessndash point-to-point link between Ethernet switch and host
bull broadcast (shared wire or medium) multiple sending and receiving nodes all connected to the same single shared broadcast channel Any one node transmits a frame the channel broadcasts the frame and each of other nodes receives a copyndash old-fashioned Ethernetndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
13
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by nodes
interference ndash collision if node receives two or more signals at the same
time
multiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
8
Link Layer Servicesbull framing link access
ndash encapsulate datagram into frame adding header trailerndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
bull reliable delivery between adjacent nodesndash we learned how to do this alreadyndash seldom used on low bit-error link (fiber some twisted
pair)ndash wireless links high error rates
bull Q why both link-level and end-end reliability
9
Link Layer Services (more)bull flow control
ndash pacing between adjacent sending and receiving nodesbull error detection
ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame bull error correction
ndash receiver identifies and corrects bit error(s) without resorting to retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but
not at same time
10
Where is the link layer implemented
bull in each and every hostbull link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI
card 80211 cardndash implements link
physical layerbull attaches into hostrsquos
system busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
11
Adaptors Communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes to
upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point
ndash PPP ( point-to-point protocol) for dial-up accessndash point-to-point link between Ethernet switch and host
bull broadcast (shared wire or medium) multiple sending and receiving nodes all connected to the same single shared broadcast channel Any one node transmits a frame the channel broadcasts the frame and each of other nodes receives a copyndash old-fashioned Ethernetndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
13
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by nodes
interference ndash collision if node receives two or more signals at the same
time
multiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
9
Link Layer Services (more)bull flow control
ndash pacing between adjacent sending and receiving nodesbull error detection
ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame bull error correction
ndash receiver identifies and corrects bit error(s) without resorting to retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but
not at same time
10
Where is the link layer implemented
bull in each and every hostbull link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI
card 80211 cardndash implements link
physical layerbull attaches into hostrsquos
system busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
11
Adaptors Communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes to
upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point
ndash PPP ( point-to-point protocol) for dial-up accessndash point-to-point link between Ethernet switch and host
bull broadcast (shared wire or medium) multiple sending and receiving nodes all connected to the same single shared broadcast channel Any one node transmits a frame the channel broadcasts the frame and each of other nodes receives a copyndash old-fashioned Ethernetndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
13
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by nodes
interference ndash collision if node receives two or more signals at the same
time
multiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
10
Where is the link layer implemented
bull in each and every hostbull link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI
card 80211 cardndash implements link
physical layerbull attaches into hostrsquos
system busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
11
Adaptors Communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes to
upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point
ndash PPP ( point-to-point protocol) for dial-up accessndash point-to-point link between Ethernet switch and host
bull broadcast (shared wire or medium) multiple sending and receiving nodes all connected to the same single shared broadcast channel Any one node transmits a frame the channel broadcasts the frame and each of other nodes receives a copyndash old-fashioned Ethernetndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
13
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by nodes
interference ndash collision if node receives two or more signals at the same
time
multiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
11
Adaptors Communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes to
upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point
ndash PPP ( point-to-point protocol) for dial-up accessndash point-to-point link between Ethernet switch and host
bull broadcast (shared wire or medium) multiple sending and receiving nodes all connected to the same single shared broadcast channel Any one node transmits a frame the channel broadcasts the frame and each of other nodes receives a copyndash old-fashioned Ethernetndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
13
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by nodes
interference ndash collision if node receives two or more signals at the same
time
multiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
12
Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point
ndash PPP ( point-to-point protocol) for dial-up accessndash point-to-point link between Ethernet switch and host
bull broadcast (shared wire or medium) multiple sending and receiving nodes all connected to the same single shared broadcast channel Any one node transmits a frame the channel broadcasts the frame and each of other nodes receives a copyndash old-fashioned Ethernetndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
13
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by nodes
interference ndash collision if node receives two or more signals at the same
time
multiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
13
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by nodes
interference ndash collision if node receives two or more signals at the same
time
multiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
14
MAC Protocols a taxonomy
Three broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive use
bull Random Accessndash channel not divided allow collisions
ndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash nodes take turns but nodes with more to send can
take longer turns
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
15
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)
bull Examples of random access MAC protocolsndash CSMA CSMACD CSMACA
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
16
CSMACD Protocol
All hosts transmit amp receive on one channelPackets are of variable size
When a host has a packet to transmit1 Carrier Sense Check that the line is quiet before
transmitting2 Collision Detection Detect collision as soon as possible If
a collision is detected stop transmitting wait a random time then return to step 1
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
17
Token Passing
bull A token rotates around a ring to each node in turn
bull All nodes (computers routers etc) copy all data and tokens and repeat them along the ring
bull When a node wishes to transmit packet(s) it grabs the token as it passes
bull It holds the token while it transmitsbull When it is done it releases the token again and
sends it on its way
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
18
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead
ndash latency
ndash single point of failure (master)
master
slaves
poll
data
data
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
19
ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed
from one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure
(token)
T
data
(nothingto send)
T
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
20
MAC Addressesbull 32-bit IP address
ndash network-layer addressndash used to get datagram to destination IP subnet
bull MAC (or LAN or physical or Ethernet) address ndash function get frame from one interface to another
physically-connected interface (same network)ndash 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
21
LAN AddressesEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD( hexadecimal)
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
22
LAN Address (more)bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
(a) MAC address like Social Security Number
(b) IP address like postal addressbull MAC flat address portability
ndash can move LAN card from one LAN to another
bull IP hierarchical address NOT portablendash address depends on IP subnet to which node is attached
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
23
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble bull 7 bytes with pattern 10101010 (ldquowake uprdquo)
followed by one byte with pattern 10101011bull used to synchronize receiver sender clock rates
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
24
Ethernet Frame Structure (more)bull Addresses 6 bytes
ndash if adapter receives frame with matching destination address or with broadcast address it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame
is dropped
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
25
Ethernet Unreliable connectionless
bull connectionless No handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnrsquot send acks to sending NICndash stream of datagrams passed to network layer can
have gaps (missing datagrams)ndash gaps will be filled if app is using TCPndash otherwise app will see gaps
bull Ethernetrsquos MAC protocol unslotted CSMACD
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
26
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal( to make sure all other
transmitters are aware of collision 48 bits)
5 During aborting after mth collision NIC chooses K at random from012hellip2m-1 NIC waits K512 bit times(Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec) returns to Step 2
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
27
The Original Ethernet
Thick copper coaxial cable
ml 1500
10Mbs
In practice minimum packet size = 512 bitsbull allows for extra time to detect collisionsbull allows for ldquorepeatersrdquo that can boost signal
Repeaters every 500m
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
28
8023 Ethernet Standards Link amp Physical Layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
29
8023 Medium Notation
bull Notation formatltdata rate in Mbpsgtltsignaling methodgtltmaximum segment length in hundreds of metersgt
bull eg 10Base5 provides 10Mbps baseband up to 500 meters
bull T and F are used in place of segment length for twisted pair and fiber
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
30
8023 10BaseX Media Options
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
31
An Ethernet Network
RouterRouterOutsideworldProblem
Shared network limits throughputLots of collisions reduces efficiency
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
32
Ethernet Switching
RouterRouter Outsideworld
Benefits Number of collisions is reduced If only one computer per port no collisions can take place (each cable is now a self-contained point-to-point Ethernet link) Capacity is increased the switch can forward multiple frames to different computers at the same time
EthernetSwitch
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
33
Switchbull link-layer device smarter than hubs take active role
ndash store forward Ethernet framesndash examine incoming framersquos MAC address selectively forward
frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
34
One Ethernet Switch in ldquoPackardrdquo
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
35
Star topologybull bus topology popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull today star topology prevailsndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
36
Switch allows multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on
each incoming link but no collisions full duplexndash each link is its own
collision domainbull switching A-to-Arsquo and B-to-Brsquo
simultaneously without collisions ndash not possible with dumb
hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
37
Switch Tablebull Q how does switch know that
Arsquo reachable via interface 4 Brsquo reachable via interface 5
bull A each switch has a switch table each entryndash (MAC address of host interface
to reach host time stamp)
bull looks like a routing table
bull Q how are entries created maintained in switch table ndash something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
38
Switch self-learningbull switch learns which
hosts can be reached through which interfacesndash when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)TTL time-to-live
A 1 60
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
39
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host
2 index switch table using MAC dest address
3 if entry found for destination then
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated
else flood
forward on all but the interface on which the frame arrived
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
40
Self-learning
forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination unknownflood
Arsquo A
bull destination A location known
Arsquo 4 60
selective send
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
41
Interconnecting switches
bull switches can be connected together
A
B
bull Q sending from A to F - how does S1 know to forward frame destined to F via S4 and S3
bull A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
42
Institutional Network
to externalnetwork
router
IP subnet
mail server
web server
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
43
Switches vs Routersbull both store-and-forward devices
ndash routers network layer devices (examine network layer headers)ndash switches are link layer devices
bull routers maintain routing tables implement routing algorithmsbull switches maintain switch tables implement filtering learning
algorithms
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
44
Hubs
bull Alternative to bus topologybull Each station is connected to the hub by two lines
(transmit and receive)
bull When a single station transmits the hub repeats the signal on the outgoing line to each station
bull Physically a star logically a busbull Hubs can be cascaded in a hierarchical
configuration
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
45
Hubshellip physical-layer (ldquodumbrdquo) repeaters
ndash bits coming in one link go out all other links at same ratendash all nodes connected to hub can collide with one anotherndash no frame bufferingndash no CSMACD at hub host NICs detect collisions
twisted pair
hub
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
46
Interconnecting LANs
bull Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router
bull By partitioning a large LAN into multiple smaller networks there are fewer collisions and more parallel communications
bull Provide a number of advantagesndash Reliability Creates self-contained unitsndash Performance Less contentionndash Security Not all data broadcast to all usersndash Geography Allows long-distance links
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
47
Key Aspects of Bridge Function
bull Makes no modification to content or format of frames it receives simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN
bull Should contain enough buffer space to meet peak demands
bull Must contain addressing and routing intelligence bull May connect more than two LANs
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
48
Bridge Operation
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
49
Bridge Functions
bull Read all frames from each network
bull Accept frames from sender on one network that are addressed to a receiver on the other network
bull Retransmit frames from sender using MAC protocol for receiver
bull Must have some routing information stored in order to know which frames to pass
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
50
Summary comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
51
Two-Level Hub Topology
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
52
Summary
Topic 10 - Ethernet
bull Describe the characteristics of Ethernet networks
bull Discuss the operation of CSMACD
bull Discuss the operation of bridges hubs and switches
bull Describe the characteristics of fast Ethernet standards
