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5 DataLink Layer 5-1
Chapter 5Link Layer and LANs
Part B
Computer Networking A Top Down Approach 4th edition Jim Kurose Keith RossAddison-Wesley July 2007
5 DataLink Layer 5-2
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-3
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-4
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
5 DataLink Layer 5-5
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-6
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-7
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-8
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-9
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-10
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-11
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-12
Collision Detection
On baseband bus collision produces much higher signal voltage than signal
Collision detected if cable signal greater than single station signal
Signal attenuated over distance Limit distance to 500m (10Base5) or 200m
(10Base2) For twisted pair (star-topology) activity on
more than one port is collision Special collision presence signal
5 DataLink Layer 5-13
Algorithm (cont)
If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes
of data) delay and try again
bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected
k=02n - 1bull give up after several tries (usually 16)bull exponential backoff
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-2
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-3
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-4
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
5 DataLink Layer 5-5
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-6
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-7
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-8
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-9
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-10
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-11
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-12
Collision Detection
On baseband bus collision produces much higher signal voltage than signal
Collision detected if cable signal greater than single station signal
Signal attenuated over distance Limit distance to 500m (10Base5) or 200m
(10Base2) For twisted pair (star-topology) activity on
more than one port is collision Special collision presence signal
5 DataLink Layer 5-13
Algorithm (cont)
If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes
of data) delay and try again
bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected
k=02n - 1bull give up after several tries (usually 16)bull exponential backoff
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-3
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-4
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
5 DataLink Layer 5-5
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-6
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-7
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-8
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-9
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-10
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-11
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-12
Collision Detection
On baseband bus collision produces much higher signal voltage than signal
Collision detected if cable signal greater than single station signal
Signal attenuated over distance Limit distance to 500m (10Base5) or 200m
(10Base2) For twisted pair (star-topology) activity on
more than one port is collision Special collision presence signal
5 DataLink Layer 5-13
Algorithm (cont)
If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes
of data) delay and try again
bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected
k=02n - 1bull give up after several tries (usually 16)bull exponential backoff
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-4
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
5 DataLink Layer 5-5
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-6
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-7
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-8
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-9
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-10
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-11
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-12
Collision Detection
On baseband bus collision produces much higher signal voltage than signal
Collision detected if cable signal greater than single station signal
Signal attenuated over distance Limit distance to 500m (10Base5) or 200m
(10Base2) For twisted pair (star-topology) activity on
more than one port is collision Special collision presence signal
5 DataLink Layer 5-13
Algorithm (cont)
If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes
of data) delay and try again
bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected
k=02n - 1bull give up after several tries (usually 16)bull exponential backoff
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-5
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-6
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-7
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-8
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-9
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-10
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-11
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-12
Collision Detection
On baseband bus collision produces much higher signal voltage than signal
Collision detected if cable signal greater than single station signal
Signal attenuated over distance Limit distance to 500m (10Base5) or 200m
(10Base2) For twisted pair (star-topology) activity on
more than one port is collision Special collision presence signal
5 DataLink Layer 5-13
Algorithm (cont)
If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes
of data) delay and try again
bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected
k=02n - 1bull give up after several tries (usually 16)bull exponential backoff
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-6
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-7
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-8
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-9
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-10
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-11
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-12
Collision Detection
On baseband bus collision produces much higher signal voltage than signal
Collision detected if cable signal greater than single station signal
Signal attenuated over distance Limit distance to 500m (10Base5) or 200m
(10Base2) For twisted pair (star-topology) activity on
more than one port is collision Special collision presence signal
5 DataLink Layer 5-13
Algorithm (cont)
If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes
of data) delay and try again
bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected
k=02n - 1bull give up after several tries (usually 16)bull exponential backoff
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-7
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-8
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-9
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-10
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-11
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-12
Collision Detection
On baseband bus collision produces much higher signal voltage than signal
Collision detected if cable signal greater than single station signal
Signal attenuated over distance Limit distance to 500m (10Base5) or 200m
(10Base2) For twisted pair (star-topology) activity on
more than one port is collision Special collision presence signal
5 DataLink Layer 5-13
Algorithm (cont)
If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes
of data) delay and try again
bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected
k=02n - 1bull give up after several tries (usually 16)bull exponential backoff
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-8
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-9
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-10
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-11
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-12
Collision Detection
On baseband bus collision produces much higher signal voltage than signal
Collision detected if cable signal greater than single station signal
Signal attenuated over distance Limit distance to 500m (10Base5) or 200m
(10Base2) For twisted pair (star-topology) activity on
more than one port is collision Special collision presence signal
5 DataLink Layer 5-13
Algorithm (cont)
If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes
of data) delay and try again
bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected
k=02n - 1bull give up after several tries (usually 16)bull exponential backoff
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-9
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-10
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-11
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-12
Collision Detection
On baseband bus collision produces much higher signal voltage than signal
Collision detected if cable signal greater than single station signal
Signal attenuated over distance Limit distance to 500m (10Base5) or 200m
(10Base2) For twisted pair (star-topology) activity on
more than one port is collision Special collision presence signal
5 DataLink Layer 5-13
Algorithm (cont)
If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes
of data) delay and try again
bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected
k=02n - 1bull give up after several tries (usually 16)bull exponential backoff
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-10
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-11
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-12
Collision Detection
On baseband bus collision produces much higher signal voltage than signal
Collision detected if cable signal greater than single station signal
Signal attenuated over distance Limit distance to 500m (10Base5) or 200m
(10Base2) For twisted pair (star-topology) activity on
more than one port is collision Special collision presence signal
5 DataLink Layer 5-13
Algorithm (cont)
If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes
of data) delay and try again
bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected
k=02n - 1bull give up after several tries (usually 16)bull exponential backoff
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-11
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-12
Collision Detection
On baseband bus collision produces much higher signal voltage than signal
Collision detected if cable signal greater than single station signal
Signal attenuated over distance Limit distance to 500m (10Base5) or 200m
(10Base2) For twisted pair (star-topology) activity on
more than one port is collision Special collision presence signal
5 DataLink Layer 5-13
Algorithm (cont)
If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes
of data) delay and try again
bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected
k=02n - 1bull give up after several tries (usually 16)bull exponential backoff
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-12
Collision Detection
On baseband bus collision produces much higher signal voltage than signal
Collision detected if cable signal greater than single station signal
Signal attenuated over distance Limit distance to 500m (10Base5) or 200m
(10Base2) For twisted pair (star-topology) activity on
more than one port is collision Special collision presence signal
5 DataLink Layer 5-13
Algorithm (cont)
If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes
of data) delay and try again
bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected
k=02n - 1bull give up after several tries (usually 16)bull exponential backoff
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-13
Algorithm (cont)
If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes
of data) delay and try again
bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected
k=02n - 1bull give up after several tries (usually 16)bull exponential backoff
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-14
Binary Exponential Backoff
Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and
reports error As congestion increases stations back off by larger
amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff
efficient over wide range of loads Low loads 1-persistence guarantees station can
seize channel once idle High loads at least as stable as other techniques
Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-15
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-16
CSMACD efficiency Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap
transprop tt 51
1efficiency
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-17
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-18
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps 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
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-19
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-20
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-21
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-22
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-23
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-24
100Mbps Fast Ethernet
Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications
from FDDI Two physical links between nodes
bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP
bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP
bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a
time
Star-wire topology Similar to 10BASE-T
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-25
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-26
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-27
Gigabit Ethernet
Gigabit Ethernet cabling
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-28
Wireless Link CharacteristicsDifferences from wired link hellip
decreased signal strength radio signal attenuates as it propagates through matter (path loss)
interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well
multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times
hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-29
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access)
AB
C
Hidden terminal problem B A hear each other B C hear each other A C can not hear each
othermeans A C unaware of their
interference at B
A B C
Arsquos signalstrength
space
Crsquos signalstrength
Signal fading B A hear each other B C hear each other A C can not hear each other
interferring at B
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-30
IEEE 80211 Wireless LAN
80211b 24-5 GHz unlicensed
radio spectrum up to 11 Mbps direct sequence
spread spectrum (DSSS) in physical layer
bull all hosts use same chipping code
widely deployed using base stations
80211a 5-6 GHz range up to 54 Mbps
80211g 24-5 GHz range up to 54 Mbps
All use CSMACA for multiple access
All have base-station and ad-hoc network versions
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-31
Figure 3-12
ISM bands
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-32
80211 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set (BSS)
(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base
station ad hoc mode hosts
only
BSS 1
BSS 2
Internet
hub switchor routerAP
AP
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-33
80211 Channels association 80211b 24GHz-2485GHz spectrum divided
into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as
that chosen by neighboring AP host must associate with an AP
scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address
selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in
APrsquos subnet
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-34
IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same
time 80211 CSMA - sense before transmitting
donrsquot collide with ongoing transmission by other node
80211 no collision detection difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) canrsquot sense all collisions in any case hidden terminal
fading goal avoid collisions CSMAC(ollision)A(voidance)
AB
CA B C
Arsquos signalstrength
space
Crsquos signalstrength
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-35
IEEE 80211 MAC Protocol CSMACA
80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff
interval repeat 2
80211 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-36
Avoiding collisions (more)
idea allow sender to ldquoreserverdquo channel rather than random access of data frames avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyrsquore
short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-37
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-38
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-39
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-40
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-41
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-42
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-43
Learning Bridges Do not forward when unnecessary Maintain forwarding table
HostPort
A 1 B 1 C 1 X 2 Y 2 Z 2
Learn table entries based on source address Table is an optimization need not be complete Always forward broadcast frames
A
Bridge
B C
X Y Z
Port 1
Port 2
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-44
Spanning Tree Algorithm Problem loops
Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 8021 specification
A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
(a) (b)
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-45
Algorithm Overview Each bridge has unique id (eg B1 B2
B3) Select bridge with smallest id as root Select bridge on each LAN closest to root
as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-46
Algorithm Details
Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be
root bridge distance (hops) from sending bridge to root
bridge Each bridge records current best
configuration message for each port Initially each bridge believes it is the root
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-47
Algorithm Detail (cont) When learn not root stop generating config
messages in steady state only root generates configuration messages
When learn not designated bridge stop forwarding config messages in steady state only designated bridges forward config
messages
Root continues to periodically send config messages If any bridge does not receive config message after
a period of time it starts generating config messages claiming to be the root
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-48
Broadcast and Multicast Forward all broadcastmulticast frames
current practice Learn when no group members
downstream Accomplished by having each member of
group G send a frame to bridge multicast address with G in source field
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-49
Limitations of Bridges
Do not scale spanning tree algorithm does not scale broadcast does not scale
Do not accommodate heterogeneity
Caution beware of transparency
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-50
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames 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
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-51
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-52
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-53
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
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)
A 1 60
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-54
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-56
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
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-57
FilteringForwardingWhen switch receives a frame
index switch table using MAC dest addressif 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
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-58
Switch example
Suppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-59
Switch example
Suppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-60
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-61
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-62
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-63
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-64
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-65
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
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
5 DataLink Layer 5-66
IEEE 802 Standards
The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up
