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5 DataLink Layer 5-1
Link Layer
Link layer services Error detection and
correction Multiple access
protocols 54 Link-Layer
Addressing 55 Ethernet
56 Hubs and switches
5 DataLink Layer 5-2
Recap Error Detection and MAP
Link Layer Services Framing reliable transfer error detectioncorrection
Error Detection and Correction Techniques Parity Checks Checksum Methods Cyclic Redundancy Check (CRC)
Multiple Access Protocols Channel Partitioning Protocols by time frequency code (CDMA) Random Access Protocols
bull ALOHA S-ALOHA CSMA bull CSMACD in Ethernetbull CSMACA in 80211
Taking Turns Protocols token ring Local Area Networks link-layer services
5 DataLink Layer 5-3
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (Media Access Control or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-4
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-5
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) httpstandardsieeeorgregauthouiindexshtml
Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-6
ARP Address Resolution Protocol [RFC 826]
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
How to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-7
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-8
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-9
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-10
DHCP (Dynamic Host Configuration Protocol)
The DHCP relay agent (implemented in the IP router) records the subnet from which the message was received in the DHCP message header for use by the DHCP server
5 DataLink Layer 5-11
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 5-12
Ethernet
ldquodominantrdquo wired LAN technology cheap $ 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 5-13
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-2
Recap Error Detection and MAP
Link Layer Services Framing reliable transfer error detectioncorrection
Error Detection and Correction Techniques Parity Checks Checksum Methods Cyclic Redundancy Check (CRC)
Multiple Access Protocols Channel Partitioning Protocols by time frequency code (CDMA) Random Access Protocols
bull ALOHA S-ALOHA CSMA bull CSMACD in Ethernetbull CSMACA in 80211
Taking Turns Protocols token ring Local Area Networks link-layer services
5 DataLink Layer 5-3
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (Media Access Control or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-4
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-5
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) httpstandardsieeeorgregauthouiindexshtml
Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-6
ARP Address Resolution Protocol [RFC 826]
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
How to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-7
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-8
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-9
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-10
DHCP (Dynamic Host Configuration Protocol)
The DHCP relay agent (implemented in the IP router) records the subnet from which the message was received in the DHCP message header for use by the DHCP server
5 DataLink Layer 5-11
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 5-12
Ethernet
ldquodominantrdquo wired LAN technology cheap $ 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 5-13
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-3
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (Media Access Control or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-4
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-5
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) httpstandardsieeeorgregauthouiindexshtml
Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-6
ARP Address Resolution Protocol [RFC 826]
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
How to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-7
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-8
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-9
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-10
DHCP (Dynamic Host Configuration Protocol)
The DHCP relay agent (implemented in the IP router) records the subnet from which the message was received in the DHCP message header for use by the DHCP server
5 DataLink Layer 5-11
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 5-12
Ethernet
ldquodominantrdquo wired LAN technology cheap $ 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 5-13
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-4
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-5
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) httpstandardsieeeorgregauthouiindexshtml
Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-6
ARP Address Resolution Protocol [RFC 826]
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
How to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-7
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-8
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-9
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-10
DHCP (Dynamic Host Configuration Protocol)
The DHCP relay agent (implemented in the IP router) records the subnet from which the message was received in the DHCP message header for use by the DHCP server
5 DataLink Layer 5-11
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 5-12
Ethernet
ldquodominantrdquo wired LAN technology cheap $ 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 5-13
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-5
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) httpstandardsieeeorgregauthouiindexshtml
Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-6
ARP Address Resolution Protocol [RFC 826]
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
How to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-7
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-8
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-9
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-10
DHCP (Dynamic Host Configuration Protocol)
The DHCP relay agent (implemented in the IP router) records the subnet from which the message was received in the DHCP message header for use by the DHCP server
5 DataLink Layer 5-11
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 5-12
Ethernet
ldquodominantrdquo wired LAN technology cheap $ 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 5-13
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-6
ARP Address Resolution Protocol [RFC 826]
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
How to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-7
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-8
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-9
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-10
DHCP (Dynamic Host Configuration Protocol)
The DHCP relay agent (implemented in the IP router) records the subnet from which the message was received in the DHCP message header for use by the DHCP server
5 DataLink Layer 5-11
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 5-12
Ethernet
ldquodominantrdquo wired LAN technology cheap $ 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 5-13
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-7
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-8
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-9
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-10
DHCP (Dynamic Host Configuration Protocol)
The DHCP relay agent (implemented in the IP router) records the subnet from which the message was received in the DHCP message header for use by the DHCP server
5 DataLink Layer 5-11
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 5-12
Ethernet
ldquodominantrdquo wired LAN technology cheap $ 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 5-13
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-8
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
5 DataLink Layer 5-9
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-10
DHCP (Dynamic Host Configuration Protocol)
The DHCP relay agent (implemented in the IP router) records the subnet from which the message was received in the DHCP message header for use by the DHCP server
5 DataLink Layer 5-11
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 5-12
Ethernet
ldquodominantrdquo wired LAN technology cheap $ 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 5-13
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-9
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-10
DHCP (Dynamic Host Configuration Protocol)
The DHCP relay agent (implemented in the IP router) records the subnet from which the message was received in the DHCP message header for use by the DHCP server
5 DataLink Layer 5-11
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 5-12
Ethernet
ldquodominantrdquo wired LAN technology cheap $ 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 5-13
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-10
DHCP (Dynamic Host Configuration Protocol)
The DHCP relay agent (implemented in the IP router) records the subnet from which the message was received in the DHCP message header for use by the DHCP server
5 DataLink Layer 5-11
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 5-12
Ethernet
ldquodominantrdquo wired LAN technology cheap $ 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 5-13
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-11
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 5-12
Ethernet
ldquodominantrdquo wired LAN technology cheap $ 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 5-13
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-12
Ethernet
ldquodominantrdquo wired LAN technology cheap $ 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 5-13
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-13
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-14
Ethernet Frame Structure (IEEE 8023)
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
ratesEthernet 8023 header overhead is 26 bytes
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-15
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 (2 bytes) indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC (4 bytes) checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-16
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 because frames fails CRC check will be dropped
gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-17
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 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-18
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle (no signal energy for 96 bit times) it starts to transmit frame If it senses channel busy waits until channel idle (plus 96 bit times) and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends 48-bit jam signal
5 After aborting adapter enters exponential backoff after the nth collision adapter chooses a K at random from 012hellip2n-1 Adapter waits K 512 bit times and returns to Step 2
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-19
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 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-20
CSMACD efficiency Efficiency of Ethernet is the fraction of time during which frames are being transmitted on the channel without collisions when there
is a large number of active nodes with large number of frames to send tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame (approx 12 msecs on 10Mbps Ethernet)
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 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-21
Ethernet Technologies 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 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-22
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 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-23
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 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-25
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes (100 meters) but
individual segment collision domains become one large domain Canrsquot interconnect 10BaseT amp 100BaseT Limited number of nodes in a collision domain
hub
hubhub
hub
Segment 1 Segment 2
LAN in multi-tier hub design
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-26
Switch Link layer device operate on Ethernet frames
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment Isolated collision domains Unlimited size (in theory) of each LAN
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-27
Forwarding
bull How do determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Segment 1Segment 2
LAN with multiple segments
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-28
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp)bull When the node was placed in the table
stale entries in table dropped (TTL can be 60 min)
Address Interface Time
62-FE-F7-11-89-A3 1 932
7C-BA-B2-B4-91-10 3 936
Portion of a switch table for a LAN
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-29
Self learning (contrsquo)
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
Forwardingfiltering rules which builds the table automatically dynamically and autonomously without any intervention from a network administrator or from a configuration protocol --- self-learning
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-30
FilteringForwardingWhen switch receives a frameindex 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
The switch deletes an address if no frames are received with that Address as a source after some period of time (aging time)
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-31
Switch example
Suppose C sends frame to D
Switch receives frame from C destined to D notes in switch table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3 (flood)
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-32
Switch example
Suppose D replies back with frame to C
Switch receives frame from D notes in switch 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 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-33
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 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-34
Switches dedicated access Switch with many interfaces Hosts have direct connection
to switch No collisions full duplex
Assume two pairs of twisted-pair cooper wire one for upstream and on for downstream
Store-and-forward swtich will transmit at most one frame at a time onto any dowstream pairs
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-35
More on Switches cut-through switching frame forwarded from
input to output port without first collecting entire frame it is forwarded through the switch when the output link is free No difference if the output port is busy slight reduction in latency if output port is
idle
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-36
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-37
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 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-38
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 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-39
Ethernet (a) hub and (b) switch topologies using twisted pair cabling
Physical Medium Twisted pair category 35 for Fast Ethernet (100m)Optical fiber single or multimode for Gigabit Ethernet (550m or 5km)Two optical fibers singlemultimode for 10Gigabit (300m ~ 40km)
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-40
Deployment of Ethernet in a campus network
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
5 DataLink Layer 5-41
Summary
Link-Layer Addressing MAC Addresses Address Resolution Protocol (ARP) Dynamic Host Configuration Protocol (DHCP)
Ethernet Ethernet Frame Structure CSMACD Ethernetrsquos Multiple Access Protocol Ethernet Technologies
Interconnections Hubs and Switches Hubs Link-Layer Switches Routers vs Switches
