data link layer “taking turns” mac protocolsyzhu /courses/comnet/slides/lec23.pdfdata link layer...
TRANSCRIPT
9
Data Link Layer
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
Data Link Layer 49
Data Link Layer
ldquoTaking Turnsrdquo MAC protocols
Token passing control token passed
from one node to next sequentially
token message
concernstoken overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
Data Link Layer 50
Data Link Layer
Summary of MAC protocols
channel partitioning by time frequency Time Division Frequency Division
random access ALOHA S-ALOHA CSMA CSMACD
Collision detection easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns polling from central site token passing
Bluetooth FDDI IBM Token Ring
Data Link Layer 51
Data Link Layer
Link Layer
1 Link Layer and services
2 Error detection and correction
3 Multiple access protocols
4 Link-layer addressing
5 Ethernet
6 Link-layer switches
7 Link virtualization ATM
Data Link Layer 52
Data Link Layer
MAC Addresses
32-bit IP address network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to another
physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software settable
Permanent globally uniqueData Link Layer 53
Data Link Layer
LAN Addresses
Each 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)
Data Link Layer 54
10
Data Link Layer
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is attached
Data Link Layer 55
Data Link Layer
Why both MAC and IP address
Answer to keep the layers independent
(1) The data link layer does not only serve IP protocol including many others
(2) if adapters use network-layer address then the adapter needs to be reconfigured every time it moves
(3) if not use MAC add at all then all received frames should be forwarded to network layer Great overhead
Data Link Layer 56
Data Link Layer
ARP Address Resolution Protocol
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)
Question how does A 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
137196723
137196778
137196714
137196788
A
BData Link Layer 57
Data Link Layer
Arp table on each node
Data Link Layer 58
Data Link Layer
How ARP works Case 1 Same LAN
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
Data Link Layer 59
Data Link Layer
How ARP works Case 2 routing to another LAN
Data Link Layer 60
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
two ARP tables in router R one for each IP network (LAN)
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
11
Data Link Layer
A creates IP 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 NIC sends frame
Rrsquos NIC 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
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
Data Link Layer 61
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-62
Illustration
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-63
Illustration
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-64
Illustration
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-65
Illustration R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-66
Illustration R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
1
Data Link Layer
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
Data Link Layer (Part B)
Yanmin Zhu
Department of Computer Science and Engineering
COMNETS CSE 1
Data Link Layer
Link Layer
1 Introduction and services
2 Error detection and correction
3 Multiple access protocols
4 Link-layer Addressing
5 Ethernet
6 Link-layer switches
7 Link virtualization ATM
COMNETS CSE 2
Data Link Layer
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet
sketch
COMNETS CSE 3
Data Link Layer
Inventors of Ethernet
Robert Metcalfe PhD Harvard
1973
David Boggs PhD Stanford
1982
Mr Metcalfe generating the ideas
Mr Boggs figuring out how to build the system
COMNETS CSE 4
David Boggs Robert Metcalfe
Data Link Layer
Xerox PARC
Ethernet
Laser Printing
GUI
Object-oriented Programming (SmallTalk)
WYSIWYG
helliphellip
COMNETS CSE 5
Data Link Layer
Metcalfes law
Value of a telecommunications network is proportional to the square of the number of connected users of the system (n2)
COMNETS CSE 6
2
Data Link Layer
Bus Topology
Old fashioned Based on Coax Bus topology popular through mid 90s
bus coaxial cable
COMNETS CSE 7
Data Link Layer
Star topology
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet
protocol (nodes do not collide with each other)
switch
star
COMNETS CSE 8
Data Link Layer
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
COMNETS CSE 9
Data Link Layer
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 network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
multiplexing COMNETS CSE 10
Data Link Layer
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICs
unreliable receiving NIC doesnrsquot send acks or nacks to sending NIC stream of datagrams passed to network layer can have gaps
(missing datagrams)
gaps will be filled if app is using TCP
otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
COMNETS CSE 11
Data Link Layer
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 Carrier sensing If NIC senses channel idle starts frame
transmission
If NIC senses channel busy waits until channel idle then transmits
COMNETS CSE 12
3
Data Link Layer
Ethernet CSMACD algorithm (2)
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2 demo
COMNETS CSE 13
Data Link Layer
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
COMNETS CSE 14
httpmediapearsoncmgcomawaw_kurose_networ
k_2appletscsmacdcsmacdhtml
Data Link Layer
Why Exponential
When there are a small number of competitors resolve completion in short time =gt wait a short time
When there are a larger number of competitors resolve completion in longer time
When experiencing more collisions be aware of more competitors
COMNETS CSE 15
Data Link Layer
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
COMNETS CSE 16
Data Link Layer
Minimum Frame Length
(1) To make it easier to distinguish valid frames from garbage Ethernet requires that valid frames must be at least 64 bytes long from destination address to checksum
(2) Another is to prevent a station from completing the transmission of a short frame before a potential collision could be detected
COMNETS CSE 17
Data Link Layer
What is the longest delay for A detecting the collision from B after Arsquos transmission of data
Minimum Frame Length (64 bytes)
COMNETS CSE 18
4
Data Link Layer
All frames must take more than 2τ to send so that the transmission is still taking place when the noise burst gets back to the sender in the worst case
Minimum Frame Length (64 bytes)
COMNETS CSE 19
Data Link Layer
Minimum Frame Length (64 bytes)
min
min
2
2 2
F
B
lF B B
v
2 Collision detection can take as long as
COMNETS CSE 20
Data Link Layer
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bps different physical layer media fiber cable
application
transport
network
link
physical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twister
pair) physical layer COMNETS CSE 21
Data Link Layer
Manchester encoding
used in 10BaseT
each bit has a transition
allows clocks in sending and receiving nodes to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
COMNETS CSE 22
Data Link Layer
Development of Ethernet
1973
Invented
Fast Ethernet
IEEE 8023u
1995 1998
Giga Ethernet
IEEE 8023z
2003
10g Ethernet
IEEE 8023ae
1983
IEEE 8023
(thick coax)
COMNETS CSE 23
Data Link Layer
Link Layer
1 Introduction and services
2 Error detection and correction
3 Multiple access protocols
4 Link-layer Addressing
5 Ethernet
6 Link-layer switches
7 Link virtualization ATM
COMNETS CSE 24
10
Data Link Layer
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy
(a) MAC address like Social Security Number
(b) IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is attached
Data Link Layer 55
Data Link Layer
Why both MAC and IP address
Answer to keep the layers independent
(1) The data link layer does not only serve IP protocol including many others
(2) if adapters use network-layer address then the adapter needs to be reconfigured every time it moves
(3) if not use MAC add at all then all received frames should be forwarded to network layer Great overhead
Data Link Layer 56
Data Link Layer
ARP Address Resolution Protocol
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)
Question how does A 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
137196723
137196778
137196714
137196788
A
BData Link Layer 57
Data Link Layer
Arp table on each node
Data Link Layer 58
Data Link Layer
How ARP works Case 1 Same LAN
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
Data Link Layer 59
Data Link Layer
How ARP works Case 2 routing to another LAN
Data Link Layer 60
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
two ARP tables in router R one for each IP network (LAN)
walkthrough send datagram from A to B via Rassume A knows Brsquos IP address
11
Data Link Layer
A creates IP 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 NIC sends frame
Rrsquos NIC 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
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
Data Link Layer 61
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-62
Illustration
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-63
Illustration
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-64
Illustration
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-65
Illustration R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-66
Illustration R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
1
Data Link Layer
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
Data Link Layer (Part B)
Yanmin Zhu
Department of Computer Science and Engineering
COMNETS CSE 1
Data Link Layer
Link Layer
1 Introduction and services
2 Error detection and correction
3 Multiple access protocols
4 Link-layer Addressing
5 Ethernet
6 Link-layer switches
7 Link virtualization ATM
COMNETS CSE 2
Data Link Layer
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet
sketch
COMNETS CSE 3
Data Link Layer
Inventors of Ethernet
Robert Metcalfe PhD Harvard
1973
David Boggs PhD Stanford
1982
Mr Metcalfe generating the ideas
Mr Boggs figuring out how to build the system
COMNETS CSE 4
David Boggs Robert Metcalfe
Data Link Layer
Xerox PARC
Ethernet
Laser Printing
GUI
Object-oriented Programming (SmallTalk)
WYSIWYG
helliphellip
COMNETS CSE 5
Data Link Layer
Metcalfes law
Value of a telecommunications network is proportional to the square of the number of connected users of the system (n2)
COMNETS CSE 6
2
Data Link Layer
Bus Topology
Old fashioned Based on Coax Bus topology popular through mid 90s
bus coaxial cable
COMNETS CSE 7
Data Link Layer
Star topology
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet
protocol (nodes do not collide with each other)
switch
star
COMNETS CSE 8
Data Link Layer
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
COMNETS CSE 9
Data Link Layer
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 network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
multiplexing COMNETS CSE 10
Data Link Layer
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICs
unreliable receiving NIC doesnrsquot send acks or nacks to sending NIC stream of datagrams passed to network layer can have gaps
(missing datagrams)
gaps will be filled if app is using TCP
otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
COMNETS CSE 11
Data Link Layer
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 Carrier sensing If NIC senses channel idle starts frame
transmission
If NIC senses channel busy waits until channel idle then transmits
COMNETS CSE 12
3
Data Link Layer
Ethernet CSMACD algorithm (2)
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2 demo
COMNETS CSE 13
Data Link Layer
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
COMNETS CSE 14
httpmediapearsoncmgcomawaw_kurose_networ
k_2appletscsmacdcsmacdhtml
Data Link Layer
Why Exponential
When there are a small number of competitors resolve completion in short time =gt wait a short time
When there are a larger number of competitors resolve completion in longer time
When experiencing more collisions be aware of more competitors
COMNETS CSE 15
Data Link Layer
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
COMNETS CSE 16
Data Link Layer
Minimum Frame Length
(1) To make it easier to distinguish valid frames from garbage Ethernet requires that valid frames must be at least 64 bytes long from destination address to checksum
(2) Another is to prevent a station from completing the transmission of a short frame before a potential collision could be detected
COMNETS CSE 17
Data Link Layer
What is the longest delay for A detecting the collision from B after Arsquos transmission of data
Minimum Frame Length (64 bytes)
COMNETS CSE 18
4
Data Link Layer
All frames must take more than 2τ to send so that the transmission is still taking place when the noise burst gets back to the sender in the worst case
Minimum Frame Length (64 bytes)
COMNETS CSE 19
Data Link Layer
Minimum Frame Length (64 bytes)
min
min
2
2 2
F
B
lF B B
v
2 Collision detection can take as long as
COMNETS CSE 20
Data Link Layer
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bps different physical layer media fiber cable
application
transport
network
link
physical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twister
pair) physical layer COMNETS CSE 21
Data Link Layer
Manchester encoding
used in 10BaseT
each bit has a transition
allows clocks in sending and receiving nodes to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
COMNETS CSE 22
Data Link Layer
Development of Ethernet
1973
Invented
Fast Ethernet
IEEE 8023u
1995 1998
Giga Ethernet
IEEE 8023z
2003
10g Ethernet
IEEE 8023ae
1983
IEEE 8023
(thick coax)
COMNETS CSE 23
Data Link Layer
Link Layer
1 Introduction and services
2 Error detection and correction
3 Multiple access protocols
4 Link-layer Addressing
5 Ethernet
6 Link-layer switches
7 Link virtualization ATM
COMNETS CSE 24
11
Data Link Layer
A creates IP 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 NIC sends frame
Rrsquos NIC 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
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
Data Link Layer 61
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-62
Illustration
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-63
Illustration
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-64
Illustration
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-65
Illustration R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
Data Link Layer
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-66
Illustration R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
1
Data Link Layer
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
Data Link Layer (Part B)
Yanmin Zhu
Department of Computer Science and Engineering
COMNETS CSE 1
Data Link Layer
Link Layer
1 Introduction and services
2 Error detection and correction
3 Multiple access protocols
4 Link-layer Addressing
5 Ethernet
6 Link-layer switches
7 Link virtualization ATM
COMNETS CSE 2
Data Link Layer
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet
sketch
COMNETS CSE 3
Data Link Layer
Inventors of Ethernet
Robert Metcalfe PhD Harvard
1973
David Boggs PhD Stanford
1982
Mr Metcalfe generating the ideas
Mr Boggs figuring out how to build the system
COMNETS CSE 4
David Boggs Robert Metcalfe
Data Link Layer
Xerox PARC
Ethernet
Laser Printing
GUI
Object-oriented Programming (SmallTalk)
WYSIWYG
helliphellip
COMNETS CSE 5
Data Link Layer
Metcalfes law
Value of a telecommunications network is proportional to the square of the number of connected users of the system (n2)
COMNETS CSE 6
2
Data Link Layer
Bus Topology
Old fashioned Based on Coax Bus topology popular through mid 90s
bus coaxial cable
COMNETS CSE 7
Data Link Layer
Star topology
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet
protocol (nodes do not collide with each other)
switch
star
COMNETS CSE 8
Data Link Layer
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
COMNETS CSE 9
Data Link Layer
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 network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
multiplexing COMNETS CSE 10
Data Link Layer
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICs
unreliable receiving NIC doesnrsquot send acks or nacks to sending NIC stream of datagrams passed to network layer can have gaps
(missing datagrams)
gaps will be filled if app is using TCP
otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
COMNETS CSE 11
Data Link Layer
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 Carrier sensing If NIC senses channel idle starts frame
transmission
If NIC senses channel busy waits until channel idle then transmits
COMNETS CSE 12
3
Data Link Layer
Ethernet CSMACD algorithm (2)
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2 demo
COMNETS CSE 13
Data Link Layer
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
COMNETS CSE 14
httpmediapearsoncmgcomawaw_kurose_networ
k_2appletscsmacdcsmacdhtml
Data Link Layer
Why Exponential
When there are a small number of competitors resolve completion in short time =gt wait a short time
When there are a larger number of competitors resolve completion in longer time
When experiencing more collisions be aware of more competitors
COMNETS CSE 15
Data Link Layer
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
COMNETS CSE 16
Data Link Layer
Minimum Frame Length
(1) To make it easier to distinguish valid frames from garbage Ethernet requires that valid frames must be at least 64 bytes long from destination address to checksum
(2) Another is to prevent a station from completing the transmission of a short frame before a potential collision could be detected
COMNETS CSE 17
Data Link Layer
What is the longest delay for A detecting the collision from B after Arsquos transmission of data
Minimum Frame Length (64 bytes)
COMNETS CSE 18
4
Data Link Layer
All frames must take more than 2τ to send so that the transmission is still taking place when the noise burst gets back to the sender in the worst case
Minimum Frame Length (64 bytes)
COMNETS CSE 19
Data Link Layer
Minimum Frame Length (64 bytes)
min
min
2
2 2
F
B
lF B B
v
2 Collision detection can take as long as
COMNETS CSE 20
Data Link Layer
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bps different physical layer media fiber cable
application
transport
network
link
physical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twister
pair) physical layer COMNETS CSE 21
Data Link Layer
Manchester encoding
used in 10BaseT
each bit has a transition
allows clocks in sending and receiving nodes to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
COMNETS CSE 22
Data Link Layer
Development of Ethernet
1973
Invented
Fast Ethernet
IEEE 8023u
1995 1998
Giga Ethernet
IEEE 8023z
2003
10g Ethernet
IEEE 8023ae
1983
IEEE 8023
(thick coax)
COMNETS CSE 23
Data Link Layer
Link Layer
1 Introduction and services
2 Error detection and correction
3 Multiple access protocols
4 Link-layer Addressing
5 Ethernet
6 Link-layer switches
7 Link virtualization ATM
COMNETS CSE 24
1
Data Link Layer
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
Data Link Layer (Part B)
Yanmin Zhu
Department of Computer Science and Engineering
COMNETS CSE 1
Data Link Layer
Link Layer
1 Introduction and services
2 Error detection and correction
3 Multiple access protocols
4 Link-layer Addressing
5 Ethernet
6 Link-layer switches
7 Link virtualization ATM
COMNETS CSE 2
Data Link Layer
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet
sketch
COMNETS CSE 3
Data Link Layer
Inventors of Ethernet
Robert Metcalfe PhD Harvard
1973
David Boggs PhD Stanford
1982
Mr Metcalfe generating the ideas
Mr Boggs figuring out how to build the system
COMNETS CSE 4
David Boggs Robert Metcalfe
Data Link Layer
Xerox PARC
Ethernet
Laser Printing
GUI
Object-oriented Programming (SmallTalk)
WYSIWYG
helliphellip
COMNETS CSE 5
Data Link Layer
Metcalfes law
Value of a telecommunications network is proportional to the square of the number of connected users of the system (n2)
COMNETS CSE 6
2
Data Link Layer
Bus Topology
Old fashioned Based on Coax Bus topology popular through mid 90s
bus coaxial cable
COMNETS CSE 7
Data Link Layer
Star topology
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet
protocol (nodes do not collide with each other)
switch
star
COMNETS CSE 8
Data Link Layer
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
COMNETS CSE 9
Data Link Layer
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 network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
multiplexing COMNETS CSE 10
Data Link Layer
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICs
unreliable receiving NIC doesnrsquot send acks or nacks to sending NIC stream of datagrams passed to network layer can have gaps
(missing datagrams)
gaps will be filled if app is using TCP
otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
COMNETS CSE 11
Data Link Layer
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 Carrier sensing If NIC senses channel idle starts frame
transmission
If NIC senses channel busy waits until channel idle then transmits
COMNETS CSE 12
3
Data Link Layer
Ethernet CSMACD algorithm (2)
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2 demo
COMNETS CSE 13
Data Link Layer
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
COMNETS CSE 14
httpmediapearsoncmgcomawaw_kurose_networ
k_2appletscsmacdcsmacdhtml
Data Link Layer
Why Exponential
When there are a small number of competitors resolve completion in short time =gt wait a short time
When there are a larger number of competitors resolve completion in longer time
When experiencing more collisions be aware of more competitors
COMNETS CSE 15
Data Link Layer
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
COMNETS CSE 16
Data Link Layer
Minimum Frame Length
(1) To make it easier to distinguish valid frames from garbage Ethernet requires that valid frames must be at least 64 bytes long from destination address to checksum
(2) Another is to prevent a station from completing the transmission of a short frame before a potential collision could be detected
COMNETS CSE 17
Data Link Layer
What is the longest delay for A detecting the collision from B after Arsquos transmission of data
Minimum Frame Length (64 bytes)
COMNETS CSE 18
4
Data Link Layer
All frames must take more than 2τ to send so that the transmission is still taking place when the noise burst gets back to the sender in the worst case
Minimum Frame Length (64 bytes)
COMNETS CSE 19
Data Link Layer
Minimum Frame Length (64 bytes)
min
min
2
2 2
F
B
lF B B
v
2 Collision detection can take as long as
COMNETS CSE 20
Data Link Layer
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bps different physical layer media fiber cable
application
transport
network
link
physical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twister
pair) physical layer COMNETS CSE 21
Data Link Layer
Manchester encoding
used in 10BaseT
each bit has a transition
allows clocks in sending and receiving nodes to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
COMNETS CSE 22
Data Link Layer
Development of Ethernet
1973
Invented
Fast Ethernet
IEEE 8023u
1995 1998
Giga Ethernet
IEEE 8023z
2003
10g Ethernet
IEEE 8023ae
1983
IEEE 8023
(thick coax)
COMNETS CSE 23
Data Link Layer
Link Layer
1 Introduction and services
2 Error detection and correction
3 Multiple access protocols
4 Link-layer Addressing
5 Ethernet
6 Link-layer switches
7 Link virtualization ATM
COMNETS CSE 24
2
Data Link Layer
Bus Topology
Old fashioned Based on Coax Bus topology popular through mid 90s
bus coaxial cable
COMNETS CSE 7
Data Link Layer
Star topology
today star topology prevailsactive switch in centereach ldquospokerdquo runs a (separate) Ethernet
protocol (nodes do not collide with each other)
switch
star
COMNETS CSE 8
Data Link Layer
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
COMNETS CSE 9
Data Link Layer
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 network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
multiplexing COMNETS CSE 10
Data Link Layer
Ethernet Unreliable connectionless
connectionless No handshaking between sending and receiving NICs
unreliable receiving NIC doesnrsquot send acks or nacks to sending NIC stream of datagrams passed to network layer can have gaps
(missing datagrams)
gaps will be filled if app is using TCP
otherwise app will see gaps
Ethernetrsquos MAC protocol unslotted CSMACD
COMNETS CSE 11
Data Link Layer
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 Carrier sensing If NIC senses channel idle starts frame
transmission
If NIC senses channel busy waits until channel idle then transmits
COMNETS CSE 12
3
Data Link Layer
Ethernet CSMACD algorithm (2)
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2 demo
COMNETS CSE 13
Data Link Layer
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
COMNETS CSE 14
httpmediapearsoncmgcomawaw_kurose_networ
k_2appletscsmacdcsmacdhtml
Data Link Layer
Why Exponential
When there are a small number of competitors resolve completion in short time =gt wait a short time
When there are a larger number of competitors resolve completion in longer time
When experiencing more collisions be aware of more competitors
COMNETS CSE 15
Data Link Layer
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
COMNETS CSE 16
Data Link Layer
Minimum Frame Length
(1) To make it easier to distinguish valid frames from garbage Ethernet requires that valid frames must be at least 64 bytes long from destination address to checksum
(2) Another is to prevent a station from completing the transmission of a short frame before a potential collision could be detected
COMNETS CSE 17
Data Link Layer
What is the longest delay for A detecting the collision from B after Arsquos transmission of data
Minimum Frame Length (64 bytes)
COMNETS CSE 18
4
Data Link Layer
All frames must take more than 2τ to send so that the transmission is still taking place when the noise burst gets back to the sender in the worst case
Minimum Frame Length (64 bytes)
COMNETS CSE 19
Data Link Layer
Minimum Frame Length (64 bytes)
min
min
2
2 2
F
B
lF B B
v
2 Collision detection can take as long as
COMNETS CSE 20
Data Link Layer
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bps different physical layer media fiber cable
application
transport
network
link
physical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twister
pair) physical layer COMNETS CSE 21
Data Link Layer
Manchester encoding
used in 10BaseT
each bit has a transition
allows clocks in sending and receiving nodes to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
COMNETS CSE 22
Data Link Layer
Development of Ethernet
1973
Invented
Fast Ethernet
IEEE 8023u
1995 1998
Giga Ethernet
IEEE 8023z
2003
10g Ethernet
IEEE 8023ae
1983
IEEE 8023
(thick coax)
COMNETS CSE 23
Data Link Layer
Link Layer
1 Introduction and services
2 Error detection and correction
3 Multiple access protocols
4 Link-layer Addressing
5 Ethernet
6 Link-layer switches
7 Link virtualization ATM
COMNETS CSE 24
3
Data Link Layer
Ethernet CSMACD algorithm (2)
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2 demo
COMNETS CSE 13
Data Link Layer
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
COMNETS CSE 14
httpmediapearsoncmgcomawaw_kurose_networ
k_2appletscsmacdcsmacdhtml
Data Link Layer
Why Exponential
When there are a small number of competitors resolve completion in short time =gt wait a short time
When there are a larger number of competitors resolve completion in longer time
When experiencing more collisions be aware of more competitors
COMNETS CSE 15
Data Link Layer
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
COMNETS CSE 16
Data Link Layer
Minimum Frame Length
(1) To make it easier to distinguish valid frames from garbage Ethernet requires that valid frames must be at least 64 bytes long from destination address to checksum
(2) Another is to prevent a station from completing the transmission of a short frame before a potential collision could be detected
COMNETS CSE 17
Data Link Layer
What is the longest delay for A detecting the collision from B after Arsquos transmission of data
Minimum Frame Length (64 bytes)
COMNETS CSE 18
4
Data Link Layer
All frames must take more than 2τ to send so that the transmission is still taking place when the noise burst gets back to the sender in the worst case
Minimum Frame Length (64 bytes)
COMNETS CSE 19
Data Link Layer
Minimum Frame Length (64 bytes)
min
min
2
2 2
F
B
lF B B
v
2 Collision detection can take as long as
COMNETS CSE 20
Data Link Layer
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bps different physical layer media fiber cable
application
transport
network
link
physical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twister
pair) physical layer COMNETS CSE 21
Data Link Layer
Manchester encoding
used in 10BaseT
each bit has a transition
allows clocks in sending and receiving nodes to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
COMNETS CSE 22
Data Link Layer
Development of Ethernet
1973
Invented
Fast Ethernet
IEEE 8023u
1995 1998
Giga Ethernet
IEEE 8023z
2003
10g Ethernet
IEEE 8023ae
1983
IEEE 8023
(thick coax)
COMNETS CSE 23
Data Link Layer
Link Layer
1 Introduction and services
2 Error detection and correction
3 Multiple access protocols
4 Link-layer Addressing
5 Ethernet
6 Link-layer switches
7 Link virtualization ATM
COMNETS CSE 24
4
Data Link Layer
All frames must take more than 2τ to send so that the transmission is still taking place when the noise burst gets back to the sender in the worst case
Minimum Frame Length (64 bytes)
COMNETS CSE 19
Data Link Layer
Minimum Frame Length (64 bytes)
min
min
2
2 2
F
B
lF B B
v
2 Collision detection can take as long as
COMNETS CSE 20
Data Link Layer
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100 Mbps
1Gbps 10G bps different physical layer media fiber cable
application
transport
network
link
physical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twister
pair) physical layer COMNETS CSE 21
Data Link Layer
Manchester encoding
used in 10BaseT
each bit has a transition
allows clocks in sending and receiving nodes to synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
COMNETS CSE 22
Data Link Layer
Development of Ethernet
1973
Invented
Fast Ethernet
IEEE 8023u
1995 1998
Giga Ethernet
IEEE 8023z
2003
10g Ethernet
IEEE 8023ae
1983
IEEE 8023
(thick coax)
COMNETS CSE 23
Data Link Layer
Link Layer
1 Introduction and services
2 Error detection and correction
3 Multiple access protocols
4 Link-layer Addressing
5 Ethernet
6 Link-layer switches
7 Link virtualization ATM
COMNETS CSE 24