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Announcements:. ECE/CS 372 – introduction to computer networks Lecture 16. Acknowledgement: slides drawn heavily from Kurose & Ross. Our goals: understand principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access - PowerPoint PPT Presentation
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Chapter 4 slide 1
ECECS 372 ndash introduction to computer networksLecture 16
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 2
Chapter 5 Data Link Layer
Our goals understand principles behind data link layer
services error detection correction sharing a broadcast channel multiple access link layer addressing
example of link layer protocol Ethernet
Chapter 5 slide 3
Link Layer (also known as layer 2)Some terminology nodes = hosts or
routers
links = communication channels that connect adjacent nodes
bull wired linksbull wireless links
frame = layer-2 packet
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
Chapter 5 slide 4
Link layer context
links may have ne protocolsframes may be delivered by different link protocols over different links eg Ethernet 80211 (WiFi)
link protocols may provide ne services eg may or may not
provide rdt over link
transportation analogy Trip Princeton to Marseille
limo Princeton to JFK plane JFK to Paris train Paris to Marseille
tourist = frame travel agent = routing
algorithm Doesnrsquot knowcare of mode
transport mode = link layer protocol ne legs may use ne modes
Chapter 5 slide 5
Link Layer Services link access and sharing
Sharing and multiple access allow mediumchannel sharing
reliable delivery between adjacent nodes transport layer-like reliability ACKrsquoing low bit-error link (eg fiber) rarely used high bit-error link (eg wireless) often used
error detection errors caused by signal attenuation noise receiver detects presence of errors
error correction receiver corrects errors wo resorting to retransmission
Chapter 5 slide 6
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 7
Error DetectionD = Data protected by error checking may include header fields EDC= Error Detection and Correction bits (redundancy)
bull error detection not 100 reliable may not detect errors (rarely)
bull larger EDC field yields better detection and correction
Chapter 5 slide 8
Example Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
In this exampleD = data bits EDC = parity bit
Chapter 5 slide 9
Checksumming Cyclic Redundancy Check
Scheme View data bits D as a binary
number
Fix r CRC bits
Choose r+1 bit pattern (generator) G
Sender choose r CRC bits R such that G exactly divides ltDRgt (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
detect burst errors less than r+1 bits
Example D= 101110
r = 3 bits
G = 1001
Find R such thatG divides ltDRgt
(modulo 2)equivalently
G divides ltD2r XOR Rgt
Chapter 5 slide 10
CRC modulo-2 arithmetic CRC calculations are done modulo-2 arithmetic
No carries in addition No borrows in subtraction=gt Addition = subtraction = bitwise-XOR XOR review
0 XOR 0 = 0 0 XOR 1 = 1 1 XOR 0 = 1 1 XOR 1 = 0=gt a XOR 0 = a a XOR a = 0 for a=01Eg 1010 XOR 0110 = 1110
Multiplications and divisions are same as in base-2 arithmetic except all additions and subtractions are done without carries or borrowsbull That is addition = subtraction = bitwise-XOR
Chapter 5 slide 11
CRC Example (find R)
R = remainder[ ]D2r
G
Want G divides ltD2r XOR Rgt
equivalently D2r XOR R = nG
equivalently (XOR both sides)
(D2r XOR R) XOR R = nG XOR R
equivalently (R XOR R = 0 amp a XOR 0 = a)
D2r = nG XOR R
equivalently (division is modulo 2 too)
since remainder[(nG XOR R)G] = R
(note R lt G) then dividing D2r by G (modulo 2) gives remainder = R
Chapter 5 slide 12
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 13
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 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable function get frame from one interface to another
physically-connected interface (same network) uses hexadecimal representation ie base-16 (ie 4
bits) it uses 16 distinct symbols 01hellip9ABCDEF 4 bits needed for each symbol 484 = 12 symbols
Eg 1A-2F-BB-76-09-AD
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 2
Chapter 5 Data Link Layer
Our goals understand principles behind data link layer
services error detection correction sharing a broadcast channel multiple access link layer addressing
example of link layer protocol Ethernet
Chapter 5 slide 3
Link Layer (also known as layer 2)Some terminology nodes = hosts or
routers
links = communication channels that connect adjacent nodes
bull wired linksbull wireless links
frame = layer-2 packet
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
Chapter 5 slide 4
Link layer context
links may have ne protocolsframes may be delivered by different link protocols over different links eg Ethernet 80211 (WiFi)
link protocols may provide ne services eg may or may not
provide rdt over link
transportation analogy Trip Princeton to Marseille
limo Princeton to JFK plane JFK to Paris train Paris to Marseille
tourist = frame travel agent = routing
algorithm Doesnrsquot knowcare of mode
transport mode = link layer protocol ne legs may use ne modes
Chapter 5 slide 5
Link Layer Services link access and sharing
Sharing and multiple access allow mediumchannel sharing
reliable delivery between adjacent nodes transport layer-like reliability ACKrsquoing low bit-error link (eg fiber) rarely used high bit-error link (eg wireless) often used
error detection errors caused by signal attenuation noise receiver detects presence of errors
error correction receiver corrects errors wo resorting to retransmission
Chapter 5 slide 6
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 7
Error DetectionD = Data protected by error checking may include header fields EDC= Error Detection and Correction bits (redundancy)
bull error detection not 100 reliable may not detect errors (rarely)
bull larger EDC field yields better detection and correction
Chapter 5 slide 8
Example Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
In this exampleD = data bits EDC = parity bit
Chapter 5 slide 9
Checksumming Cyclic Redundancy Check
Scheme View data bits D as a binary
number
Fix r CRC bits
Choose r+1 bit pattern (generator) G
Sender choose r CRC bits R such that G exactly divides ltDRgt (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
detect burst errors less than r+1 bits
Example D= 101110
r = 3 bits
G = 1001
Find R such thatG divides ltDRgt
(modulo 2)equivalently
G divides ltD2r XOR Rgt
Chapter 5 slide 10
CRC modulo-2 arithmetic CRC calculations are done modulo-2 arithmetic
No carries in addition No borrows in subtraction=gt Addition = subtraction = bitwise-XOR XOR review
0 XOR 0 = 0 0 XOR 1 = 1 1 XOR 0 = 1 1 XOR 1 = 0=gt a XOR 0 = a a XOR a = 0 for a=01Eg 1010 XOR 0110 = 1110
Multiplications and divisions are same as in base-2 arithmetic except all additions and subtractions are done without carries or borrowsbull That is addition = subtraction = bitwise-XOR
Chapter 5 slide 11
CRC Example (find R)
R = remainder[ ]D2r
G
Want G divides ltD2r XOR Rgt
equivalently D2r XOR R = nG
equivalently (XOR both sides)
(D2r XOR R) XOR R = nG XOR R
equivalently (R XOR R = 0 amp a XOR 0 = a)
D2r = nG XOR R
equivalently (division is modulo 2 too)
since remainder[(nG XOR R)G] = R
(note R lt G) then dividing D2r by G (modulo 2) gives remainder = R
Chapter 5 slide 12
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 13
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 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable function get frame from one interface to another
physically-connected interface (same network) uses hexadecimal representation ie base-16 (ie 4
bits) it uses 16 distinct symbols 01hellip9ABCDEF 4 bits needed for each symbol 484 = 12 symbols
Eg 1A-2F-BB-76-09-AD
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 3
Link Layer (also known as layer 2)Some terminology nodes = hosts or
routers
links = communication channels that connect adjacent nodes
bull wired linksbull wireless links
frame = layer-2 packet
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
Chapter 5 slide 4
Link layer context
links may have ne protocolsframes may be delivered by different link protocols over different links eg Ethernet 80211 (WiFi)
link protocols may provide ne services eg may or may not
provide rdt over link
transportation analogy Trip Princeton to Marseille
limo Princeton to JFK plane JFK to Paris train Paris to Marseille
tourist = frame travel agent = routing
algorithm Doesnrsquot knowcare of mode
transport mode = link layer protocol ne legs may use ne modes
Chapter 5 slide 5
Link Layer Services link access and sharing
Sharing and multiple access allow mediumchannel sharing
reliable delivery between adjacent nodes transport layer-like reliability ACKrsquoing low bit-error link (eg fiber) rarely used high bit-error link (eg wireless) often used
error detection errors caused by signal attenuation noise receiver detects presence of errors
error correction receiver corrects errors wo resorting to retransmission
Chapter 5 slide 6
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 7
Error DetectionD = Data protected by error checking may include header fields EDC= Error Detection and Correction bits (redundancy)
bull error detection not 100 reliable may not detect errors (rarely)
bull larger EDC field yields better detection and correction
Chapter 5 slide 8
Example Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
In this exampleD = data bits EDC = parity bit
Chapter 5 slide 9
Checksumming Cyclic Redundancy Check
Scheme View data bits D as a binary
number
Fix r CRC bits
Choose r+1 bit pattern (generator) G
Sender choose r CRC bits R such that G exactly divides ltDRgt (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
detect burst errors less than r+1 bits
Example D= 101110
r = 3 bits
G = 1001
Find R such thatG divides ltDRgt
(modulo 2)equivalently
G divides ltD2r XOR Rgt
Chapter 5 slide 10
CRC modulo-2 arithmetic CRC calculations are done modulo-2 arithmetic
No carries in addition No borrows in subtraction=gt Addition = subtraction = bitwise-XOR XOR review
0 XOR 0 = 0 0 XOR 1 = 1 1 XOR 0 = 1 1 XOR 1 = 0=gt a XOR 0 = a a XOR a = 0 for a=01Eg 1010 XOR 0110 = 1110
Multiplications and divisions are same as in base-2 arithmetic except all additions and subtractions are done without carries or borrowsbull That is addition = subtraction = bitwise-XOR
Chapter 5 slide 11
CRC Example (find R)
R = remainder[ ]D2r
G
Want G divides ltD2r XOR Rgt
equivalently D2r XOR R = nG
equivalently (XOR both sides)
(D2r XOR R) XOR R = nG XOR R
equivalently (R XOR R = 0 amp a XOR 0 = a)
D2r = nG XOR R
equivalently (division is modulo 2 too)
since remainder[(nG XOR R)G] = R
(note R lt G) then dividing D2r by G (modulo 2) gives remainder = R
Chapter 5 slide 12
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 13
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 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable function get frame from one interface to another
physically-connected interface (same network) uses hexadecimal representation ie base-16 (ie 4
bits) it uses 16 distinct symbols 01hellip9ABCDEF 4 bits needed for each symbol 484 = 12 symbols
Eg 1A-2F-BB-76-09-AD
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 4
Link layer context
links may have ne protocolsframes may be delivered by different link protocols over different links eg Ethernet 80211 (WiFi)
link protocols may provide ne services eg may or may not
provide rdt over link
transportation analogy Trip Princeton to Marseille
limo Princeton to JFK plane JFK to Paris train Paris to Marseille
tourist = frame travel agent = routing
algorithm Doesnrsquot knowcare of mode
transport mode = link layer protocol ne legs may use ne modes
Chapter 5 slide 5
Link Layer Services link access and sharing
Sharing and multiple access allow mediumchannel sharing
reliable delivery between adjacent nodes transport layer-like reliability ACKrsquoing low bit-error link (eg fiber) rarely used high bit-error link (eg wireless) often used
error detection errors caused by signal attenuation noise receiver detects presence of errors
error correction receiver corrects errors wo resorting to retransmission
Chapter 5 slide 6
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 7
Error DetectionD = Data protected by error checking may include header fields EDC= Error Detection and Correction bits (redundancy)
bull error detection not 100 reliable may not detect errors (rarely)
bull larger EDC field yields better detection and correction
Chapter 5 slide 8
Example Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
In this exampleD = data bits EDC = parity bit
Chapter 5 slide 9
Checksumming Cyclic Redundancy Check
Scheme View data bits D as a binary
number
Fix r CRC bits
Choose r+1 bit pattern (generator) G
Sender choose r CRC bits R such that G exactly divides ltDRgt (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
detect burst errors less than r+1 bits
Example D= 101110
r = 3 bits
G = 1001
Find R such thatG divides ltDRgt
(modulo 2)equivalently
G divides ltD2r XOR Rgt
Chapter 5 slide 10
CRC modulo-2 arithmetic CRC calculations are done modulo-2 arithmetic
No carries in addition No borrows in subtraction=gt Addition = subtraction = bitwise-XOR XOR review
0 XOR 0 = 0 0 XOR 1 = 1 1 XOR 0 = 1 1 XOR 1 = 0=gt a XOR 0 = a a XOR a = 0 for a=01Eg 1010 XOR 0110 = 1110
Multiplications and divisions are same as in base-2 arithmetic except all additions and subtractions are done without carries or borrowsbull That is addition = subtraction = bitwise-XOR
Chapter 5 slide 11
CRC Example (find R)
R = remainder[ ]D2r
G
Want G divides ltD2r XOR Rgt
equivalently D2r XOR R = nG
equivalently (XOR both sides)
(D2r XOR R) XOR R = nG XOR R
equivalently (R XOR R = 0 amp a XOR 0 = a)
D2r = nG XOR R
equivalently (division is modulo 2 too)
since remainder[(nG XOR R)G] = R
(note R lt G) then dividing D2r by G (modulo 2) gives remainder = R
Chapter 5 slide 12
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 13
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 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable function get frame from one interface to another
physically-connected interface (same network) uses hexadecimal representation ie base-16 (ie 4
bits) it uses 16 distinct symbols 01hellip9ABCDEF 4 bits needed for each symbol 484 = 12 symbols
Eg 1A-2F-BB-76-09-AD
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 5
Link Layer Services link access and sharing
Sharing and multiple access allow mediumchannel sharing
reliable delivery between adjacent nodes transport layer-like reliability ACKrsquoing low bit-error link (eg fiber) rarely used high bit-error link (eg wireless) often used
error detection errors caused by signal attenuation noise receiver detects presence of errors
error correction receiver corrects errors wo resorting to retransmission
Chapter 5 slide 6
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 7
Error DetectionD = Data protected by error checking may include header fields EDC= Error Detection and Correction bits (redundancy)
bull error detection not 100 reliable may not detect errors (rarely)
bull larger EDC field yields better detection and correction
Chapter 5 slide 8
Example Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
In this exampleD = data bits EDC = parity bit
Chapter 5 slide 9
Checksumming Cyclic Redundancy Check
Scheme View data bits D as a binary
number
Fix r CRC bits
Choose r+1 bit pattern (generator) G
Sender choose r CRC bits R such that G exactly divides ltDRgt (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
detect burst errors less than r+1 bits
Example D= 101110
r = 3 bits
G = 1001
Find R such thatG divides ltDRgt
(modulo 2)equivalently
G divides ltD2r XOR Rgt
Chapter 5 slide 10
CRC modulo-2 arithmetic CRC calculations are done modulo-2 arithmetic
No carries in addition No borrows in subtraction=gt Addition = subtraction = bitwise-XOR XOR review
0 XOR 0 = 0 0 XOR 1 = 1 1 XOR 0 = 1 1 XOR 1 = 0=gt a XOR 0 = a a XOR a = 0 for a=01Eg 1010 XOR 0110 = 1110
Multiplications and divisions are same as in base-2 arithmetic except all additions and subtractions are done without carries or borrowsbull That is addition = subtraction = bitwise-XOR
Chapter 5 slide 11
CRC Example (find R)
R = remainder[ ]D2r
G
Want G divides ltD2r XOR Rgt
equivalently D2r XOR R = nG
equivalently (XOR both sides)
(D2r XOR R) XOR R = nG XOR R
equivalently (R XOR R = 0 amp a XOR 0 = a)
D2r = nG XOR R
equivalently (division is modulo 2 too)
since remainder[(nG XOR R)G] = R
(note R lt G) then dividing D2r by G (modulo 2) gives remainder = R
Chapter 5 slide 12
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 13
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 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable function get frame from one interface to another
physically-connected interface (same network) uses hexadecimal representation ie base-16 (ie 4
bits) it uses 16 distinct symbols 01hellip9ABCDEF 4 bits needed for each symbol 484 = 12 symbols
Eg 1A-2F-BB-76-09-AD
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 6
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 7
Error DetectionD = Data protected by error checking may include header fields EDC= Error Detection and Correction bits (redundancy)
bull error detection not 100 reliable may not detect errors (rarely)
bull larger EDC field yields better detection and correction
Chapter 5 slide 8
Example Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
In this exampleD = data bits EDC = parity bit
Chapter 5 slide 9
Checksumming Cyclic Redundancy Check
Scheme View data bits D as a binary
number
Fix r CRC bits
Choose r+1 bit pattern (generator) G
Sender choose r CRC bits R such that G exactly divides ltDRgt (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
detect burst errors less than r+1 bits
Example D= 101110
r = 3 bits
G = 1001
Find R such thatG divides ltDRgt
(modulo 2)equivalently
G divides ltD2r XOR Rgt
Chapter 5 slide 10
CRC modulo-2 arithmetic CRC calculations are done modulo-2 arithmetic
No carries in addition No borrows in subtraction=gt Addition = subtraction = bitwise-XOR XOR review
0 XOR 0 = 0 0 XOR 1 = 1 1 XOR 0 = 1 1 XOR 1 = 0=gt a XOR 0 = a a XOR a = 0 for a=01Eg 1010 XOR 0110 = 1110
Multiplications and divisions are same as in base-2 arithmetic except all additions and subtractions are done without carries or borrowsbull That is addition = subtraction = bitwise-XOR
Chapter 5 slide 11
CRC Example (find R)
R = remainder[ ]D2r
G
Want G divides ltD2r XOR Rgt
equivalently D2r XOR R = nG
equivalently (XOR both sides)
(D2r XOR R) XOR R = nG XOR R
equivalently (R XOR R = 0 amp a XOR 0 = a)
D2r = nG XOR R
equivalently (division is modulo 2 too)
since remainder[(nG XOR R)G] = R
(note R lt G) then dividing D2r by G (modulo 2) gives remainder = R
Chapter 5 slide 12
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 13
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 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable function get frame from one interface to another
physically-connected interface (same network) uses hexadecimal representation ie base-16 (ie 4
bits) it uses 16 distinct symbols 01hellip9ABCDEF 4 bits needed for each symbol 484 = 12 symbols
Eg 1A-2F-BB-76-09-AD
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 7
Error DetectionD = Data protected by error checking may include header fields EDC= Error Detection and Correction bits (redundancy)
bull error detection not 100 reliable may not detect errors (rarely)
bull larger EDC field yields better detection and correction
Chapter 5 slide 8
Example Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
In this exampleD = data bits EDC = parity bit
Chapter 5 slide 9
Checksumming Cyclic Redundancy Check
Scheme View data bits D as a binary
number
Fix r CRC bits
Choose r+1 bit pattern (generator) G
Sender choose r CRC bits R such that G exactly divides ltDRgt (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
detect burst errors less than r+1 bits
Example D= 101110
r = 3 bits
G = 1001
Find R such thatG divides ltDRgt
(modulo 2)equivalently
G divides ltD2r XOR Rgt
Chapter 5 slide 10
CRC modulo-2 arithmetic CRC calculations are done modulo-2 arithmetic
No carries in addition No borrows in subtraction=gt Addition = subtraction = bitwise-XOR XOR review
0 XOR 0 = 0 0 XOR 1 = 1 1 XOR 0 = 1 1 XOR 1 = 0=gt a XOR 0 = a a XOR a = 0 for a=01Eg 1010 XOR 0110 = 1110
Multiplications and divisions are same as in base-2 arithmetic except all additions and subtractions are done without carries or borrowsbull That is addition = subtraction = bitwise-XOR
Chapter 5 slide 11
CRC Example (find R)
R = remainder[ ]D2r
G
Want G divides ltD2r XOR Rgt
equivalently D2r XOR R = nG
equivalently (XOR both sides)
(D2r XOR R) XOR R = nG XOR R
equivalently (R XOR R = 0 amp a XOR 0 = a)
D2r = nG XOR R
equivalently (division is modulo 2 too)
since remainder[(nG XOR R)G] = R
(note R lt G) then dividing D2r by G (modulo 2) gives remainder = R
Chapter 5 slide 12
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 13
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 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable function get frame from one interface to another
physically-connected interface (same network) uses hexadecimal representation ie base-16 (ie 4
bits) it uses 16 distinct symbols 01hellip9ABCDEF 4 bits needed for each symbol 484 = 12 symbols
Eg 1A-2F-BB-76-09-AD
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 8
Example Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
In this exampleD = data bits EDC = parity bit
Chapter 5 slide 9
Checksumming Cyclic Redundancy Check
Scheme View data bits D as a binary
number
Fix r CRC bits
Choose r+1 bit pattern (generator) G
Sender choose r CRC bits R such that G exactly divides ltDRgt (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
detect burst errors less than r+1 bits
Example D= 101110
r = 3 bits
G = 1001
Find R such thatG divides ltDRgt
(modulo 2)equivalently
G divides ltD2r XOR Rgt
Chapter 5 slide 10
CRC modulo-2 arithmetic CRC calculations are done modulo-2 arithmetic
No carries in addition No borrows in subtraction=gt Addition = subtraction = bitwise-XOR XOR review
0 XOR 0 = 0 0 XOR 1 = 1 1 XOR 0 = 1 1 XOR 1 = 0=gt a XOR 0 = a a XOR a = 0 for a=01Eg 1010 XOR 0110 = 1110
Multiplications and divisions are same as in base-2 arithmetic except all additions and subtractions are done without carries or borrowsbull That is addition = subtraction = bitwise-XOR
Chapter 5 slide 11
CRC Example (find R)
R = remainder[ ]D2r
G
Want G divides ltD2r XOR Rgt
equivalently D2r XOR R = nG
equivalently (XOR both sides)
(D2r XOR R) XOR R = nG XOR R
equivalently (R XOR R = 0 amp a XOR 0 = a)
D2r = nG XOR R
equivalently (division is modulo 2 too)
since remainder[(nG XOR R)G] = R
(note R lt G) then dividing D2r by G (modulo 2) gives remainder = R
Chapter 5 slide 12
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 13
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 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable function get frame from one interface to another
physically-connected interface (same network) uses hexadecimal representation ie base-16 (ie 4
bits) it uses 16 distinct symbols 01hellip9ABCDEF 4 bits needed for each symbol 484 = 12 symbols
Eg 1A-2F-BB-76-09-AD
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 9
Checksumming Cyclic Redundancy Check
Scheme View data bits D as a binary
number
Fix r CRC bits
Choose r+1 bit pattern (generator) G
Sender choose r CRC bits R such that G exactly divides ltDRgt (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
detect burst errors less than r+1 bits
Example D= 101110
r = 3 bits
G = 1001
Find R such thatG divides ltDRgt
(modulo 2)equivalently
G divides ltD2r XOR Rgt
Chapter 5 slide 10
CRC modulo-2 arithmetic CRC calculations are done modulo-2 arithmetic
No carries in addition No borrows in subtraction=gt Addition = subtraction = bitwise-XOR XOR review
0 XOR 0 = 0 0 XOR 1 = 1 1 XOR 0 = 1 1 XOR 1 = 0=gt a XOR 0 = a a XOR a = 0 for a=01Eg 1010 XOR 0110 = 1110
Multiplications and divisions are same as in base-2 arithmetic except all additions and subtractions are done without carries or borrowsbull That is addition = subtraction = bitwise-XOR
Chapter 5 slide 11
CRC Example (find R)
R = remainder[ ]D2r
G
Want G divides ltD2r XOR Rgt
equivalently D2r XOR R = nG
equivalently (XOR both sides)
(D2r XOR R) XOR R = nG XOR R
equivalently (R XOR R = 0 amp a XOR 0 = a)
D2r = nG XOR R
equivalently (division is modulo 2 too)
since remainder[(nG XOR R)G] = R
(note R lt G) then dividing D2r by G (modulo 2) gives remainder = R
Chapter 5 slide 12
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 13
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 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable function get frame from one interface to another
physically-connected interface (same network) uses hexadecimal representation ie base-16 (ie 4
bits) it uses 16 distinct symbols 01hellip9ABCDEF 4 bits needed for each symbol 484 = 12 symbols
Eg 1A-2F-BB-76-09-AD
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 10
CRC modulo-2 arithmetic CRC calculations are done modulo-2 arithmetic
No carries in addition No borrows in subtraction=gt Addition = subtraction = bitwise-XOR XOR review
0 XOR 0 = 0 0 XOR 1 = 1 1 XOR 0 = 1 1 XOR 1 = 0=gt a XOR 0 = a a XOR a = 0 for a=01Eg 1010 XOR 0110 = 1110
Multiplications and divisions are same as in base-2 arithmetic except all additions and subtractions are done without carries or borrowsbull That is addition = subtraction = bitwise-XOR
Chapter 5 slide 11
CRC Example (find R)
R = remainder[ ]D2r
G
Want G divides ltD2r XOR Rgt
equivalently D2r XOR R = nG
equivalently (XOR both sides)
(D2r XOR R) XOR R = nG XOR R
equivalently (R XOR R = 0 amp a XOR 0 = a)
D2r = nG XOR R
equivalently (division is modulo 2 too)
since remainder[(nG XOR R)G] = R
(note R lt G) then dividing D2r by G (modulo 2) gives remainder = R
Chapter 5 slide 12
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 13
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 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable function get frame from one interface to another
physically-connected interface (same network) uses hexadecimal representation ie base-16 (ie 4
bits) it uses 16 distinct symbols 01hellip9ABCDEF 4 bits needed for each symbol 484 = 12 symbols
Eg 1A-2F-BB-76-09-AD
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 11
CRC Example (find R)
R = remainder[ ]D2r
G
Want G divides ltD2r XOR Rgt
equivalently D2r XOR R = nG
equivalently (XOR both sides)
(D2r XOR R) XOR R = nG XOR R
equivalently (R XOR R = 0 amp a XOR 0 = a)
D2r = nG XOR R
equivalently (division is modulo 2 too)
since remainder[(nG XOR R)G] = R
(note R lt G) then dividing D2r by G (modulo 2) gives remainder = R
Chapter 5 slide 12
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 13
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 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable function get frame from one interface to another
physically-connected interface (same network) uses hexadecimal representation ie base-16 (ie 4
bits) it uses 16 distinct symbols 01hellip9ABCDEF 4 bits needed for each symbol 484 = 12 symbols
Eg 1A-2F-BB-76-09-AD
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 12
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 13
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 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable function get frame from one interface to another
physically-connected interface (same network) uses hexadecimal representation ie base-16 (ie 4
bits) it uses 16 distinct symbols 01hellip9ABCDEF 4 bits needed for each symbol 484 = 12 symbols
Eg 1A-2F-BB-76-09-AD
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 13
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 48 bit MAC address (for most LANs)
bull burned in NIC ROM also sometimes software settable function get frame from one interface to another
physically-connected interface (same network) uses hexadecimal representation ie base-16 (ie 4
bits) it uses 16 distinct symbols 01hellip9ABCDEF 4 bits needed for each symbol 484 = 12 symbols
Eg 1A-2F-BB-76-09-AD
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 14
MAC AddressesEach adapter on LAN has unique MAC (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)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 15
A encapsulates BrsquoMAC into the frame
A sends the frame into the medium
All nodes will hear the frame
Only B grabs the frame
All other nodes discard the frame
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
Assume bull A knows Brsquos MAC addressbull A wants to send a frame to B
Node A
Node B
MAC Addresses
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 16
ARP Address Resolution Protocol
Solution ARP Each IP node (host router)
on LAN has ARP table ARP table maps IP MAC
address for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) remove
mapping after TTL (typically 20 min)
A consults its table to determine Brsquos MAC given it knows Brsquos IP
Q how to construct these ARP tables
Question if A doesnrsquot know Brsquos MAC address how does it determine this MAC 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
Node A
Node B
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 17
ARP Case 1mdashSame LAN (network) A wants to send
datagram to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP 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
A sends frame to B since it knows its MAC now
A caches 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
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 18
ARP Case 2mdashrouting to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
how does ARP work now Would the previous scenario work
datagram needs to go from A to B via R assume A knows Brsquos IP address
A
B
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 19
Herersquos how it works (A wants to send IP datagram to B)
A knows that B belongs to a different subnet by checking Brsquos IP address
A also knows IP address of router R (routing table at layer 3) A uses ARP to get Rrsquos MAC address for 111111111110 (IP address of
router) A creates frame with Rs MAC address as dest frame contains A-to-B
IP datagram Arsquos NIC sends frame and Rrsquos NIC receives it R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address (R has Brsquos IP address included in
the just received frame) R creates frame containing A-to-B IP datagram and sends it to B
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
222222222222
49-BD-D2-C7-56-2A
ARP Case 2mdashrouting to another LAN
A
B
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Lecture 15 chapter 5 slide 20
Link layer Addressing last words manufacturer buys portion of MAC address space (to assure
uniqueness of MAC addresses) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat addressing portability
can move LAN card from one LAN to another not true for IP
Q why canrsquot we just use IP addresses ie no MAC addresses LANs designed for all network layer protocols not just IP IP address will have to be storedreconfigured to the adapter
every time the adaptercomputer is moved
Q Why canrsquot we processcheck frames at network layer instead Overhead each and every frame will be processed
(including those destined to others)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 21
Link Layer
1 Error detection and correction 2 Link-layer Addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 22
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Ethernet 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at aparty
(shared air acoustical)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 23
Multiple Access protocolsneed for sharing of mediumchannel single channel
needs be used by all nodes interferencecollision
two or more simultaneous transmissions lead to collided signals
multiple access protocol allows multiple concurrent access
algorithm that nodes use to share channel ie determines when a node can transmit
no coordination no out-of-band channelagreeing about channel sharing must use channel itself
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 24
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 25
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
Random Access channel not divided allow collisions need to know how to ldquorecoverrdquo from collisions
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 26
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle
Eg 6-station LAN 134 have pkt slots 256 idle
1 3 4 1 3 4
6-slotframe
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 27
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle Eg 6-station LAN 134 have pkt frequency
bands 256 idle fr
eq
uency
bands time
FDM cable
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 28
ldquoTaking Turnsrdquo MAC protocols
Polling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 29
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
If token then send message
concerns token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 4 slide 30
ECECS 372 ndash introduction to computer networksLecture 17
Announcements
Acknowledgement slides drawn heavily from Kurose amp Ross
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 31
Random Access Protocols distributed
unlike TDMA or FDMA no coordination among nodes
one node at a time when a node transmits it does so at full data rate R
collisions can occur two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions
examples of random access MAC protocols ALOHA CSMACD CSMACA (CSMA Carrier Sense Multiple Access CD Collision Detection CA Collision Avoidance)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 32
Slotted ALOHA
Assumptions all frames same size
time divided into slots slot = time to transmit 1
frame
start transmit at beginning of slot only
nodes are synchronized
if multiple nodes transmit in slot all can detect collision
Operation when node gets a fresh frame
transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 33
Slotted ALOHA
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots wasting
slots clock synchronization
LegendC =
collisionE =
emptyidle
S = success
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 34
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that a given node has success in a slot p(1-p)N-1
prob that any node has a success Np(1-p)N-1
Efficiency = Np(1-p)N-1
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 35
Slotted Aloha efficiency Efficiency = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
p = 1N
Max Eff = (1-1N)N-1
When N increases to max eff = (1-1N)N-1 goes
to 1e = 37
At best channel used for 37 useful transmission time
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 36
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 37
Pure Aloha efficiency
P(success by given node) = P(node transmits) x
P(no other node transmits in [t0-1t0]
x
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting N -gt infinity
= 1(2e) = 18
(to be derived in homework problem)even worse than slotted Aloha
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 38
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 39
CSMA collisionscollisions can still occurNode Brsquos transmission collides with Node Drsquos transmissioncollision is a wasteA cannot hear Brsquos signalC cannot hear Drsquos signal
spatial layout of nodes
noterole of distance amp propagation delay in collision likelihood
- Longer distances =gt weaker signals =gt collision may be recovered
- Longer delays =gt collision may be avoided
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 40
CSMACD (Collision Detection)CD (collision detection)
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
CSMACD (CSMA w Collision Detection) collisions detectable colliding transmissions aborted reducing channel wastage
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 41
CSMACD collision detection
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 42
Link Layer
1 Error detection and correction 2 Link-layer addressing3 Multiple access protocols4 Ethernet
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 43
Ethernet topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
bus coaxial cable
switch
star
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 44
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble total 8 bytes 7 bytes each with pattern
10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates (senderrsquos data rate matches receiverrsquos data rate)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 45
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
Ethernet Frame Structure
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 46
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 CSMACD(more on this nexthellip)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 47
CSMACD vs slotted ALOHA
CSMACD1 Unsychronized
NIC (adapater) may transmit at anytime no notion of timeslots
2 Carrier-sense Never transmit if others are transmitting
3 Collision detectionstop transmitting as soon as collision is detected
4 Random backoffwait a random time before retransmitting (more later)
Slotted ALOHA1 Sychronized
transmit at beginning of a timeslot only
2 No carrier-sense No check for whether others transmit or not
3 No collision detectionno stop during collision
4 No random backoff
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 48
Notion of bit time
Example consider a 10 Mbps Ethernet linkQ1 what is a ldquobit timerdquoA1 1(10x10^6) second = 01 microsecond
Q2 how much time is ldquo96 bit timerdquoA2 96 x 01 = 96 microsecond
Before describing CSMACD letrsquos introduce
bit time = time to transmit one bit on a Ethernet link
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 49
Ethernet CSMACD algorithm
1 adapter receives datagram from network layer creates frame
2 If adapter senses channel idle for 96 bit time starts frame transmission(gap to allow interface recovery)
3 If adapter senses channel busy waits until channel idle (plus 96 bit time) then transmits
4 If adapter transmits entire frame without detecting another transmission adapter is done with frame
5 If adapter detects another transmission while transmitting aborts and sends a 48-bit jam signal(to make sure all nodes are aware of collision)
6 After aborting (after sending jam signal) adapter enters exponential backoff adapter chooses K at random(next slide is explained how) adapter waits K512 bit times returns to Step 2
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 50
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Exponential Backoff Goal adapt retrans attempts to estimate current load
heavy load random wait will be longer Light load random wait will be shorter
first collision choose K from 01hellip after 2nd collision choose K from 0123hellip after 3rd collision choose K from 01234567hellip after 10th collision choose K from 01234hellip1023hellip after mth collision choose K from 012hellip2m-1 Then delay transmission until K 512 bit times
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 51
Example A and B are connected via an Ethernet link of 10 Mbps Propagation delay between them = 224 bit time At time t=0 both transmit which results in collision After collision A chooses K=0 and B chooses K=1
A B10Mbps
Q1 how much is ldquobit timerdquo =110 = 01 microsecond Q2 at what time does collision occur
=(2242) x 01 = 112 microsecond Q3 at what time does bus become idle
=224 (both A amp B detect collision) + 48 (A amp B finish sending jam signals) + 224 (last bit of Brsquos jam signal arrives at A) = 496 bit time
=496x01 (bit time)= 496 microseccond
Q4 at what time does A begin retransmission[496 (bus becomes idle) + 96]x01 (bit time)= 592 microsecond
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
Chapter 5 slide 52
Distance
A and B are connected via an Ethernet link of R bps A and B are separated by distance d meters Speed of propagation is s metersecond Length of each frame is L bits
A BR (bps)
Suppose A starts transmitting a frame and before it finishes B starts also
Q1 can A finish transmitting before it detects that B starts transmitting Basically is it possible of A not to detect collisionAnswer yes- if frame is not long enough A canrsquot detect collision
Q1 find relationship between L R s and d so A can detect collisionHints1 B canrsquot send if medium is busy listen-before-talk policy2 Think of worst case when just before the first bit coming from A
arrives to B B starts sending
d (m) s (ms)L (bits)
