IEEE 802 LANs - TKK Tietoliikennelaboratorio MAC Protocols (802.x) June 2004 1 IEEE 802 LANs • LAN: Local Area Network • What is a local area network? – A LAN is a network that

HMG/HUT MAC Protocols (802.x) June 2004

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IEEE 802 LANs• LAN: Local Area Network• What is a local area network?

– A LAN is a network that resides in a geographically restricted area

– LANs usually span a building or a campus


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Characteristics of LANs• Short propagation delays

• Small number of users

• Single shared medium (usually)

• Inexpensive


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Common LANs• Bus-based LANs

– Ethernet (*)– Token Bus (*)

• Ring-based LANs– Token Ring (*)

• Switch-based LANs– Switched Ethernet– ATM LANs

(*) IEEE 802 LANs


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IEEE 802 Standards802.1: Introduction802.2: Logical Link Control (LLC)802.3: CSMA/CD (Ethernet)802.4: Token Bus802.5: Token Ring802.6: DQDB 802.11: CSMA/CA (Wireless LAN)


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IEEE 802 Standards (cont’d)• 802 standards define:

– Physical layer protocol

– Data link layer protocol• Medium Access (MAC) Sublayer

• Logical Link Control (LLC) Sublayer


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OSI Layers and IEEE 802

802.2 Logical Link Control

802.3 802.4 802.5Medium Access Control

Data Link Layer

Physical Layer

Higher Layers

OSI layers IEEE 802 LAN standards

Higher Layers

CSMA/CD Token-passing Token-passingbus bus ring


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IEEE 802 LANs (cont’d)• Ethernet• Token Ring


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Ethernet (CSMA/CD)• IEEE 802.3 defines Ethernet• Layers specified by 802.3:

– Ethernet Physical Layer– Ethernet Medium Access (MAC) Sublayer


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Ethernet (cont’d)• Possible Topologies:

1. Bus2. Branching non-rooted tree for large Ethernets


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Ethernet: MAC Layer• Data encapsulation

– Frame Format– Addressing– Error Detection

• Link Management– CSMA/CD– Backoff Algorithm


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Ethernet Frame Format

PreamblePreamble SFDSFD DADA SASA TypeType DataData PadPad CRCCRC

7 1 6 6 2 0-1500 0-46 4

1. Preamble: trains clock-recovery circuits2. Start of Frame Delimiter: indicates start of frame3. Destination Address: 48-bit globally unique address

assigned by manufacturer.1b: unicast/multicast1b: local/global address

4. Type: Indicates protocol of encapsulated data (e.g. IP = 0x0800)5. Pad: Zeroes used to ensure minimum frame length6. Cyclic Redundancy Check: check sequence to detect bit errors.

Bytes:


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Ethernet MAC Frame Address Field

• Destination and Source Addresses:– 6 bytes each

• Two types of destination addresses– Physical address: Unique for each user– Multicast address: Group of users– First bit of address determines which type of address

is being used0 = physical address1 = multicast address


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Ethernet MAC FrameOther Fields

• Length Field– 2 bytes in length– determines length of data payload

• Data Field: between 0 and 1500 bytes• Pad: Filled when Length < 46• Frame Check Sequence Field

– 4 bytes– Cyclic Redundancy Check (CRC-32)


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CSMA/CD• Recall:

– CSMA/CD is a “carrier sense” protocol.• If channel is idle, transmit immediately• If busy, wait until the channel becomes idle

– CSMA/CD can detect collections.• Abort transmission immediately if there is a

collision• Try again later according to a backoff algorithm


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CSMA/CD (cont’d)• Carrier sense reduces the number of

collisions• Collision detection reduces the impact of

collisions


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CSMA/CD and Ethernet• Ethernet:

– Short end-to-end propagation delay– Broadcast channel

• Ethernet access protocol:– 1-Persistent CSMA/CD– with Binary Exponential Backoff Algorithm


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Ethernet Backoff Algorithm:Binary Exponential Backoff

• If collision,– Choose one slot randomly from 2k slots, where k is

the number of collisions the frame has suffered.– One contention slot length = 2 x end-to-end

propagation delay

This algorithm can adapt to changes in network load.


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Binary Exponential Backoff (cont’d)

slot length = 2 x end-to-end delay = 15 µs

A B

t=0µs: Assume A and B collide (kA = kB = 1)A, B choose randomly from 21 slots: [0,1]Assume A chooses 1, B chooses 1

t=30µs: A and B collide (kA = kB = 2)A, B choose randomly from 22 slots: [0,3]Assume A chooses 2, B chooses 0

t=45µs: B transmits successfullyt=75µs: A transmits successfully


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Binary Exponential Backoff (cont’d)• In Ethernet,

– Binary exponential backoff will allow a maximum of 15 retransmission attempts

– If 16 backoffs occur, the transmission of the frame is considered a failure.


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Ethernet Performance


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Ethernet Features and Advantages1. Passive interface: No active element2. Broadcast: All users can listen3. Distributed control: Each user makes own

decision

SimpleReliable

Easy to reconfigure


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Ethernet Disadvantages

• Lack of priority levels

• Cannot perform real-time communication

• Security issues


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Ethernet Switching• Recent development: Connect many

Ethernet segments or subnets through an “Ethernet switch”

to segment 1

to segment 2to segment 3

to segment 4


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Why Ethernet switching?• LANs may grow very large

– The switch has a very fast backplane– It can forward frames very quickly from one

segment to another• Cheaper than upgrading all host interfaces

to use a faster network


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Token Ring• IEEE 802.5 Standard• Layers specified by 802.5:

– Token Ring Physical Layer– Token Ring MAC Sublayer


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Token Ring (cont’d)• Token Ring, unlike Ethernet, requires an

active interface

HostRinginterface


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Token Ring Configuration


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Token Ring Configuration


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Token Ring MAC Sublayer• Token passing protocol• Frame format• Token format


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Token Passing Protocol• A token (8 bit pattern) circulates around the ring• Token state:

– Busy: 11111111– Idle: 11111110


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Token Passing Protocol (cont’d)• General Procedure:

– Sending host waits for and captures an idle token– Sending host changes the token to a frame and

circulates it– Receiving host accepts the frame and continues to

circulate it– Sending host receives its frame, removes it from the

ring, and generates an idle token which it then circulates on the ring


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Token Ring Frame and Token Formats

SD AC ED

SD AC FC DestinationAddress

SourceAddress Data Checksum ED FS

Token Format

Frame Format

1 1 1

1 1 1 2/6 2/6 unlimited 4 1 1

Bytes


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Token Ring Delimiters

• SD = Starting Delimiter• ED = Ending Delimiter• They contains invalid differential Manchester

codes

SD AC ED




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Token Ring Access Control Field

• P = Priority bits– provides up to 8 levels of priority when accessing the

ring

• T = Token bit– T=0: Token – T=1: Frame

SD AC ED

P P P T M R R R

(Note: The AC fieldis also used in frames)


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Token Ring Access Control Field(cont’d)

SD AC ED

P P P T M R R R

• M = Monitor Bit– Prevents tokens and frames from circulating indefinitely– All frames and tokens are issued with M=0– On passing through the “monitor station,” M is set to 1– All other stations repeat this bit as set– A token or frame that reaches the monitor station with M=1 is

considered invalid and is purged


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Token Ring Access Control Fields(cont’d)

SD AC ED

P P P T M R R R

• R = Reservation Bits– Allows stations with high priority data to request (in

frames and tokens as they are repeated) that the next token be issued at the requested priority


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Token Ring Frame Control Field

• FC = Frame Control Field– Defines the type of frame being sent– Frames may be either data frames or some type of

control frame. Example control frames:• Beacon: Used to locate breaks in the ring• Duplicate address test: Used to test if two stations have the

same address




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Token Ring Address & Data Fields

• Address Fields:– Indicate the source and destination hosts– Broadcast:

• Set all destination address bits to 1s.

• Data– No fixed limit on length– Caveat: Hosts may only hold the token for a limited

amount of time (10 msec)




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Token Ring Checksum and Frame Status

• Checksum: 32-bit CRC• FS = Frame Status

– Contains two bits, A and C– When the message arrives at the destination, it sets

A=1– When the destination copies the data in the message,

it sets C=1




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The Token Ring Monitor Station• One station on the ring is designated as the

“monitor station”• The monitor station:

– marks the M bit in frames and tokens– removes marked frames and tokens from the ring– watches for missing tokens and generates new ones

after a timeout period


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Using Priority in Token Ring• If a host wants to send data of priority n, it may only grab

a token with priority value n or lower.• A host may reserve a token of priority n by marking

setting the reservation bits in the AC field of a passing token or frame– Caveat: The host may not make the reservation if the token or

frame’s AC field already indicates a higher priority reservation

• The next token generated will have a priority equal to the reserved priority


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• When a new token is generated (i.e., when a sender finishes sending and releases an idle token), or when a sender sends a data frame, RRR is set to the lowest priority.


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Priority Transmission: Example

A

DC

B

Host B has 1 frame of priority 3 to send to AHost C has 1 frame of priority 2 to send to AHost D has 1 frame of priority 4 to send to AToken starts at host A with priority 0 and circulates

clockwiseHost C is the monitor station(priority 0: lowest priority in this example)


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Example (cont’d)Event Token/Frame AC FieldA generates a token P=0, M=0, T=0, R=0B grabs the token and sets themessage destination to A P=3, M=0, T=1, R=0Frame arrives at C, and C reservespriority level 2. Monitor bit set. P=3, M=1, T=1, R=2Frame arrives at D, andD attempts to reserve priority level 4: P=3, M=1, T=1, R=4Frame arrives at A, and Acopies it P=3, M=1, T=1, R=4Frame returns to B, so B removesit, and generates a new token P=4, M=0, T=0, R=0Token arrives at C, but its priority istoo high. C reserves priority 2. M bit. P=4, M=1, T=0, R=2


45

Example (cont’d)Event Token/Frame AC FieldToken arrives at D, and D grabsit, sending a message to A P=4, M=0, T=1, R=0Frame arrives at A, and A copies it P=4, M=0, T=1, R=0Frame arrives at B, which doesnothing to it P=4, M=0, T=1, R=0Frame arrives at C, which sets themonitor bit P=4, M=1, T=1, R=2Frame returns to D, so D removesit and generates a new token with P=2 P=2, M=0, T=0, R=0

etc… Attempt to complete this scenario on your own.


46

TOKEN RING Performance

• Ring Topology• A bit pattern token (1111 1111) floats on the ring• Station captures token, converts to connector

(11 11 1110), transmits frame• Intermediate stations relay message/token.• Token is released when(a) Leading edge of frame is received, and(b) Frame is transmitted.


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Throughput : Simple AnalysisTime required by a bit to traverse the whole ring

a = ----------------------------------------------------Frame transmission time

Number of active stations : NAverage time to pass token to the next station: a/N


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t0

(a)

t0+a

(b)

t0+1

(c)

t0+a+1

(d)

Fig for case 1


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Case 1 ( a<1)(a) Frame transmission begins(b) Leading edge received.© Total frame is transmitted and token is released.(d) Total frame is received.

Average time to transmit frame

S = ---------------------------------------------------(Time elapsed between a token is transmitted + Average

Token Passing Time)

Na+1

1S =


50

Fig for case 2 ( a > 1)

t0

(a)

t0+1

(b)

t0+a

(c)t0+a+1

(d)


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Case 2 (a > 1)(a) Frame transmission begins(b) Frame transmission completed.© Leading edge received and token is released.(d) Total frame is received.

S = for a > 1Naa +

1


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Delay and stability

No. of stations (equally spaced) = NMean time for token to travel round the ring = RMean Token Cycle Time = TMean Packet Transmission Time =During T• All N queues are served.• Mean number of packets are transmitted = Q• Token rotates (with mean value R)

T = R + Q

X

X


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For stable systemDepartures = ArrivalsQ = N λ T

ρ = λ (for one station)

- Token is free with probability (1-Nρ)

- Token is in use with probability Nρ• Nρ<1

• ρ<1/N

X

( ) RNT =− ρ1


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• Average number of packets transmitted from a queue in T = Q / N

• In limited service (IEEE 802.5 has THT) λT < m– m packets served per token visit

• Tagged job methodology and residual service time analysis gives

Average waiting delay (excluding service delay),W as

[ ]( )

( )( )RN

RRN

xNWλρ

ρλρ

ρ−−

++−−+

Ε=121

12

2


55

Flavor #1: Release After Reception (RAR)

• Computer captures token, transmits data, waits for data to successfully travel around ring, then releases token again.

• Allows computer to detect errored frames and retransmit them.

Data

time

TRANSPToken

PROP

Token arrives at host 1

Token departs from host 1


l1/c l2/c lN/c l1/c

TRANST

DataTRANSP


l3/c

Token

l2/c

TRANST

Example time evolution in which host 1 and host 3 have packets to transmit:


56

Efficiency of RAR

TRANSTTRANSPTRANSP

PROPaa

PROPTRANSTPROPTRANSPNTRANSPN

PROPTRANSTPROPTRANSPN

clTRANSTPROPTRANSPNT

clTRANSTPROPTRANSPT

RAR

i i

>>=+

≈

+++≤∴

+++=

+++≤∴

+++≤

∑

,,11

)()(

)(

/)(

/

1,1

12,1

η

Recall: Efficiency, η, is the fraction of time spent sending useful data.

Define: Ti,j to be the time from when the token arrives at host i until it next arrives at host j.


57

Flavor #2: Release After Transmission (RAT)

• Computer captures token, transmits data, then releases token again.

Data

time

TRANSPToken


Token departs from host 1


l1/c

TRANST

DataTRANSP


Token

l2/c

TRANST

Example time evolution in which host 1 and host 3 have packets to transmit:

Token


58

Efficiency of RAT

TRANSTTRANSPTRANSP

PROPaNa

PROPTRANSTTRANSPNTRANSPN

PROPTRANSTTRANSPN

clTRANSTTRANSPNT

clTRANSTTRANSPT

RAT

i i

>>=+

≈

++≤∴

++=

++≤∴

++≤

∑

,,/11

)()(

)(

/)(

/

1,1

12,1

η

Documents

IEEE 802 LANs - TKK Tietoliikennelaboratorio MAC Protocols (802.x) June 2004 1 IEEE 802 LANs • LAN: Local Area Network • What is a local area network? – A LAN is a network that