P. Bhagwat

802.11 Specificationoverview

P. Bhagwat

802.11 Specifications

PLCP SublayerPHY layer ManagementPMD Sublayer

MAC sublayer

MAC LayerManagement

PHY ServiceInterface

PHY Mgmt ServiceInterface

LLCMAC ServiceInterface

MAC Mgmt ServiceInterface

LLC

MIB

DSSS FH IR OFDM

PHY

MACWEP MAC

Mgmt

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802.11 System Architecture

Basic Service Set (BSS): a set of stations which communicate with one another

Independent Basic Service Set (IBSS)

• only direct communication possible

• no relay function

Infrastructure Basic Service Set (BSS)

• AP provides • connection to wired network• relay function

• stations not allowed to communicate directly

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802.11 MAC

Carrier sensing (CSMA)Rules:

carrier ==> do not transmit

no carrier ==> OK to transmit

But the above rules do not always apply to wireless.

Solution: RTS/CTS

Collision detection (CD)Does not work over wireless

Therefore, use collision avoidance (CA) random backoff

priority ack protocol

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802.11 - MAC layer

Prioritiesdefined through different inter frame spacesno guaranteed, hard prioritiesSIFS (Short Inter Frame Spacing)

highest priority, for ACK, CTS, polling response

PIFS (PCF IFS)medium priority, for time-bounded service using PCF

DIFS (DCF, Distributed Coordination Function IFS) lowest priority, for asynchronous data service

t

medium busySIFS

PIFS

DIFSDIFS

next framecontention

direct access if medium is free DIFS

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802.11 MAC protocol: CSMA/CA

Use CSMA with collision Avoidance Based on carrier sense function in PHY called Clear Channel Assessment

(CCA)

Reduce collision probability where mostly needed Efficient backoff algorithm stable at high loads Possible to implement different fixed priority levels

Busy medium

Defer access

DIFS

contentionwindow

slot time

Next Frame

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802.11 MAC : Contention window

63127

255

511

1023

CW min

CW max

Initial attemptFirst retransmission

Second retransmissionThird retransmission

Fourth retransmissionFifth retransmission

31

For DSSS PHYSlot time = 20 s

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Backoff procedure

Immediate access when medium is free >= DIFS When medium is not free, defer until the end of current frame trasnsmission + DIFS period To begin backoff procedure

Choose a random number in ( 0, Cwindow) Use carrier sense to determine if there is activity during each slot Decrement backoff time by one slot if no activity is detected during that slot

Suspend backoff procedure if medium is determined to be busy at anytime during a backoff slot

Resume backoff procedure after the end of current frame transmission

A

DIFS

Frame

B

FrameC

D

DIFS

defer

defer

defer

CWindow

DIFS

Frame

CWindow

DIFS

Frame

DIFS

Frame

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CSMA/CA + ACK protocol

Defer access based on carrier sense

Direct access when medium is sensed free longer than DIFS

Receiver of directed frames to return an ACK immediately when CRC is correct When no ACK received then retransmit frame after a random backoff

SIFSSrc

DIFS

ACK

Data

Dest

Next Frame

contentionwindow

Other

DIFS

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Fragments transmission

SourceFragment 0

Destination

SIFS

Fragment transmission supported to improve transmission reliability under noisy environments

Transmitter holds the channel until the end of fragment transmission burst

If the source does not receive and ACK frame, it will transmit the failed MPDU after performing the backoff procedure and the contention process

Receiver may receive duplicate fragments and is responsible for detecting and discarding duplicate fragments

ACK 0

SIFS

Fragment 1

ACK 1

SIFS

Fragment 2

ACK 2

SIFSDIFS

SIFSBackoff window

Fragment burst

SIFS

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Problems with carrier sensing

Z

W

YX

Exposed terminal problem

Z is transmittingto W

Y will not transmit to Xeven though it cannot interfere

Presence of carrier ===> hold off transmission/

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Problems with carrier sensing

Y

Z

W

Hidden terminal problem

W finds that medium is freeand it transmits a packet to Z

no carrier ===> OK to transmit/

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Solving Hidden Node problem with RTS/CTS

Y

ZX

W

RTS CTS

listen RTS ==> transmitter is close to melisten CTS ==> receiver is close to me

- listen RTS- wait long enough

for the requestedstation to respondwith CTS

- if (timeout) thenready to transmit

- listen CTS- wait long enoughfor the transmitter to send its data

Note: RTS/CTS does not solve exposed terminal problem. In the example above, X can send RTS, but CTS from the responder will collide with Y’s data.

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RTS/CTS exchange example

RTS + CTS + Frame + ACK exchange invoked when frame size is large Overhead estimation

RTS -- 18 bytes (PCLP Preamble) + 6 bytes (PCLP Header) + 20 bytes (RTS) 192 µs + 160 µs = 352 µs

CTS -- 18 bytes (PCLP Preamble) + 6 bytes (PCLP Header) + 14 bytes (RTS) 192 µs + 112 µs = 304 µs

SIFS – 10 µs

NAV (Network Allocation Vector) NAV maintains prediction of future traffic on the medium based on duration information

that is announced in RTS/CTS frames prior to actual exchange of data

Src

DIFS

ACK

RTS

Dest

Frame

CTS

352 µs 10

µs

SIFS

8192 s

Dest NAV (RTS)

NAV (CTS)

304µs 10

µs

10µs

304µs