MAC for WLANDoug Young Suh

suh@khu.ac.krLast update : Aug 1, 2009

WLAN DCF PCF

IEEE802.11 Wireless MACMAC Service and functionsMAC AddressingFragmentationCarrier sense mechanismsChannel Access Protocols

Distributed Coordination Function (DCF)Point Coordination Function (PCF)Extended MAC for QoS (802.11e)

Power managementSecurity

WLAN DCF PCF

PHYMAC sublayerLLC sublayer

Network layer

MSDUDLL

MAC service and functionsTransmitting/receiving MSDU between PHY and LLCSpecifying various PHY layer parametersUnicast, multicast, broadcast dataFragmentation and defragmentation

WLAN DCF PCF

MAC Addressing48-bit IEEE 802 addresseⅹ4Each BSS also has a BSS ID

the MAC address of the APIf not, randomly generated

Addr 1 Addr 2 Addr 3 Addr 4

From A to AP1

33.44 11.11 22.22 n/a

From AP2 to B

22.22 55.66 11.11 n/a

From AP1 to AP2

55.66 33.44 22.22 11.11

WLAN DCF PCF

0b.cd.ef.gh.55.66

0b.cd.ef.gh.33.55

0b.cd.ef.gh.11.11

0b.cd.ef.gh.22.22B

AP2

AP1

A

Dest Ad Src Ad BSS ID special

Fragmentation/defragmentation

One MSDU multi MPDUsEven if an MPDU is lost, previous ones are valid.<SA, MSDU seq #, MPDU fragment #>Timer and aMaxMSDUtransmitLifetime

WLAN DCF PCF

MSDU

Frame bodyMACHDR

CRC

Frame bodyMACHDR

CRC

Frame bodyMACHDR

CRC

Frame body

MACHDR

CRC

Carrier Sense MechanismThe PHY-CCA.indicate primitive indicates to the MAC whether the channel is BUSY or IDLE.NAV (Network Allocation Vector)

The length of time that is to elapse before the channel becomes IDLE (available).The duration field in frame contains the length of time for which the transmitting station expects the channel to be busy.

WLAN DCF PCF

Channel Access Protocols

WLAN DCF PCF

Principles of Channel Access Protocols

Prioritized CDMA/CARandom backoff for the transmission timePriority : The higher, the shorter idle time.

IFS (Inter-Frame Space)SIFS (Short IFS) between ACKS and consecutive fragments of the same MSDUPIFS (PCF IFS), DIFS (DPCF IFS)EIFS (Extended IFS) : for DCF after errorsSIFS < PIFS < DIFS

WLAN DCF PCF

IFS (Inter Frame Space) Interframe spacing in 802.11.

WLAN DCF PCF

DCFThe station’s timer runs after DIFS.

WLAN DCF PCF

BUSY BUSYDIFS DIFS

A B C D

At A, a frame from LLC, the station senses the medium busy, and calculates a backoff time of 12 slots.

From B, the station’s timer runs until the medium busy, and re-starts at C

At D, the timer expires and the station transmits.

Backoff time

Random variable within [0, CW]CW (Contention Window)CW varies CWmin to CWmax After a successful transmission, CW=Cwmin

At unsuccessful transmission, CW *=2 Note that worse station becomes to have worse chance for increasing overall throughput. This is problem for realtime multimedia service?

WLAN DCF PCF

Transmitting multiple fragments

ACKs are sent after only SIFSNext fragment is sent if and only is an ACK is received.A station may maintain control of the medium to transmit multiple fragments of the same MSDU.

WLAN DCF PCF

Fragment 1

SIFS

ACK

SIFS

Fragment 2

SIFS

ACK

SIFS

Fragment3 (last)

SIFS

ACK

* All fragments but last are in the same size.

RTS/CTSSeize the medium for a long unicast sequence

Request-To-Send, Clear-To-SendRTS has the Duration field to set the total time required for the sequence and the last ACK.All other stations receives the RTS/CTS to update their NAV and do not transmit until the end of the transmission.Overcomes ‘hidden node’ problem

ABC : When C is not seen to A, for ABNo RTS/CTS for multicast/broadcast

WLAN DCF PCF

Fragment 0

SIFS

ACK

SIFS

Fragment 1

SIFS

ACK

SIFSFragment

2 (last)

SIFS

ACKCTS

SIFS

RTS

SIFSNAV of others

SIFS

DIFSPIFS

Contention window

Point Coordination Function

(PCF)

WLAN DCF PCF

PCF Overview In a Point Coordinator, part of an APNot in 11a, 11b, or 11g, but in 11e Backward compatible to DCF in 11a, 11b, and 11gContention-free period and contention period

PCF during CFP, in every CFP repetition intervalDCF during CP

From the PC to other stations after PIFSFrom other station to PC in response to a poll from the PCTypes of frames during the PCF : piggy-back allowed

Data, CF-Poll, CF-ACK, Null, CF-EndNull : negative response to CF-Poll

WLAN DCF PCF

PCF Frame Sequence

Dx = frames sent by Point CoordinatorUx = frames sent by polled stations

WLAN DCF PCF

beacon

SIFS

D1+pol1

SIFS

U1+ack

SIFSPIFS SIFS

D2+ack+pol2

SIFS

U2+ack

SIFSSIFS

D3+ack+pol3

PIFS

D4+pol4

SIFS

U4+ack

SIFSSIFS

CF-End

Contention-free repetition intervalContention-free period

No response to CF-Poll 3

NAV of other stations

Reset NAV

CF_Max_Duration

Contention period

CF-PollingCF-Pollable and Non-CF-Pollable

Every station registers to the AP to be a CF-Pollable station .Non-CF-Pollable in the DCF rules.

Polling list

* CF (Contention Free)

WLAN DCF PCF