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