MAC for WLAN Doug Young Suh Last update : Aug 1, 2009 WLAN DCF PCF

IEEE Wireless MAC MAC Service and functions MAC Addressing Fragmentation Carrier sense mechanisms Channel Access Protocols Distributed Coordination Function (DCF) Point Coordination Function (PCF) Extended MAC for QoS (802.11e) Power management Security WLAN DCF PCF PHY MAC sublayer LLC sublayer Network layer MSDU DLL

MAC service and functions Transmitting/receiving MSDU between PHY and LLC Specifying various PHY layer parameters Unicast, multicast, broadcast data Fragmentation and defragmentation WLAN DCF PCF

MAC Addressing 48-bit IEEE 802 addresse ⅹ 4 Each BSS also has a BSS ID the MAC address of the AP If not, randomly generated Addr 1Addr 2Addr 3Addr 4 From A to AP n/a From AP2 to B n/a From AP1 to AP WLAN DCF PCF 0b.cd.ef.gh b.cd.ef.gh b.cd.ef.gh b.cd.ef.gh B AP2 AP1 A Dest AdSrc AdBSS IDspecial

Fragmentation/defragmentation One MSDU  multi MPDUs Even if an MPDU is lost, previous ones are valid. Timer and aMaxMSDUtransmitLifetime WLAN DCF PCF MSDU Frame body MAC HDR CRCCRC Frame body MAC HDR CRCCRC Frame body MAC HDR CRCCRC Frame body MAC HDR CRCCRC

Carrier Sense Mechanism The 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/CA Random backoff for the transmission time Priority : The higher, the shorter idle time. IFS (Inter-Frame Space) SIFS (Short IFS) between ACKS and consecutive fragments of the same MSDU PIFS (PCF IFS), DIFS (DPCF IFS) EIFS (Extended IFS) : for DCF after errors SIFS < PIFS < DIFS WLAN DCF PCF

IFS (Inter Frame Space) Interframe spacing in WLAN DCF PCF

DCF The station’s timer runs after DIFS. WLAN DCF PCF BUSY DIFS ABCD  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 CW min to CW max After a successful transmission, CW=Cw min 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 SIFS Next 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 ACKACK Fragment 2 SIFS ACKACK Fragment 3 (last) SIFS ACKACK * All fragments but last are in the same size.

RTS/CTS Seize the medium for a long unicast sequence Request-To-Send, Clear-To-Send RTS 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 A  B  C : When C is not seen to A, for A  B No RTS/CTS for multicast/broadcast WLAN DCF PCF Fragment 0 SIFS ACK SIFS Fragment 1 SIFS ACK SIFS Fragment 2 (last) SIFS ACK CTS SIFS RTS SIFS NAV of others SIFS DIFS PIFS Contention window

Point Coordination Function (PCF) WLAN DCF PCF

PCF Overview In a Point Coordinator, part of an AP Not in 11a, 11b, or 11g, but in 11e Backward compatible to DCF in 11a, 11b, and 11g Contention-free period and contention period PCF during CFP, in every CFP repetition interval DCF during CP From the PC to other stations after PIFS From other station to PC in response to a poll from the PC Types of frames during the PCF : piggy-back allowed Data, CF-Poll, CF-ACK, Null, CF-End Null : negative response to CF-Poll WLAN DCF PCF

PCF Frame Sequence Dx = frames sent by Point Coordinator Ux = frames sent by polled stations WLAN DCF PCF beacon SIFS D1+pol1 SIFS U1+ack SIFSPIFSSIFS D2+ack +pol2 SIFS U2 +ack SIFS D3+ack +pol3 PIFS D4 +pol4 SIFS U4+ack SIFS CF-End Contention-free repetition interval Contention-free period No response to CF-Poll 3 NAV of other stations Reset NAV CF_Max_Duration Contention period

CF-Polling CF-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