IEEE 802.11: Wireless LANs ALOHA, Slotted ALOHA Carrier Sense Multiple Access (CSMA), CSMA/CD MACA, MACAW, FAMA, DFWMAC Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) Difficult to detect packet collisions (near-far problem) Optional RTS-CTS handshaking to avoid hidden node problem
IEEE 802.11 MAC Mandatory Distributed Coordination Function (DCF) For distributed contention-based channel Optional Point Coordination Function (PCF) For centralized contention-free channel access
Distributed Coordination Function (DCF)
Random Backoff Procedure Choose a Backoff Time over the interval [0, CW] Backoff Time = Random() × aSlotTime (e.g., Backoff Time = 6 × 20 = 120 µs) If the medium is idle for a backoff slot, the backoff time is decremented by aSlotTime If the medium is determined to be busy during a backoff slot, the backoff procedure is suspended until the medium is determined to be idle for DIFS period Whenever the Backoff Timer reaches zero, a packet transmission begins
Random Backoff Procedure Successful Packet Transmissions : minimum contention window size
Random Backoff Procedure Retransmission Case (i.e., Collisions or Transmission Failures) : increase contention window size
Binary Exponential Backoff
Wasting Factors in Backoff Procedure IDLE SLOTS : small # of active stations with large contention window size (ex, 2 stations in DSSS MinCW=31) COLLISIONS : large # of active stations with small contention window size (ex, 100 stations in HSSS MinCW=15)
Example for Idle Slots Large minimum contention window size + small number of stations large wasting idle slots
Point Coordination Function (PCF) Two consecutive frames are separated by SIFS CFP lengths depend on traffic amount Maximum length announced by
Problems of Legacy MAC No notion of QoS and related signaling Restricted polling scheduling Superframe with alternating CFP and CP needs to be short for short delay bound AP assuming the full control over the medium during CFP: overlapping WLANs? Uncontrollable/unpredictable frame transmission times Large wasting ilde slots when # of active stations is small Rapid performance degradation when # of active stations is large too slow to resolve collisions
Hybrid Coordination Function (HCF) Contention-based channel access Enhanced Distributed Coordination Function (EDCF) for prioritized QoS Variation of legacy DCF provide differentiated, distributed access to the WM for 8 user priorities By using different AIFS, CWmin,CWmax values, Controlled channel access QoS is characterized by a set of parameters A traffic stream (TS) is set up between transmitter and receiver (and HC – located within QoS AP) Polling mode plus HC’s prioritized channel access for parameterized QoS Variation of legacy PCF
Access Category Access category (AC) as a virtual DCF 4 ACs implemented within a QSTA to support 8 user priorities Multiple ACs contend independently The winning AC transmits a frame
AIFS (Arbitration-time inter-frame space) AIFS is the deferral time for backoff count-down that is used to achieve QoS differentiation AIFS is an actual IFS of priority-dependent duration For stations with classification i= 0,1,… AIFSi = aSIFSTime + aAIFSi x aSlotTime where aAIFSi is the AIFS slot count for class i Example: For the top-priority class aAIFS0 = 1 and AIFS0 = PIFS For legacy stations aAIFS0 = 2 and AIFS = DIFS *
Default QoS Parameter Set
EDCF Inter-Frame Space
HC Controlled Channel Access Traffic Specification (TSPEC) Element
HC Controlled Channel Access During CFP HC assumes the full control over the medium Similar to PCF During CP HC can grab the channel after a PIFS idle time Polled TXOP can exist in both CFP and CP Superframe size needs not be very small anymore QoS (+)CF-Poll specifies the polled TXOP limit During a polled TXOP, the TXOP holder can transmit whatever frames it wants NAV protects a polled TXOP
HC Controlled Channel Access HC scheduling Mixture of downlink and polled TXOP scheduling QSTA scheduling During a polled TXOP, schedule frame transmissions Admission control by HC To decide whether to admit a TS or not