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IEEE 802.11: Wireless LANs

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)

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IEEE 802.11: Wireless LANs

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  1. 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

  2. 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

  3. Distributed Coordination Function (DCF)

  4. 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

  5. Random Backoff Procedure • Successful Packet Transmissions • : minimum contention window size

  6. Random Backoff Procedure • Retransmission Case (i.e., Collisions or Transmission Failures) • : increase contention window size

  7. Binary Exponential Backoff

  8. 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)

  9. Example for Idle Slots • Large minimum contention window size + small number of stations •  large wasting idle slots

  10. Point Coordination Function (PCF) • Two consecutive frames are separated by SIFS • CFP lengths depend on traffic amount • Maximum length announced by

  11. 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

  12. 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

  13. 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

  14. 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 • whereaAIFSiisthe AIFS slot count for class i • Example: • For the top-priority class aAIFS0= 1 andAIFS0 = PIFS • For legacy stationsaAIFS0= 2 and AIFS = DIFS *

  15. Default QoS Parameter Set

  16. EDCF Inter-Frame Space

  17. HC Controlled Channel Access • Traffic Specification (TSPEC) Element

  18. 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

  19. 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

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