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
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 *

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