1. Enhanced Backoff Scheme in CSMA/CA for IEEE 802.11
Actor: Hsien-pang Tsai
2019/6/28

2. Outline Introduction Proposed Backoff Scheme Analysis Conclusion
Reference

3. DCF & PCF DCF is defined for asynchronous data transmissions
DCF shares access to the medium based on the CSMA/CA protocol.
PCF is used to support time-bounded data transfer.

4. CSMA/CA
Station is unable to detect the channel and transmit data simultaneously.
Listen before talk.
The receiving station sends the acknowledgment packet after a SIFS.

5. CSMA/CA (1)

6. Carrier-sense Mechanism
Physical and virtual carrier-sense functions are used to determine the state of the medium.
A virtual carrier-sense mechanism is referred to as the network allocation vector (NAV).

7. RTS & CTS To incorporate the virtual carrier sensing mechanism.
Exchange of short RTS/CTS frames before data transmission is to avoid long collisions and reduce the amount of bandwidth wasted in collisions.

8. RTS & CTS (1)

9. Backoff Procedure
The backoff delay is an integer uniformly chosen in the range [0, W-1].
The initial backoff window size is set to W=CWmin + 1.
If collisions occur, the backoff window size is doubled until it reaches the maximum value, W=CWmax + 1.
W will be reset to CWmin after every successful transmission.

10. Backoff Procedure (1)

11. Proposed Backoff Scheme
Search the optimal value of CW To optimize the performance of the CSMA/CA protocol.
Propose a dynamic CW resetting scheme to let the backoff counter oscillate around the optimal value.

12. Proposed Backoff Scheme (1)
Transmission successes, w is set to the value max[w/2, CWmin +1].
Transmission fails, w is set to the value min[2w, CWmax +1].

13. The Markov chain of the proposed scheme
{s(t), b(t)}, is a discrete-time Markov chain.
b(t), the random backoff window value at slot time t.
s(t), the backoff stage i for the same station at time t.
p for transmitted packets to collide.
pb for the channel to be in a busy status.

14. The Markov chain of the proposed scheme (2)
Collision
Proposed
Reseting to CMin

15. Analysis Parameters

16. Analysis
Basic access mode (m=3)
Throughput

17. Analysis (1)
RTS-CTS access mode (m=3)
Throughput

18. Analysis (2)
Basic access mode (m=3)
Delay

19. Analysis (3)
RTS-CTS access mode (m=3)
Delay

20. Analysis (4)
Basic access mode (w=16)
Throughput

21. Analysis (5)
RTS-CTS access mode (w=16)
Throughput

22. Analysis (6)
Basic access mode (w=16)
Delay

23. Analysis (7)
RTS-CTS access mode (w=16)
Delay

24. Conclusion
CSMA/CA.
Proposed Backoff Scheme.
Analysis Result.

25. Reference
IEEE standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, ISO/IEC :1999(E), Aug
Wen-Kuang Kuo and C.-C. Jay Kuo,” Enhanced Backoff Scheme in CSMA/CA for IEEE ”.