1. Performance Analysis of IEEE802.11 Distributed Coordination Function (DCF) Author : Giuseppe Bianchi Presented by: 李政修 December 23, 2003

2. March 26, 2003Math884 project/wqh2 Outline Overview of IEEE 802.11 DCF Mathematical model Notations Bi-dimensional Markov Chain One step transition probabilities Stationary distribution Performance evaluation of DCF Conclusion and future work

3. March 26, 2003Math884 project/wqh3 Overview of IEEE 802.11 MAC and PHY layers specifications for wireless LANs MAC Protocols Fundamental: Distributed Coordination Function (DCF) CSMA/CA based Binary Exponential Backoff rules Optional: Point Coordination Function (PCF)

4. March 26, 2003Math884 project/wqh4 Overview of IEEE 802.11 DCF Two access techniques Basic mechanism: 2 way handshaking RTS/CTS mechanism: 4 way handshaking Source Dest DATA ACK Source DESt RTS CTS DATA ACK

5. March 26, 2003Math884 project/wqh5 Example of RTS/CTS Access Scheme BUSY RTS CTS NAV (RTS) DATA ACK NAV(CTS) RTS SIFS A B C BO=3 (set) BO=8 (set) DIFS BO=5 (set) BO=5(resume) BO=5 (suspend) BO=0 collision DIFS BO=15 (set) BO=10 (set) CSMA/CA based CSMA: listen at least DIFS before talk CA: defer transmission for random back-off time after DIFS

6. March 26, 2003Math884 project/wqh6 Overview of IEEE 802.11 DCF Backoff procedure—BEB algorithm Backoff counter: Initial: uni~[0,CW-1] Non zero: decremented for each idle slot Zero: transmit C c c s c c c c ccs s CW min CW max CW t

7. March 26, 2003Math884 project/wqh7 Analytical Model to Evaluate DCF Saturation throughput: The limit reached by the system throughput as the offered load increases, and represents the maximum load that the system can carry in stable conditions Assumptions Constant & independent collision probability for each transmitted packet Ideal channel condition (no hidden terminals and capture) Fixed number of stations operated under overload (saturation condition)

8. March 26, 2003Math884 project/wqh8 Bi-dimensional Markov Chain model Behavior of a single station Notations Time scale: Discrete and integer, t, beginning of a slot time, when backoff time counter decrements or regenerated [t, t+1], interval between 2 consecutive slot time, can be variable length Makovian State: B(t) ={s(t), b(t)} b(t): backoff time counter at time t s(t): backoff stage at time t CWi = 2 i CWmin m: maximum backoff stage, CWmax = 2 m CWmin p: prob.of each transmitted packet being collided

9. March 26, 2003Math884 project/wqh9

10. March 26, 2003Math884 project/wqh10 One step transition probabilities (1) 1) P{i,k|i,k+1}=1, k : [0,Wi-2], i : [0,m] At beginning of t Backoff counter not reach zero, no transmission Channel sensed idle for 1 mini-slot till t+1 At beginning of t+1 Backoff counter decremented by 1 i, ki, k+1 1

11. March 26, 2003Math884 project/wqh11 One step transition probabilities (2) P{0,k|i,0}=(1-p)/W 0, k : [0,W 0 -1], i : [0,m] At beginning of t Backoff counter reaches zero, successful transmitted [t,t+1] At beginning of t+1 Contention window reset to CWmin (backoff stage = 0) Backoff counter chosen randomly in [0,W 0 -1] P{i+1,k|i,0}= p/W i+1, k : [0,W i+1 -1], i : [1,m-1] At beginning of t Backoff counter reaches zero, transmit in [t,t+1], collision Contention window < CWmax At beginning of t+1 contention window doubled Backoff counter chosen randomly in [0,W i+1 -1]

12. March 26, 200312 State transits upon backoff counter reach zero (Contention Window <CWmax) i+1, 1 i+1, 0 i+1,W i+1 -1 i+1,W i+1 -2 0, 00, 1 … 0, W 0 -20, W 0 -1 i, 0......... (1-p)/W 0 p/W i+1 Tx Success collision

13. March 26, 2003Math884 project/wqh13 One step transition probabilities (3) P{m,k|i,0}= p/W m, k : [0,W m -1], i = m At beginning of t Backoff counter reaches zero, transmit in [t,t+1], collision Contention Window = CWmax At beginning of t+1 Contention Window remains at CWmax Backoff time counter chosen randomly in [0,W m -1]

14. March 26, 2003Math884 project/wqh14 State transits upon backoff counter reach zero (Contention Window = CWmax) 0, 00, 1 … 0, W 0 -20, W 0 -1...... (1-p)/W 0 m, 0 m, 1 m, W m -2m, W m -1 p/W m …

15. 15 Bi-dimensional Markov Chain model One step transition diagram

16. March 26, 2003Math884 project/wqh16 Results obtained from the model (1) Stationary distribution tao, Probability of a station transmit in a randomly chosen slot time p, Probability of a transmitted packet encounters a collision Using numerical techniques to solve tao, p

17. March 26, 2003Math884 project/wqh17 Results obtained from the model (2) System throughput: as a function of tao (similar to that derived in class)

18. March 26, 2003Math884 project/wqh18 Length of T s and T collision

19. March 26, 2003Math884 project/wqh19 Performance evaluation of 802.11DCF (1) ParametersBasic AccessRTS/CTS Network sizeSensitiveInsensitive Prob.taoSensitiveInsensitive CWminDependentIndependent CWmaxMarginal effect Negligible effect Packet size More effective for longer packets

20. March 26, 2003Math884 project/wqh20 Network size vs. throughput Basic and RTS/CTS access schemes Performance evaluation of 802.11DCF (2)

21. March 26, 2003Math884 project/wqh21 Probability tao vs. Throughput Basic Access RTS/CTS Performance evaluation of 802.11DCF (3)

22. March 26, 2003Math884 project/wqh22 CWmin vs. Throughput Basic Access RTS/CTS Performance evaluation of 802.11DCF (4)

23. March 26, 2003Math884 project/wqh23 CWmax vs. throughput (CWmin = 32) Performance evaluation of 802.11DCF (5)

24. March 26, 2003Math884 project/wqh24 Packet length vs. throughput Performance evaluation of 802.11DCF (6)

25. March 26, 2003Math884 project/wqh25 Conclusion and future work Major contributions of the introduced paper Proposed analytical model Accurate: verified by comparison with simulations Simple Account for all exponential backoff details Evaluate basic and RTS/CTS access schemes Performance evaluation on saturation throughput What to improve considering the upper limit of retransmission times

26. March 26, 2003Math884 project/wqh26 Conclusion and future work Extend one hop to multihop For fixed topology Find a mathematical solution