1. 600.647 Advanced Wireless Antonello Cruz FMAC/CSR: a Fair MAC Protocol for Wireless Ad-hoc Networks Zhifei Li Anil K. Gupta Sukumar Nandi

2. 600.647 Advanced Wireless Antonello Cruz Unfairness Based on the length of time the system is observed Short-term fairness => long-term fairness Short-term unfairness => long-term unfairness Goal is short-term fairness

3. 600.647 Advanced Wireless Antonello Cruz Current Protocol IEEE 802.11 Distributed Coordination Function (DCF) 2-way handshake (data/ACK) 4-way handshake (RTS/CTS/data/SCK) Assume 4-way handshake to cope with common hidden-terminal problems

4. 600.647 Advanced Wireless Antonello Cruz Unfairness in IEEE 802.11 IEEE 802.11 Distributed Coordination Function (DCF) Concealed Information Problem Imprecise Collision Detection High Contention

5. 600.647 Advanced Wireless Antonello Cruz Medium Contention Two flows contend the medium if either sender or receiver of one flow is within transmission range of the other

6. 600.647 Advanced Wireless Antonello Cruz Concealed Information Problem Neither sender is within range of the receiver of the other flow Only one of the senders is within the range of the receiver of the other flow Both senders are within range of the receiver of the other flow

7. 600.647 Advanced Wireless Antonello Cruz Concealed Information Problem Neither sender is within range of the receiver of the other flow Short-term unfairness

8. 600.647 Advanced Wireless Antonello Cruz Concealed Information Problem Only one of the senders is within the range of the receiver of the other flow

9. 600.647 Advanced Wireless Antonello Cruz Concealed Information Problem Both senders are within range of the receiver of the other flow

10. 600.647 Advanced Wireless Antonello Cruz Concealed Information Problem When the contention period occurs? Starts after the transmission of ACK Ends at transmission of RTS

11. 600.647 Advanced Wireless Antonello Cruz Imprecise Collision Detection S A and S B send RTS at the same time R A detects collision and doesn’t send CTS R B can’t hear S A ’s RTS and sends CTS S B captured the medium

12. 600.647 Advanced Wireless Antonello Cruz High-Contention

13. 600.647 Advanced Wireless Antonello Cruz General Framework to Achieve MAC Fairness Fairness Model Fair share over certain duration/window (W) Flow-level fairness Consider packets to have same length All application have same requirements Compensation Model Aggressive Normal Restrictive

14. 600.647 Advanced Wireless Antonello Cruz Estimation of the of the Number of Active Flows List of active flows from tuples (RTS/CTS/Data/ACK) with timestamps Stale entry older than W e

15. 600.647 Advanced Wireless Antonello Cruz Imprecise Estimations Potential problems New Flow. Other nodes are unaware. (under- estimation) Last packet of a flow waits one W e to delete. (over-estimation) We need smallest W e Flow can’t transmit within W e and have its entry deleted by the other nodes (under- estimation) We need larger W e

16. 600.647 Advanced Wireless Antonello Cruz Estimation of Actual Share Keep history of flows Check how many times you transmitted History = {A, B, A, C, B, A, D, E, C, …} n = 5 (from the flow ID list) Share w for flows A, B, C, D, E are 2/5, 2/5, 1/5, 0, and 0 Sender and receiver don’t need to know other flows, history = {A, -1, A, -1, -1}

17. 600.647 Advanced Wireless Antonello Cruz Detection of Unfairness Measure how much a flow has been over/under used Degree by which flow should be –Aggressive N a –Restrictive N r Normal use, N a and N r are set to 0

18. 600.647 Advanced Wireless Antonello Cruz Detection of Unfairness To find N a and N r slide window in history N a : slide until the flow x enters the window For D, N a = 2 {A, B, [A, C, B, A, D], E, C, …} For E, N a = 3 {A, B, A, [C, B, A, D, E], C, …} N r : slide until only 1 flow x in the window For A, N r = 3 {A, B, A, [C, B, A, D, E], C, …} For B, N r = 2 {A, B, [A, C, B, A, D], E, C, …}

19. 600.647 Advanced Wireless Antonello Cruz FMAC/CSR Based on cooperation between sender and receiver Sender and Receiver estimate parameters Estimated values may differ in sender and receiver

20. 600.647 Advanced Wireless Antonello Cruz FMAC/CSR: Sender If flow in aggressive mode, back-off [0, X], X = max (n, 2n - N a ) If flow in normal mode back-off [2n, CW] If flow in restrictive mode, defer by (N r + 1) x TxTime(packet). Then back- off [2n, CW x N r ]

21. 600.647 Advanced Wireless Antonello Cruz FMAC/CSR: Sender Regardless of mode, when back-off timer expires and medium is idle, transmit Recompute mode when becomes idle after a busy state The CW is manipulated as in the Binary Exponential Back-off

22. 600.647 Advanced Wireless Antonello Cruz FMAC/CSR: Sender

23. 600.647 Advanced Wireless Antonello Cruz Receiver Feedback Implicit Restrictive-notification If a flow has over-used the medium, it must have just transmitted a packet. The restrictive-notification can be piggybacked in the ACK frame The receiver sends N r

24. 600.647 Advanced Wireless Antonello Cruz Receiver Feedback Explicit Aggressive-notification If the flow is under-used, there will be no packet received recently. The notification has to be sent explicitly Use frame similar to RTS - it will contend for the medium Prioritize medium access –Sender in aggressive mode –Receiver in aggressive mode –Sender in normal mode –Sender in restrictive mode

25. 600.647 Advanced Wireless Antonello Cruz FMAC/CSR Summary

26. 600.647 Advanced Wireless Antonello Cruz Simulation Results

27. 600.647 Advanced Wireless Antonello Cruz Simulation Results

28. 600.647 Advanced Wireless Antonello Cruz Simulation Results

29. 600.647 Advanced Wireless Antonello Cruz Simulation Results

30. 600.647 Advanced Wireless Antonello Cruz Simulation Results

31. 600.647 Advanced Wireless Antonello Cruz Simulation Results

32. 600.647 Advanced Wireless Antonello Cruz Final Thoughts Ingenious solution, solves unfairness problem with very small reduction in performance Potential obstacles for widely adoption How to interact with nodes that don’t talk FMAC/CSR? Different packet sizes Become an endorsed IEEE standard