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FMAC/CSR: a Fair MAC Protocol for Wireless Ad-hoc Networks

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

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FMAC/CSR: a Fair MAC Protocol for Wireless Ad-hoc Networks

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

  2. 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. 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. Unfairness in IEEE 802.11 IEEE 802.11 Distributed Coordination Function (DCF) • Concealed Information Problem • Imprecise Collision Detection • High Contention

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

  6. 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. Concealed Information Problem • Neither sender is within range of the receiver of the other flow Short-term unfairness

  8. Concealed Information Problem • Only one of the senders is within the range of the receiver of the other flow

  9. Concealed Information Problem • Both senders are within range of the receiver of the other flow

  10. Concealed Information Problem When the contention period occurs? Starts after the transmission of ACK Ends at transmission of RTS

  11. Imprecise Collision Detection • SA and SB send RTS at the same time • RA detects collision and doesn’t send CTS • RB can’t hear SA’s RTS and sends CTS • SB captured the medium

  12. High-Contention

  13. General Frameworkto 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. Estimation of theof the Number of Active Flows • List of active flows from tuples (RTS/CTS/Data/ACK) with timestamps • Stale entry older than We

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

  16. 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. Detection of Unfairness • Measure how much a flow has been over/under used • Degree by which flow should be • Aggressive Na • Restrictive Nr • Normal use, Na and Nr are set to 0

  18. Detection of Unfairness To find Na and Nr slide window in history Na: slide until the flow x enters the window For D, Na = 2 {A, B, [A, C, B, A, D], E, C, …} For E, Na = 3 {A, B, A, [C, B, A, D, E], C, …} Nr: slide until only 1 flow x in the window For A, Nr = 3 {A, B, A, [C, B, A, D, E], C, …} For B, Nr = 2 {A, B, [A, C, B, A, D], E, C, …}

  19. FMAC/CSR • Based on cooperation between sender and receiver • Sender and Receiver estimate parameters • Estimated values may differ in sender and receiver

  20. FMAC/CSR: Sender • If flow in aggressive mode, back-off [0, X], X = max (n, 2n - Na) • If flow in normal mode back-off [2n, CW] • If flow in restrictive mode, defer by(Nr + 1) x TxTime(packet). Then back-off [2n, CW x Nr]

  21. 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. FMAC/CSR: Sender

  23. 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 Nr

  24. 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. FMAC/CSR Summary

  26. Simulation Results

  27. Simulation Results

  28. Simulation Results

  29. Simulation Results

  30. Simulation Results

  31. Simulation Results

  32. 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

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