FMAC/CSR: a Fair MAC Protocol for Wireless Ad-hoc Networks
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FMAC/CSR: a Fair MAC Protocol for Wireless Ad-hoc Networks
Zhifei Li
Anil K. Gupta
Sukumar Nandi
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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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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
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Unfairness in IEEE 802.11
IEEE 802.11
Distributed Coordination Function (DCF)
• Concealed Information Problem
• Imprecise Collision Detection
• High Contention
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Medium Contention
Two flows contend the medium if either sender or receiver of one flow is within transmission range of the other
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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
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Concealed Information Problem
• Neither sender is within range of the receiver of the other flow
Short-term unfairness
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Concealed Information Problem
• Only one of the senders is within the range of the receiver of the other flow
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Concealed Information Problem
• Both senders are within range of the receiver of the other flow
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Concealed Information Problem
When the contention period occurs?
Starts after the transmission of ACK
Ends at transmission of RTS
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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
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High-Contention
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General Framework to Achieve MAC Fairness
• Fairness ModelFair share over certain duration/window (W)
Flow-level fairness
Consider packets to have same length
All application have same requirements
• Compensation ModelAggressive
Normal
Restrictive
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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
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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
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Estimation of Actual Share
• Keep history of flows
• Check how many times you transmittedHistory = {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}
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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
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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, …}
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FMAC/CSR
• Based on cooperation between sender and receiver
• Sender and Receiver estimate parameters
• Estimated values may differ in sender and receiver
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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]
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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
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FMAC/CSR: Sender
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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
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Receiver FeedbackExplicit Aggressive-notificationIf 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
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FMAC/CSR Summary
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Simulation Results
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Simulation Results
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Simulation Results
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Simulation Results
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Simulation Results
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Simulation Results
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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