fmac/csr: a fair mac protocol for wireless ad-hoc networks
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
600.647 Advanced Wireless Antonello Cruz
FMAC/CSR: a Fair MAC Protocol for Wireless Ad-hoc Networks
Zhifei Li
Anil K. Gupta
Sukumar Nandi
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
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
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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
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
600.647 Advanced Wireless Antonello Cruz
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
600.647 Advanced Wireless Antonello Cruz
Simulation Results
600.647 Advanced Wireless Antonello Cruz
Simulation Results
600.647 Advanced Wireless Antonello Cruz
Simulation Results
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