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

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

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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

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

600.647 Advanced Wireless Antonello Cruz

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

600.647 Advanced Wireless Antonello Cruz

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}

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 Na

– Restrictive Nr

• Normal use, Na and Nr are set to 0

600.647 Advanced Wireless Antonello Cruz

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, …}

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

600.647 Advanced Wireless Antonello Cruz

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]

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

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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

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

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