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

600.647 Advanced Wireless Antonello Cruz

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


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


Unfairness in IEEE 802.11

IEEE 802.11

Distributed Coordination Function (DCF)

• Concealed Information Problem

• Imprecise Collision Detection

• High Contention


Medium Contention

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


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



• Neither sender is within range of the receiver of the other flow

Short-term unfairness



• 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



When the contention period occurs?

Starts after the transmission of ACK

Ends at transmission of RTS


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


High-Contention


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


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


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


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}


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


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, …}


FMAC/CSR

• Based on cooperation between sender and receiver

• Sender and Receiver estimate parameters

• Estimated values may differ in sender and receiver


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]


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


FMAC/CSR: Sender


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


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


FMAC/CSR Summary


Simulation Results


Simulation Results


Simulation Results


Simulation Results


Simulation Results


Simulation Results


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

Documents

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