Performance Analysis of the IEEE 802.11 Distributed Coordination Function

Giuseppe BianchiIEEE Journal on Selected Areas in

CommunicationsMarch 2000

Outline

• Introduction• 802.11 Distributed Coordination Function• Maximum & Saturation Throughput Performance• Throughput Analysis• Model Validation• Maximum Saturation Throughput• Performance Evaluation• Conclusion

Introduction

• 802.11• Distributed Coordination Function– The fundamental mechanism to access the

medium– Based on CSMA/CA– Two techniques• Basic Access Mechanism• RTS/CTS Mechanism

802.11 DCF

• Two access techniques– Basic mechanism: 2 way handshaking– RTS/CTS mechanism: 4 way handshaking

Source Dest

DATA

ACK

SourceDESt

RTS

CTS

DATA

ACK

802.11 DCF

802.11 DCF

802.11 DCF

Maximum and Saturation Throughput Performance

• Maximum throughput performance• Saturation throughput performance– Maximum load in stable condition

Throughput Analysis

• Assumption– Fixed # of stations– Always having a packet available for transmission• Transmission queues are always nonempty

– Two parts of analysis• Study the behavior of single station with a Markov model• Study the events that occur within a generic slot time &

expressed throughput for both Basic & RTS/CTS access method– Obtain the stationary probability

Throughput Analysis

• n stations– Each station always has a packet available for

transmission• b(t)– Stochastic Process representing backoff time counter

• W = ; = W• s(t)– Stochastic Process representing backoff stage– (0,m)

Throughput Analysis

• Each packet collide with constant and independent probability p

• Model bi-dimensional process {s(t) , b(t)} with discrete-time Markov chain

Markov Chain model

Markov Chain model

• Stationary distribution of the chain

• i ϵ ( 0, m ) , k ϵ ( 0, -1 )

Markov Chain model

• Probability τ – a station transmits in randomly chosen slot time

Markov Chain model

• Some note– If m = 0 , • Independent of p

• In general, τ depends on p

Throughput

• Normalized system throughput S• Probability of transmission – At least one transmission in the slot time

• Probability of successful transmission – Transmit successfully

Throughput

• E[P]: average packet payload size• : average time the channel is sensed busy because of a

successful transmission• : average time the channel is sensed busy by each stationi

during a collusion

Throughput

Throughput

Maximum Saturation Throughput

• Optimal

–

Model Validation

Performance Evaluation

Basic RTS/CTS

Performance Evaluation

Basic RTS/CTS

Performance Evaluation

Performance Evaluation

Performance Evaluation

Conclusion

• Evaluated the 802.11 DCF throughput performance

• Model suited for both Basic Access and RTS/CTS Access mechanisms

• The model is extremely accurate in predicting the system throughput

• Basic Access strongly depends on n and w• RTS/CTS is better in large network scenarios