performance analysis of the ieee 802.11 distributed coordination function giuseppe bianchi ieee...

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