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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
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