of the IEEE 802.11 Distributed Coordination Function Performance Analysis of the IEEE 802.11 Distributed Coordination Function (By Giuseppe Bianchi, 2000) Sunghwa Son

Outline Introduction 802.11 DCF Maximum & Saturation Throughput Performance Throughput Analysis Model Validation Maximum Saturation Throughput Performance Evaluation Conclusions

Introduction Distribution Coordination Function (DCF) Fundamental mechanism to access medium Based on CSMA/CA Wireless LAN uses half-duplex Describes two techniques Basic Access Mechanism RTS/CTS Mechanism

Basic Access Mechanism Channel Idle for a period of time Distribution Inter Frame Space (DIFS) Station generates random backoff interval Employs discrete-time backoff scale Transmission take at beginning of time slot Backoff time chosen between (0,w-1) Backoff time counter decremented to 0

Basic Access Mechanism

RTS/CTS Waits until channel sensed idle Transmits short frame called RTS Receiving station send CTS Carry information of length of packet Solves hidden terminal problems

RTS/CTS

Saturation Throughput Limit reached by system throughput as offered load increases Maximum load that the system can carry in stable conditions

Throughput Analysis Analytical Evaluation of Saturation Throughput Assumption Finite number of terminals and ideal channel conditions Fixed number of stations, each always having packet for transmission

Throughput Analysis Two cases Packet Transmission Probability Markov Chain model Throughput System throughput

Packet Transmission Probability Each packet collide with constant and independent probability p p will be referred to as conditional collision probability Model bidimensional process {s(t) , b(t)} with discrete-time Markov chain Stochastic Process b(t) representing backoff time counter Stochastic Process s(t) representing backoff stage

Markov Chain Model

Markov Chain Model Closed-form solution for Markov chain

Markov Chain Model Probability τ that a station transmits in a randomly chosen slot time

Markov Chain Model When m =0, i.e., no exponential backoff is considered, probability τ results to be independent of p In general τ depends on conditional collision probability p

Throughput Normalized system throughput S Probability that there is at least one transmission in the considered slot time Probability that exactly one station transmits on the channel, conditioned on the fact that at least one station transmits

Throughput Normalized system throughput E[P] is the average packet payload size Specify Ts and Tc to compute throughput for DCF access mechanism

Throughput Considering System via Basic Access mechanism Packet header H = PHYhrd +MAChrd Propagation delay δ

Throughput Packet transmission via RTS/CTS Access mechanism

Model Validation

Maximum Saturation Throughput τ depends on n (network size), W, and m. As n is not a directly controllable variable, the only way to achieve optimal performance is to employ adaptive techniques to tune the values m and W In the 802.11 standard, the values W and m are hardwired in the PHY layer details, and thus they cannot be made dependent on n

Maximum Saturation Throughput For n sufficiently large Results to be independent of n

Performance Evaluation Throughput of Basic Access mechanism depends on W Optimal value of W depends on n High value of W gives excellent throughput performance in case of 50 contending stations

Performance Evaluation Throughput obtained with RTS/CTS mechanism is almost independent of the value

Performance Evaluation Number of transmissions per packet increases as W reduces & network size n increases.

Conclusion Model suited for both Basic Access and RTS/CTS Access mechanisms Performance of Basic Access method depends on W and n Performance is only marginally dependent on the system parameters when the RTS/CTS mechanism is considered RTS/CTS scheme solved hidden terminal problem