A performance model of the IEEE 802.11 distributed coordination function under finite load condition

Modeling of the IEEE 802.11 wireless LAN MAC protocol has received considerable research attention recently. Most of the previous works have concerned with the derivation of system throughput for the Distributed Coordination Function (DCF) of the MAC protocol. There have been very few works investigating the delay performance of the DCF scheme. This thesis presents an analytical model of the DCF under un-saturated homogeneous and heterogeneous conditions. Our model allows stations to have either the same or different packet arrival and transmission rates. The arrival of packets is assumed to be according to a Poisson process. We model each station's MAC buffer as an M/G/1 queue and the service time of each station as a three dimensional Markov chain. The dependency between stations is taken into account through the backoff counter freezing and packet collision during transmission. We derive the probability generating function (PGF) of the packet service time distribution and obtain the closed form expression of the mean packet delay by applying the M/G/1 queuing result. We also investigate the impact of different contention window sizes on the delay performance. The accuracy of the model has been verified by extensive simulations.