Variability of Service Times and Throughput Efficiency Trade-Off in IEEE 802.11 DCF

First order metrics (throughput, average delay) of the IEEE 802.11 DCF MAC protocol have been extensively analyzed. In this work the probability distribution of the service time is derived and it is used to highlight the source of service time burstiness. It is proved that the variance of the service time is minimized by taking the same contention window value at each back off stage, under the constraint that the saturation throughput has a given value. A trade off between throughput efficiency and variance of the service times is computed and the binary exponential back off mechanism is found to be responsible of the bursty service process of IEEE 802.11 DCF. Implication of this result on the design and tuning of IEEE 802.11 DCF parameters is discussed.