  The Distributed Coordination Function (DCF) aims at fair and efficient medium
access in IEEE 802.11. In face of its success, it is remarkable that there is
little consensus on the actual degree of fairness achieved, particularly
bearing its impact on quality of service in mind. In this paper we provide an
accurate model for the fairness of the DCF. Given M greedy stations we assume
fairness if a tagged station contributes a share of 1/M to the overall number
of packets transmitted. We derive the probability distribution of fairness
deviations and support our analytical results by an extensive set of
measurements. We find a closed-form expression for the improvement of long-term
over short-term fairness. Regarding the random countdown values we quantify the
significance of their distribution whereas we discover that fairness is largely
insensitive to the distribution parameters. Based on our findings we view the
DCF as emulating an ideal fair queuing system to quantify the deviations from a
fair rate allocation. We deduce a stochastic service curve model for the DCF to
predict packet delays in IEEE 802.11. We show how a station can estimate its
fair bandwidth share from passive measurements of its traffic arrivals and
departures.
