We present in this paper an analytical model that accounts for the positions of stations with respect to the Access Point while evaluating the performance of 802.11 MAC layer. Our model is based on the Bianchi's work where the performance of 802.11 MAC layer is computed using a discrete time Markov chain, but where all stations are implicitly assumed to be located at the same distance to the Access Point. In our model, given the position of one station, we compute its saturation throughput while conditioning on the positions of the other concurrent stations. Our model also gives the total saturation throughput of the medium. We solve the model numerically and we show that the saturation throughput of one station is strongly dependent on its own position and on the positions of the other stations. One station achieves a high throughput when it is close to the Access Point and loses when it moves away. There is some distance threshold above which the lost in the throughput is quick and important. When a station is far from the Access Point compared to the other stations, it will end up by contending for the bandwidth not used by the other stations. We believe that our model is a good tool to dimension 802.11 wireless access networks and to study their capacities and their performances.