An experimental study of MANET using 802.11b has shown that there exist several configurations where this standard does not guarantee fairness for the channel access. In this paper, we investigate the performance of the WiFi protocol in such configurations using the analytical modelling technique PEPA. The proposed model appears to be generic as it can be used to model any mobile node implementing this protocol. To provide a better understanding of such a problem we analyse the protocol behaviour and evaluate its cost in terms of medium utilisation and throughput for the communicating pairs. The results obtained are compared to simulation results. Categories and Subject Descriptors: I.6.5 [Model Development]: Modeling methodologies General Terms: Performance