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IEEE 802.11 MAC protocol has been the standard for wireless LANs, and also adopted in many network simulation packages for wireless multihop ad hoc networks. However, it is well known that, as the traffic goes up, the performance of IEEE 802.11 MAC drops dramatically in terms of delay and throughput, especially when the station approaches its saturation state. To explore the inherent problems in this protocol, it is important to characterize the probability distribution of the packet service time at the MAC layer. In this paper, by modeling the exponential backoff process as a Markov chain, we can use the signal transfer function of the generalized state transition diagram to derive the probability distribution of the MAC layer service time. We then present the discrete probability distribution for MAC layer packet service time, which is shown to accurately match the simulation data from network simulations. Finally, based on the probability model for the MAC layer service time, we are able to analyze a few performance metrics of the wireless LAN and give better explanation to the performance degradation in delay and throughput at various traffic load.