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In this work we develop a general analytical framework to study the fairness characteristics of the IEEE 802.11a MAC protocol. We prove that equally sharing the channel bandwidth among the nodes leads to a significant inefficiency because nodes transmitting at low bit rates (i.e., having bad channel conditions) consume more channel resources than nodes transmitting at high bit rates. To solve this problem we extend the 802.11 backoff algorithm to guarantee equal temporal shares to all the nodes in the network. This is achieved by prioritizing the channel access using different contention windows. We analytically describe the behavior of the modified MAC protocol, called TF-802.11a protocol, and we exploit our analytical results to determine how to select the average backoff for each available bit rates in order to achieve a channel utilization fairness. An extensive set of simulations confirms the throughput gain provided by the proposed temporal fair MAC protocol with respect to the standard protocol.