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In this paper, a slot-based MAC protocol is proposed for cognitive radio wireless networks. This protocol makes cognitive users sense licensed channels periodically, and each period consists of a fixed number of slots. When cognitive users identify un-used licensed channels, they use IEEE 802.11 distribution coordination function (DCF) to contend the spare channels. According to 802.11 DCF, all other cognitive users freeze their backoff counters when a cognitive user transmits data. So the decrement of backoff counter, which equals to the decrement of the number of slots in one period, depends on the traffic of cognitive users. When the slots of one period decrease to zero, this period ends. So the duration of each period may be variable with different busy conditions of cognitive users. We use the Markov chain model to estimate the saturation throughput of our proposed MAC protocol.