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The proposed IEEE 802.11e draft standard defines new MAC protocols for QoS in wireless networks, mainly HCF (hybrid coordination function) and EDCF (enhanced distributed coordination function). EDCF is a contention-based channel access scheme and is part of HCF for infrastructure networks and may be used as a separate coordination function for wireless ad-hoc networks. We propose to extend EDCF with a dynamic adaptation algorithm for the minimum contention window (CWmin) that enables each station to tune the size of the CWmin used in its back-off algorithm at run time. The purpose of our scheme is to reduce delay and jitter and increase the efficiency of the transmission channel. Priorities between access categories are provisioned by tuning the size of the CWmin according to application requirements and channel conditions. The performances of the IEEE 802.11e EDCF, enhanced with our adaptation algorithm, are extensively investigated by simulation. Results obtained indicate that the CWmin adaptation scheme outperforms the 802.11e EDCF standard in terms of channel utilization, throughput, and packet delay. Indeed, the delay for the high priority access category decreases by up to 30%, total throughput increases by up to 18%, and channel capacity is at least 15% higher.