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The proposed IEEE 802.11e draft standard defines new MAC protocols for QoS in wireless networks, mainly EDCF and HCF. In this paper, we propose to extend EDCF with a dynamic adaptation algorithm of the maximum contention window (CWmax) that enables each station to tune the size of the CWmax used in its back-off algorithm at run time. The purpose of our scheme is to reduce delay and jitter and increases the efficiency of the transmission channel. Priorities between access categories are provisioned by updating the size of the CWmax according to application requirements and channel conditions. The performance of the enhanced EDCF are extensively investigated by simulation. Results obtained indicate that the delay and jitter for high priority traffic, i.e., audio traffic, improve and delay decreases by up to 43%. Furthermore, delay for lower priority traffic, e.g., video traffic, remains stable. Throughput in CWmax adaptation, compared to EDCF, is stable in light and medium system load and increases by up to 7% especially at high system load.