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Enhanced distributed control function (EDCF) can guarantee inter-AC (access category) differentiation for wireless ad hoc networks. However, it ignores the intra-AC differentiation. This paper proposes a new function to adjust the contention window in 802.11e to guarantee both inter-AC differentiation and intra-AC one. Our goal is to alleviate the delay, collisions and increase the throughput by decreasing collisions when the loads of the network are heavy. The main contribution of our algorithm, which takes flow priority and nodes density into consideration and can be adaptively adjusted according to recent network conditions, is to smooth the collision by tuning the contention window. We also present a theoretical throughput analysis on our new function. The experiment results indicate that our scheme outperforms IEEE802.11e EDCF in terms of packet delay, throughput and collisions.