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The primary medium access control (MAC) technique of IEEE 802.11 is called Distributed Coordination Function (DCF), which is adaptive for low traffic loads and nodes' density. However, this does not hold when the traffic loads are high. This paper proposes a new function to adjust the contention window (CW) in 802.11 ad hoc networks in order to extend the DCF. Our goal is to alleviate the delay, collisions and the packet loss rate and increase the throughput by decreasing collisions when the loads of the network is under heavy loads and collisions among nodes are frequent. The contribution of our proposed algorithm which takes consideration of nodes' density and data rate and can be adaptively adjusted based on the recent networks conditions, is to smooth the collision through tuning the contention window. The performances of the IEEE 802.11 DCF, enhanced with our adaptation function, are extensively evaluated. The experiment results indicate that our proposed scheme outperforms the 802.11 DCF standards in terms of packet delay, throughput, packet loss rate and collisions. The delay decreases by up to 23.97%, total throughput increases up to 14.77%, collisions decrease at least 34.3% and loss rate is 27.12% lower.