The exponential growth of wireless technologies attracted many research to improve the quality of service (QoS) of global wireless networks such as 802.11 and 802.16. In specific, research targeted the physical layer (PHY) parameters and medium access control (MAC) mechanisms to improve QoS. In this paper we provide a simple yet efficient model to maintain the quality of service for wireless networks with the aim to improve the capacity and throughput for non-real time applications. We developed a new model for the access of the medium in a contention-based environment taking into consideration fairness and damping of collision frequency. The proposed model is analytically proved using Markov chain, taking into account the maximum contention window size, the exponential back-off procedure as well as considering the bandwidth and time needed and the possible time out for lost packets. We validated the accuracy of the proposed model by simulation and comparing with the standard model of the IEEE 802.11.