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IEEE 802.11 is a widely used standard for MAC and PHY layers of WLANs. Unfortunately the access methods offered in this standard cannot support QoS (quality of service) for real-time traffics. It seems that the access methods employed in this standard, causes high variations in delay or jitter and also waste of bandwidth due to collisions. In this paper, we provide service differentiation by defining priority classes and modify DCF to enhance it. We modify calculating method of CW (contention window) size after a successful transmission and study the effect of CW size on performance. The results of our simulations show that performance of DCF with this modification is better than the scheme used in DCF, especially, for traffics for which throughput is the most important parameter. Also this method provides better fairness among low and high priority traffics.