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Quality of service (QoS) in wireless 802.11 networks cannot be achieved without the support for service differentiation at the medium access layer. IEEE802.11 suffers from QoS limitations that have often been summarized in the literature. We present simulation results to show the effectiveness of several proposed techniques of improving the QoS in the IEEE802.11 protocol. Specifically, we closely examine the sliding contention window (SCW) technique and present a performance study that augments the work done by the original authors, by considering the determinism of QoS achieved. We find that simple approaches such as priority queuing and differential inter-frame spacing provide service differentiation at a single station but are not successful in providing it across a number of stations. Further, we find that the SCW approach provides a high service differentiation and deterministic QoS guarantees. It achieves these by a strict separation of the contention window range of different priority classes. Further, it measures and reacts to losses suffered by high priority flows at the MAC layer and to the increase in network load. These cross layer design techniques help SCW maintain good performance even at high network load. However, this strict service differentiation comes at the cost of reduced performance of low priority flows at heavy load.