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A set of enhancements to the IEEE 802.11 standard, viz. the IEEE 802.11e have been proposed to meet the increasing demand for quality of service. The standard provides a means for service differentiation by using multiple traffic categories at each node, where each traffic category has its own individual parameters such as priority, inter-frame space and contention window size. After each successful transmission, the contention window size is decreased based on a static equation, which may result in poor channel utilization and a decrease on the system throughput. In this paper, we propose a new protocol, called enhanced distributed coordination function with dual-measurement (EDCF-DM), to address this issue. EDCF-DM is based on the idea of reducing the number of idle slots by dynamically varying the contention window size according to the current traffic state of the traffic categories at each node. Meanwhile, it carefully adapts the contention window size based on the network condition of the system to avoid incurring extra collisions. Extensive simulations are performed to evaluate the proposed protocol. Simulation results demonstrate that EDCF-DM provides a good service differentiation and outperforms the standard 802.11e in terms of channel utilization, throughput and packet delay.