The emerging IEEE 802.11 family of wireless technologies has shown tremendous growth and acceptance as a last hop wireless solution in Local Area Networks (LANs). The use of multimedia networking applications has brought more requirements to the network, creating a need for end-to-end quality of service (QoS). In this context, the standard introduces one mechanism, called EDCA which provides a prioritized traffic to guarantee the QoS involving four different Access Category (ACs). The main problem of the original EDCA is that the values of the main parameters of each AC queue (such as contention window limits) are static and do not take into account wireless channel conditions. In this paper, we present a new approach to split the contention windows per AC into different sub-windows, ensuring that this partition decreases channel collisions while maintaining a low delay and high throughput. Our technique select on the fly, the sub-windows per AC taking into account both applications requirements and network conditions. Preliminary results on ad-hoc and infrastructure wireless networks show that the presented technique enhances the channel utilization, decreasing the delay and increasing the throughput.