Due to its increasingly growing capacity, WLANs are becoming mature enough to be integrated in a real commercial multi-service offers. However, their success will mainly depend on their ability to provide quality of service for different media types (audio, video, etc.). In this paper, we devise a new protocol which provides a more strict service differentiation between different traffics. The protocol deals also with the well known 802.11 anomaly and improves network throughput by using separate contention window ranges reinforced by a packet length differentiation. Thus, each flow will be seen allocated a selected backoff interval and a packet length from variable bounded ranges to improve both throughput and delay. The different backoffs and packet lengths vary dynamically depending on the propagation conditions. Fairness is then taken into consideration as the adaptation is made sensitive to upper-layers? quality of service metrics as well as to the propagation conditions observed at the PHY layer expressed with SNR value, carrying out a crosslayer architecture.