IEEE 802.11e provides a basic mechanism for traffic differentiation in WLANs. We present a method based on monitoring the traffic on the AP that it is efficient and allows the enforcement of QoS policies (admission control, traffic prioritization, bandwidth allocation etc.) in a IEEE 802.11e access network. The approach is simple to implement, flexible and does not rely on restrictive hypothesis about the traffic. We validate the proposal through experiments on VoIP traffic conducted in a real test bed.