Currently deployed IEEE 802.11 WLANs work mostly with Distributed Coordination Function (DCF) mode at the MAC layer, which does not provide QoS support. The upcoming IEEE standard 802.11e achieves service differentiation by assigning different Channel Access Parameters (CAPs) to different traffic classes at the MAC layer. However, such relative differentiation does not yield QoS guarantee. In practice, appropriately selecting CAPs a priori is difficult. Time-varying traffic loads also make the use of fixed CAPs inefficient for both QoS support and channel utilization. In this work, we propose a novel architecture called HARMONICA, in which the access point dynamically selects the best CAPs for each traffic class to optimally match their QoS requirements. We present and discuss a simple admission control mechanism used by HARMONICA to avoid congestion. Our simulation results demonstrate that under an interference-free environment, HARMONICA can guarantee the QoS for all traffic classes while simultaneously achieving quasi-optimal channel utilization.