The Wi-Fi Alliance's WMM (Wi-Fi Multimedia) is a subset of the IEEE 802.11e QoS draft standard. WMM defines four access categories: voice, video, best-effort, and background. Each packet is tagged according to category. Packets in higher priority categories remain in queue for shorter periods, while packets in lower priority categories stay in-queue longer. This is done using extensions to the CSMA/CD Distributed Coordination Function (DCF) which adjust a station's minimum interframe spacing and random backoff wait parameters that apply to each category. To learn more about how 802.11 QoS and WMM work, read this WMM white paper.