Wi-Fi Multimedia Extensions (WME) - This mandatory mechanism refines the 802.11 Distributed Control Function to prioritize shared media access. In 802.11 WLANs, stations contend for the same channel. When a station has data to send, it checks to see if the channel is busy. If the channel is free, the station can transmit with a minimum interframe space. If the channel is busy, the station uses a backoff timer to wait for the channel to be free. With 802.11e, a new Enhanced Distributed Control Access (EDCA) mechanism will apply shorter interframe spaces and backoff timers to higher-priority traffic. Four classes of service -- Voice, Video, Best Effort and Background -- are marked with eight 802.1d tags.
Wi-Fi Scheduled Multimedia (WSM) - This optional mechanism goes beyond station contention by letting APs periodically poll stations for latency-sensitive traffic. To use this mechanism, a station must first send the AP a profile describing its QoS requirements (e.g., throughput, latency, jitter). The AP either reserves (schedules) transmit opportunities to satisfy the station's requirements, or advises the station that it cannot meet those requirements. In this way, a station can associate only with an AP able to meet its needs, or find another AP. However, the station must be able to predict those needs in advance.
Without QoS control, 802.11 WLANs offer best-effort delivery, treating all stations as equals. This is often just fine for short data transactions -- checking email, browsing the web, etc... However, best-effort may not be good enough for latency-sensitive applications like voice and video. Implementing QoS controls can better support those applications, but increases implementation complexity and the potential for interoperability problems in multi-vendor environments. The latter concern is addressed by the Wi-Fi Alliance WMM certification program which tests interoperability between products that implement a defined subset of 802.11e QoS features.