ity available through HCF and HCCA. This and other 802.11e capabilities are planned to be progressively included in the Wi-Fi Alliance's WMM certification program. Chapter Six Table 6-11: WMM Access Category Descriptions Access category Description WMM voice priority Highest priority. Allows multiple concurrent VoWLAN calls, with low latency and quality equal to a toll voice call. WMM video priority Prioritises video traffic above lower categories. One 802.11g or 802. 11a channel can support 3 to 4 standard definition TV streams or 1 high definition TV stream. WMM best effort priority Traffic from legacy devices, from applications or devices that lack QoS capabilities, or traffic such as internet surfing that is less sensitive to latency but is affected by long delays. WMM background priority Low priority traffic, such as a file download or print job, that does not have strict latency or throughput requirements. Spectrum Management at 5 GHz (802.11h) The 802.11h standard supplements the 802.11 MAC with two additional spectrum management services, transmit power control (TPC), which limits the transmitted power to the lowest level needed to ensure adequate signal strength at the farthest station, and dynamic frequency selection (DFS), which enables a station to switch to a new channel if it detects other non-associated stations or systems transmitting on the same channel. These mechanisms are required for 5 GHz WLANs operating under European regulations, in order to minimise interference with satellite communications (TPC) and military radar (DFS), and support for the 802.11h extensions was required from 2005 for all 802.1 11a compliant systems operating in Europe. In the US, compliance with 802.11h is also required for 802.11a products operating in the 12 channels from 5.47 to 5.725 GHz. IEEE 802.11h compliant networks therefore have access to 24 non-overlapping OFDM channels, resulting in a potential doubling of overall network capacity. Transmit Power Control An 802.11h compliant station indicates its transmit 