Because 802.11a wireless networks operate in the 5-GHz radio frequency band and support as many as 24 non-overlapping channels, they are less susceptible to interference than their 802.11b/g counterparts. However, regulatory requirements governing the use of the 5-GHz band vary from country to country, hampering 802.11a deployment.

In response, the International Telecom-munication Union recommended a harmonized set of rules for WLANs to share the 5-GHz spectrum with primary-use devices such as military radar systems. Approved in September, the IEEE 802.11h standard defines mechanisms that 802.11a WLAN devices can use to comply with the ITU recommendations. These mechanisms are dynamic frequency selection (DFS) and transmit power control (TPC). WLAN products supporting 802.11h will be available in the second half of this year.

DFS detects other devices using the same radio channel, and it switches WLAN operation to another channel if necessary. DFS is responsible for avoiding interference with other devices, such as radar systems and other WLAN segments, and for uniform utilization of channels.

An access point specifies that it uses DFS in the frames WLAN stations use to find access points. When a WLAN station associates or re-associates with an access point, the station reports a list of channels that it can support. When it's necessary to switch to a new channel, the access point uses this data to determine the best channel.

The access point initiates a channel switch by sending a frame to all stations associated with the access point that identifies the new channel number, the length of time until the channel switch takes effect, and whether or not transmission is allowed before the channel switch. Stations that receive the channel switch information from the access point change to the new channel after the elapsed time.

An access point measures channel activity to determine if there is other radio traffic in the channels being used for a WLAN. The access point sends a measurement request to a station or group of stations identifying the channel where activity is to be measured, the start time of the measurement and the duration of the measurement. The station performs the requested measurement of channel activity and generates a report to the access point.