• United States
by Michael See, special to Network World

802.11h helps WLANs share spectrum

Jul 19, 20044 mins
Cellular NetworksWi-Fi

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.

TPC is intended to reduce interference from WLANs to satellite services by reducing the radio transmit power WLAN devices use. TPC also can be used to manage the power consumption of wireless devices and the range between access points and wireless devices.

An access point specifies TCP support in the frames it generates to WLAN stations. These frames also specify the maximum transmit power allowed in the WLAN and the transmit power the access point currently is using. The transmit power used by stations associated with an access point cannot exceed the maximum limit that the access point sets.

When a WLAN station associates with an access point, the station indicates its transmit power capability. The access point uses this data about the stations associated with it to determine the maximum power for the WLAN segment. This means the radio power in a WLAN segment can be adjusted to reduce interference with other devices while still maintaining sufficient link margin for operation of the wireless network.

Frames also are sent between the access point and the stations to monitor the signal strength of the wireless network. The access point can dynamically adjust the radio signal strength, if necessary, to maintain wireless communications.

The original motivation for the DFS and TPC mechanisms defined in 802.11h ensure a standard method of operation under the regulatory requirements governing the 5-GHz band, which will spur deployment of 802.11a wireless networks. Along with meeting regulatory requirements, DFS and TPC can be used to improve the management, deployment and operation of WLANs.

See is CTO of the network infrastructure business unit of Alcatel’s Enterprise Solutions Division. He can be reached at