The WRC demands that a channel be vacated when a WLAN device receives a signal in a channel exceeding -62 or -64 dBm. One way to fulfill this requirement is with dynamic frequency selection. This technique is similar to the way that conversations are handled via walkie talkies. Public-safety personnel frequently detect other users on a particular channel. When the traffic reaches a certain level, they ask their partners to switch to another channel so they can continue their conversation uninterrupted.

DFS allows Wi-Fi products to avoid co-channel interference. Among the other potential uses of DFS is the uniform utilization of available channels. This aspect will become increasingly important as 802.11a products become more common. The elements that are associated with DFS are shown in TABLE 2.

Dynamic frequency selection works in the following manner: A station provides a list of supported channels to the access point. The access point then approves or rejects the request based on channel information. Whenever an access point schedules a quiet interval in order to test for radar, stations may not transmit on that channel during the quiet interval (FIG. 1).

An access point also can request measurements from a station via a measurement request. The station responds by sending a measurement report to the access point (FIG. 2). Lastly, an access point may utilize a channel-switch announcement to inform stations when they should switch to a specified channel. The station subsequently responds by switching to the new channel (FIG. 3).

The second element of 802.11h, transmit power control (TPC), is a method of lowering the transmit power used by a WLAN system. TPC lowers the "volume" of the signal on a network. WLAN access points and stations that are located near each other do not need to use the same transmit power as access points and stations that are close to the limit of their range.

Transmit power control was designed to reduce WLAN interference with satellite services in the 5-GHz band. Other potential uses of TPC include range control and the reduction of power consumption. The elements that are associated with TPC are shown in TABLE 3.

TPC is utilized in the following manner: A station sends the minimum and maximum transmit-power information (the station's power capability) to the access point (FIG. 3 ). This step occurs during the association or reassociation attempt. The access point then determines if the station meets the local regulatory requirements (power constraint). It either approves or denies the request.

Next, the access point sends a request for transmit-power and link-margin information (transmit-power-control request) to the station. The station responds by sending the information (transmit-power-control report) to the access point. The station adapts its transmit power based upon input from the access point and/or other sources.

The WRC resolution and the ratification of IEEE 802.11h form a powerful union. This combination bodes well for the success of 802.11a Wi-Fi products. In fact, the two events are likely to facilitate the spread of Wi-Fi from the 2.4-GHz band to the 5-GHz band in the upcoming year. Consequently, 802.11a and tri-mode (802.11a/b/g) products should become more prevalent in the future.

To date, regulations regarding the use of the 5-GHz spectrum have not been globally harmonized. This fact has hampered the development of a "world-mode" product. With different portions of the spectrum approved for WLAN use in different countries, manufacturers have been faced with two options: Either provide products that meet each individual country's regulatory requirements or do not provide 802.11a products at all.

With the 802.11h amendment to the 802.11a PHY and MAC layer, interference mitigation becomes a reality in the 5-GHz band. Manufacturers are now poised to implement one standards-based solution to address interference concerns around the world. This solution is vastly superior to implementing multiple technologies.

The combination of harmonized-spectrum allocations and mitigation measures will allow for greater economies of scale in 802.11a and dual-band products. These cost savings will be passed on to consumers, making Wi-Fi even more affordable. Many companies will likely produce a "world-mode" product that will cover the 5.15-to-5.25-GHz; 5.25-to-5.35-GHz; 5.47-to-5.725-GHz; and 5.725-to-5.825-GHz channels with appropriate 802.11h interference-mitigation techniques. Such a product would fulfill numerous countries' regulatory requirements. Business travelers, in particular, will soon benefit from having a product that is capable of working in multiple regulatory domains worldwide.

To date, Wi-Fi has been tremendously successful. Thanks to harmonized spectrum and standards-based interference-mitigation measures, that popularity is about to extend to 5 GHz.

Intel Corp.
200 Mission College Blvd., P.O. Box 58119, Santa Clara, CA 95052-8119; (408) 765-8080, www.intel.com.