After completing an RF site survey, you’ll have a good idea of the number and location of access points (APs) to provide adequate coverage and performance for users. Before installing the access points, however, be sure to determine what RF channels you plan to use. This will ensure that users will be able to roam throughout the facility and have the performance that they need.
802.11b/g RF channel basics
Direct communication between an 802.11 client radio and an access point occurs over a common channel frequency. You set the channel in the access point, and the radio card automatically tunes its transceiver to the frequency of the access point having the strongest signal. The radio card then continues with association and communications with the chosen AP.
To support roaming, the client radio periodically scans all access points and reassociates with the access point having the strongest signal (if the current access point signal amplitude is below a specific threshold). As a design rule, access points within range of each other should be set to channel frequencies with minimal signal overlap. Otherwise, you’ll find that roaming doesn’t work well, and performance will degrade because of interference between APs.
The 802.11b/g standards define a total of 14 frequency channels within the 2.4 GHz band. The FCC allows channels 1 through 11 within the U.S.; whereas, most of Europe can use channels 1 through 13. In Japan, all 14 channels are available. The total 2.4 GHz spectrum width is roughly 90 MHz.
An important concept to note regarding channel assignments is that the channel actually represents the center frequency that the transceiver within the client radio and access point uses (e.g., 2.412 GHz for channel 1 and 2.417 GHz for channel 2). There is only 5 MHz separation between the center frequencies, and an 802.11b signal occupies approximately 30 MHz of the frequency spectrum. The signal falls within about 15 MHz on each side of the center frequency. Consequently, an 802.11b/g signal overlaps with several adjacent channel frequencies. This leaves you with only three channels (channels 1, 6, and 11 for the U.S.) available for use without causing interference between access points (see figure below).
Jim Geier is an independent consultant and author with 30 years experience planning, designing, analyzing and implementing communications systems, wireless networks, and mobile devices. Jim is the author of over a dozen books on mobile and wireless topics, such as including Designing and Deploying 802.11ac Wireless Networks (Cisco Press), Implementing 802.1X Security Solutions (Wiley), Wireless Networking Handbook (New Riders) and Network Re-engineering (McGraw-Hill). He has been an active participant within IEEE standards organizations, such as the IEEE 802.11 Working Group, and the Wi-Fi Alliance. He has served as Chairman of the IEEE Computer Society, Dayton Section, and various conferences. He has served as a testifying expert for patent litigation cases focusing on technologies dealing with Wi-Fi, Bluetooth, mobile devices, cellular systems, wireless network protocols, network security mechanisms, location systems, and application data management protocols. Jim’s education includes bachelor’s and master’s degrees in electrical engineering, with emphasis in mobile devices / systems and wireless communications.