Assigning 802.11b Access Point Channels

By Jim Geier

February 11, 2002

The 802.11b standard gives us 14 channels to work with, right? Wrong! Sorry, it's really only three useful ones. Calm down - we explain it all right here.

After completing a RF site survey, you'll have a good idea of the number and location of access points necessary to provide adequate coverage and performance for users. Before installing the access points, however, be sure to determine what channel frequencies 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 channel basics
Direct communication between an 802.11 radio card 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 access point.

To support roaming, the radio card 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 overlap. Otherwise, you'll find that roaming doesn't work well, and performance will degrade because of interference between access points.

What channels are available? The 802.11b standard defines a total of 14 frequency channels. The FCC allows channels 1 through 11 within the U.S.; whereas, most of Europe can use channels 1 through 13. In Japan, you have only one choice: channel 14.

An important concept to note regarding channel assignments is that the channel actually represents the center frequency that the transceiver within the 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 of each side of the center frequency. As a result, an 802.11b signal overlaps with several adjacent channel frequencies. This leaves you with only three channels (channels 1, 6, and 11 for the U.S.) that you can use without causing interference between access points.

Channel assignment tips
To effectively assign access point channels, start by properly planning the location of access points. It's important to have enough access points to provide adequate signal coverage throughout the facility, but don't overdo it. Make sure that access points are far enough apart so that you'll be able to assign non-overlapping channels (i.e., 1, 6, and 11 in the U.S.) to access points that are within range of each other. As a result, it's crucial to perform a RF site survey before assigning access point channels.

Your life will be easy for smaller networks. For wireless LANs with only one access point, just set the access point to any one of the channels. In fact, the default setting shipped by the vendor will work fine in most cases. If there are two or three access points, assign any combination of channels 1, 6, and 11. This keeps the signals far enough apart in the RF spectrum to avoid problems.

For wireless LANs having greater than three access points, do some up-front planning. With single floor facilities, the access points will be spread relatively far apart, making channel assignment relatively straightforward. You can easily use a drawing to identify the position of your access points in relation to each other and assign each access point a channel. The key with larger networks is to make certain channels are assigned in a way that minimizes the overlap of signals. Just ensure that all access points within range of each other, about 300 feet or so apart, are set to channels that don't overlap (i.e., 1, 6 and 11).

If access points will be located on multiple floors, then you'll have the more difficult task of thinking three-dimensionally. You can approach this problem the same way as the single floor application, but take into consideration access points on adjacent floors. Radio waves will go through the floor and ceilings, so access points located near each other on different floors need to be set to non-overlapping channels also.

The most difficult channel assignment problem is when a different company next door has a wireless LAN. When sharing a building with other occupants, consider getting to know your neighbors and plan the use of access point channels together. If it's not possible to coexist with other 802.11b wireless LANs, then you may have a good reason to base your network on 802.11a.

Stay tuned! Next time, we'll discuss public wireless LAN technologies.

Author Biography: Geier provides independent consulting services to companies developing and deploying wireless networks. He is the author of the book, Wireless LANs (2nd Edition), and regularly instructs workshops on wireless LANs.



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