Why do only three channels not overlap?

Q: On an 802.11b access point with a total of 11 channels, why is it that only three are non-overlapping? What exactly does that mean? -- Scott, Va.

A: 802.11b defines a general channel numbering scheme that allows the standard to be used in different locations with different radio band definitions. To accommodate these differences, 802.11b defines channels (actually channel centers, known as "fc" to the engineers) to occur every 5 MHz, beginning at one half of the bandwidth of the occupied channel above the start of the 2.4 GHz ISM band, plus a little bit of a guard band.

Direct sequence spread spectrum (DSSS) is the mechanism used by 802.11b radios to transmit data, and it requires a rather large amount of bandwidth. 802.11b transmits 11 spreading “chips” for each bit of data, and occupies 22 MHz: 11 MHz above the channel center and 11 MHz below. To meet the regulations for out-of-band emissions below 2.4 GHz, 802.11b includes a 1 MHz guard band at the bottom of the band. This now gives us enough information to figure out where each of the channels is defined.

Starting at 2400 MHz (2.4 GHz), we add 1 MHz for the guard band and 11 MHz for half of the occupied signal bandwidth. The result is that 802.11b channel 1 sits at 2412 MHz. An 802.11b transmitter set for channel 1 sends a signal from 2401 MHz to 2423 MHz. Any other 802.11b signal that overlaps any part of the signal on channel 1 results in some degree of interference. The larger the signal overlap, the greater the interference. 

To completely avoid overlap, the lower edge of the next 802.11b signal must not dip below 2423 MHz. With 802.11b channels every 5 MHz above 2412 MHz, the next channel that results in no overlap is channel 5, at 2437 MHz, which occupies the band from 2426 MHz to 2448 MHz. Above channel 5, channel 9 is the next channel without overlap. However, the choice of channels 1, 5, and 9 leaves 10 MHz of band unused. So, the channels are spread out to channels 1, 6 and 11, to completely utilize the band that is available in the U.S.

The non-overlapping channels are the best choices when planning an 802.11b deployment with three or more adjacent access points. This will minimize the interference between each of the access points and the mobile devices associated with those access points. For small deployments of one or two access points, you can use any two channels that are four or more channels apart. 

Whatever channels you use, be sure to check that your neighbor does not have his own 802.11b system that can interfere with your APs, since this will affect the way that you assign channels within your network. To avoid having to worry about this, look for a wireless LAN system with built-in dynamic channel assignment capabilities, which keeps things running smoothly with minimal hands-on intervention.

Learn more about this topic

Wireless LAN throughput on the rise

A look at proposals to boost wireless bandwidth to at least 100M bit/sec. Network World, 09/26/03.

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