Building Wireless Community Networks, Second Edition

802.11b Channels and Interference

The IEEE 802.11b specification details 11 possible overlapping frequencies^[3] on which communications can take place. Much like the different channels on a cordless phone, changing the channel can help eliminate noise that degrades network performance and can even allow multiple networks to coexist in the same physical space without interfering with each other.

Rather than attempting to set up a single central access point with a high-gain omnidirectional antenna, you will probably find it more effective to set up several low-range, overlapping cells. If you use access point hardware, and all of the APs are connected to the same physical network segment, users can even roam seamlessly between cells.

This spectrum’s 11 overlapping channels are shown in Table 2-1.

Table 2-1. 802.11b channel frequencies

Channel	Center frequency (GHz)
1	2.412
2	2.417
3	2.422
4	2.427
5	2.432
6	2.437
7	2.442
8	2.447
9	2.452
10	2.457
11	2.462

The channels actually use 22MHz of signal bandwidth, so adjacent radios will need to be separated by at least five channels to see zero overlap. For example, channels 1, 6, and 11 have no overlap. Neither do 2 and 7, 3 and 8, 4 and 9, or 5 and 10. While you will ideally want to use non-overlapping channels for your access points, in a crowded setting (such as a city apartment building or office park) this is becoming less of an option. It is possible, although not ideal, to use channels that are closer together. ...