On the A/P you can nominate which channel the A/P broadcasts on. The adapters in your computers tune in to that frequency.

802.11b has 11 (& boxes that are 802.11g & backward compatible to b do same)
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#3 according to D-Link website, 802.11g uses 12 chans & these get bonded in pairs (thus down to 6 channels) when used for high-speed comms (54mbs & 108mbs)
My current 802.11g/b is a Netgear WGR614 that can do 54mbs & have ordered a D-Link 802.11g/b that can do 108mbs, also have a few 802.11a units that can do 108mbs)
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The frequecies of these channels vary between world regions so each modern A/P allows you to set the region & if you want, the base channel to operate on. Each channel frequency is only slightly different from the one below it and they overlap. Most 802.11b will use channel 11 by default 802.11g will channel 3 by default. These are well apart.

If (as we do at work) you have multiple A/P units and want roaming, then to prevent adjacent A/P interference we stagger the base channel choice in the A/Ps so that adjacent A/Ps are 5 channels apart. This provides minimal interference as our workers roam the warehouses with their 802.11b units.

The only other point (that am sure you know) is that at 2.4Ghz, the 802.11b & g units are vulnerable to interference from wireless mice, keyboards, bluetooth & many microwave ovens.

Doug

PS 802.11a has 13 channels & will get at least 11 more.

High speed WiFi is achieved by using two channels concurrently (e.g. 108 Mbs). Both the A/P & the adapter need to support the mechanism for using the 2 chans & this tends to vary among brands.

#2 #3 #4 minor corrections additions