In the 802.11b specification (in the United States), the 2.4GHz spectrum is broken into 11 overlapping channels. Ideally, as you add access points to your network, you want to allow your coverage areas (or cells) to overlap slightly, so there are no gaps in coverage. Wherever possible, you should keep a spacing of at least 25MHz (or 5 channels) in adjacent cells, as shown in Figure 4-2. Otherwise, traffic on nearby APs can interfere with each other, degrading performance.
For example, you may use channels 1, 6, and 11 in an alternating pattern to provide complete coverage without any frequency overlap. Of course, everyone else using 802.11b is trying to do the same thing, and they will probably be using one of these channels. Especially in a crowded area, perfect 25MHz spacing may be impossible. If necessary, you may be able to get away with spacing as close as two or three channels, but don't ever try to run two adjacent networks on the same channel (things may look fine at first, but will fall apart as the network load increases).
To figure out what channels your neighbors use, take a look at your signal strength meter and the other tools that your wireless card came with (the Orinoco/Agere/Proxim card, for example, ships with an excellent Site Map utility). You can also use NetStumbler, an excellent network discovery tool for Microsoft Windows. You can get it online for free at http://www.netstumbler.com/. I'll describe more tools that you can use for site surveys in Chapter 7.