Channel Spacing
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.
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