I'd like to bring to your attention a tool that, if you are doing wireless stuff, you are going to want. The tool is called Wi-Spy 2.4x and is manufactured and sold by the wonderfully named MetaGeek.
The Wi-Spy is a USB (1.1 or 2) spectrum analyzer for the radio frequency range from 2400 MHz to 2483.5 MHz (usually known as the 2.4GHz band, used by Wi-Fi and Bluetooth) with a resolution of 328 KHz. It can detect signals in the range -110 dBm to -6.5 dBm with an amplitude resolution of 0.5 dBm and has a sweep time of 165 milliseconds.
In other words, this is a pretty snazzy piece of hardware, but two other features make the Wi-Spy outstanding. The first is its software, Chanalyzer, which runs on Windows 2000 or later with .Net 2.0 installed. Linux and Mac support are also available via third-party software.
Chanalyzer's user interface shows three graphs that share a common X-axis of frequency; you can select this axis to be displayed as frequency or as Wi-Fi channels or Zigbee channels (I plan to cover Zigbee in more depth in future).
The top graph is the Spectral View. This is a "waterfall graph" (that is, one that scrolls down over time) that plots the selected time period (you can select from 15 seconds to one hour) against the frequency range, with each point color-coded by the signal's amplitude. This highlights bandwidth use over time.
The graph below that is what MetaGeek calls the Topographic View. This display shows the popularity of the spectrum by plotting the percentage activity for each frequency and amplitude over the selected time period. In other words, it is a spectrum utilisation map.
The bottom graph is the Planar View, which plots amplitude against frequency and shows current, average and maximum amplitudes. There are also two markers that you can place on the frequency axis to get data for a specific frequency.
What this deluge of data shows you is, for a given location, how the spectrum is being used and abused. For example, you can see where devices such as microwave ovens, Bluetooth devices and cordless phones might be adding noise and reducing Wi-Fi throughput.
When Wi-Fi or Zigbee channels are selected you can click on one or more of their labels on the frequency axis to overlay a highlighted zone that shows the channel frequency limits. Wi-Fi channels are 5 MHz apart, but to ensure more or less complete isolation you much choose ones which are 25 MHz apart. This is particularly important where other people's nearby access points might be a problem - most people never change their access points' default use of channel 6, so to avoid overlap you should use channel 1 or 11.
Note that the European Union also allows the use of channels 12 and 13, and channel 14 is the only one allowed to be used for Wi-Fi in Japan.
The other outstanding feature is that you can save and replay your Wi-Spy spectrum captures and share them with other users. MetaGeek has a library of user contributions that shows what the graphs generated by devices such as microwave ovens, cellular phones, baby monitors and cordless phones look like - these are extremely useful when you are trying to track down your own Wi-Fi problems.
MetaGeek also offers a freeware rewrite of that old hacker favorite, NetStumbler (last updated in 2004), called Inssider. Wi-Spy 2.4x is, as I wrote, outstanding, and at US$399 an excellent value. Highly recommended.
With the 2.4GHz band getting ever more crowded, this device could pay for itself if you need to know what is causing interference and avoid it.