To help explain why spectrum analyzers are needed, it would be best to first define Radio Frequency Interference (RFI). RFI is produced by a RF generating source on the same frequency or nearby frequencies (when considering 802.11 spread spectrum technology) that erodes the quality of another RF communications link. For example, two 802.11 wireless networks that share the same channel and coverage area would consider each other RFI. Some inadvertent examples of RFI could be a defective fluorescent light fixture that is emitting RFI and happens to be located right next to an 802.11 access point. Another example would be RFI emitted from a poorly sealed microwave oven which affects the same 802.11 access point, but only randomly. The examples are numerous and whether the source is emitting RF by design or not, it's still RFI to the wireless device that is being interfered with.It quickly becomes apparent that RFI should be one of many considerations during site surveys, network setup, and whenever the wireless network is not operating normally. By doing so, the administrator then has a complete understanding of the RF environment, which allows for knowledgeable decisions and solutions.
How is RFI located and measured? By using a spectrum analyzer, it's the only tool that gives the administrator a clear idea as to what's happening on the RF band being tested. Spectrum analyzers are also helpful in determining if, how many, and what other channels are being utilized by foreign wireless networks.
So if it has the potential to be such a useful tool, why doesn't everyone have one already? It's not that much of a puzzler—most spectrum analyzers are very expensive and purchasing one is hard to justify with tight IT budgets. Fortunately about two years ago a company called MetaGeek.net developed the Wi-Spy spectrum analyzer with a price tag that was thousands of dollars less than other analyzers. To maintain the low price, many of the features seen in the high-end analyzers are not included. But Wi-Spy is still capable enough to alert wireless network administrators of any RFI conditions.
MetaGeek.net has recently released a new spectrum analyzer called Wi-Spy2.4x. Tim Higgins published an excellent review of the new Wi-Spy2.4x, pointing out all of the improvements. The review also compares the Wi-Spy2.4x to the original Wi-Spy and the Cognio Spectrum Expert which is considered a high end spectrum analyzer. As stated in the review either of the Wi-Spy devices would be a welcome addition to the tool kit of an 802.11b/g wireless network administrator.
Like most networking tools, the Wi-Spy device has a learning curve. To help with this the MetaGeek.net website includes support for the application software. Especially useful is the web page containing actual recordings of known RF generators.