Wi-Spy spies on Wi-Fi

by Mark Gibbs

January 3, 2008 —

While we continue to investigate our problem with deferred procedure calls
(see last week's Gearhead) -- a problem that seems to have mysteriously vanished
again -- we wanted 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 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 a future
Gearhead column -- for now, see this Network World story).

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 utilization 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 they need to be 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 $399 an excellent value. Highly
recommended.

Network World