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Originally published at Internet.com
A picture, they say, is worth a thousand words - and if you're a network administrator or an RF engineer charged with managing a wireless network in a hostile environment, the right images from a spectrum analyzer could speak volumes.
I'll admit it: I don't have the typical home environment. I have two 5.8 GHz cordless phones, each with three handsets, as well as a 2.4 GHz cordless phone with two handsets. When I survey the 2.4 GHz spectrum prior to testing wireless products, I see as many as 10 APs. Who knows what else is out there. This seemed like the perfect test environment to check out AirMagnet's Spectrum Analyzer 2.0.
Priced at $3,995, this is clearly a tool aimed at enterprises that must maintain a highly reliable wireless network. It may seem hard to justify that price, but if your wireless network slows to a crawl because of interference you can't identify, the loss of productivity could easily offset the cost.
Installation of the AirMagnet Spectrum Analyzer is quite simple. You install the software, then insert the bundled dual band PC card into your notebook. The card can locate interference on all 14 channels (only 1-11 are legal in the US) in the 2.4 GHz radio band used by 802.11b/g, and the channels in the 5.150-5.350 and 5.725-5.850 GHz bands used by 802.11a.
In order to detect Wi-Fi signals, you also need a Wi-Fi card - preferably one on which you can disable the transmitter. For my tests, I used an IBM Thinkpad T40, and enabled the embedded wireless. I set the preferences to associate to a non-existent AP so that its transmissions would be limited.
Since my editor doesn't allow me an infinite number of words, I thought I would include a number of screen shots along with explanations of the test scenarios for each. And if a picture is truly worth a thousand words, well, let's hope that my editor pays me by the word... [Editor's Note: I didn't.]
If you're a user of AirMagnet's Spectrum Analyzer 1.0, you might find some of the features in 2.0, such as session record and playback, compelling enough for an upgrade. You can record, play, open, save and pause session files. Version 2.0 also includes new device classifiers, including 5 GHz microwave ovens, European Bluetooth devices, continuous FSK, OFDM, QAM, FM and CW devices, as well as burst FSK, FSK-FHSS, OFDM, QAM and QAM-FHSS. It can also differentiate between analog video, CW and FM. To help you analyze data, new charts include devices vs. channel, devices vs. time, duty cycle vs. channel and duty cycle vs time.
When you launch AirMagnet's Spectrum Analyzer, the screen says powered by Cognio. Spectrum Analyzer 2.0 is based on Cognio's Spectrum Expert 3.0. The difference in the AirMagnet version is that it integrates with AirMagnet Survey 3.0. This provides real-time FFT (Fast Fourier Transform) visibility, a spectrum snapshot for each data point, and spectrum reports when surveying with Survey 3.0.
The opening screen presents you with a default grouping of windows. While this view is quite useful, you can configure the program to display exactly the windows and charts that you want, and you can save multiple configurations for future use.
Opening Screen
On the left side of the screen is a box that shows active devices, and, if you have a Wi-Fi card in receive mode, Wi-Fi APs. Here, you can see three cordless phones and seven APs. Below that box is the control panel that controls frequencies displayed and amplitude scaling.
The top left corner graph is the Real Time FFT plot. This shows power, expressed in dBm as a functiion of frequency measured during the period of one frequency sweep. Typically, sweep time is one second. Note that the channel edges for the non-overlapping 2.4 GHz Wi-Fi channels - 1, 6 and 11 - are shown in dotted blue lines for reference.
The graph to the right of the Real Time FFT plot is the FFT duty cycle. This shows the percentage of time that the power, from both 802.11 and interfering devices, is 20dB or more above the noise floor. Again, the horizontal axis is the bandwidth being monitored. Like the real time FFT plot, channel edge markers for channels 1, 6 and 11 are shown. You'll see in other screen shots how the FFT duty cycle changes due to either 802.11 traffic or interference.
Below the Real Time FFT plot is the Swept Spectrogram plot. This plot actually combines the data shown in the first two graphs, and allows you to see power in the RF spectrum as a function of frequency, measured over time. Typically, each sweep line is one second, so this view shows you about three minutes of history. Signal strength is indicated by color. Blue is -112 dBm (noise) and red is -42 dBm (very strong signal).
The last chart is a stacked bar chart showing devices per channel.
AirMagnet Spectrum Analyzer 2.0 has five tabs that run horizontally across the top of the interface. The first tab shows your configured screen for the 2.4 GHz spectrum. The next tab shows your configured screens for the 5 GHz spectrum.
5 GHz Spectrum
Here, I chose only to view the FFT Duty Cycle, the Swept Spectogram and the Real Time FFT plots. Note the fairly clean environment, with most power levels down in the noise.
It's easy to add or remove charts or plots to the main screen. Just right-click on the screen, and the dialogue box below pops up with individual options.
Add Plot Dialogue
Add Chart Dialogue
The third tab gives you a list of devices. In this view, devices are identified and grouped together. In the screen shot below, note how the cordless phones are grouped together. To generate this screen, I made a phone call from a 2.4 GHz phone to a 5 GHz phone. As you can see from the fairly strong signals, both of the base stations and handsets were in the same room as the Spectrum Analyzer.
Device Tab
I've always heard that microwave ovens cause interference in the 2.4 GHz band, but never really had a way to test it. Now, with AirMagnet's Spectrum Analyzer 2.0, I could. For the microwave test, I took the Spectrum Analyzer notebook and placed it in the center of the kitchen, about eight feet away from the microwave oven. I put a plastic bowl with about four cups of water into the oven (to provide a load for the microwave), and turned it on high. As you can see from the screenshot below, the microwave oven was clearly identified by the Spectrum Analyzer.
Device tab showing microwave oven
A right-click on the device brought up detailed information about the source of the interference.
Microwave detail
Note the relatively strong signal (-51.2 dBm) and the affected channels (8-11).
Author: Craig Ellison
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