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Eliminating interference thru Wi-Fi spectrum analysis

As Wi-Fi matures from convenient, as-needed access to full-time support for mission critical applications, companies are becoming increasingly concerned about optimizing performance and reducing radio interference.

 

As Wi-Fi matures from convenient, as-needed access to full-time support for mission critical applications, companies are becoming increasingly concerned about optimizing performance and reducing radio interference.

WLAN traffic analyzers provide 802.11 packet capture, decode, and expert analysis to help you fine-tune AP parameters, position APs to minimize cross-channel interference, detect Wi-Fi coverage gaps, etc. But traffic analysis can take you only so far. For example, you might determine that certain 802.11 stations are experiencing high error rates due to noise. But what is the source of that interference, and how can you eliminate it? This is where RF spectrum analysis comes in.

Busy skies, noisy neighbors

The unlicensed radio frequencies utilized by 802.11a, b, and g are both a blessing and a curse. The blessing? You don't need permission to install Wi-Fi APs, configured to send and receive traffic on defined channels in the 2.4 GHz ISM and 5 GHz UNII bands. The curse? You don't have exclusive use of those channels. This radio frequency spectrum is used by many other devices, including Bluetooth, cordless phones, wireless surveillance cameras, baby monitors, garage door openers, microwave ovens, and other WLANs. RF interference is already common, and destined to grow along with wireless adoption.

Unlike traffic analyzers that operate at the data link layer, RF spectrum analyzers examine the underlying physical medium. They measure the power (amplitude) of radio signals and RF pulses, transform those measurements into numeric sequences, and employ statistical analysis to plot spectral usage, quantify "air quality," and isolate transmission sources.

RF spectrum analyzers are not at all unique to 802.11 – they have long been used by radio network engineers during equipment design and product testing, and by RF technicians who install and maintain 2G and 3G wireless networks. Some examples of traditional spectrum analyzers that cover the frequencies used by 802.11 include:

Traditional spectrum analyzers are purpose-built test equipment, housed in computer enclosures, carrying cases, or "luggable" tablets. Given the target market, these products are sophisticated, designed to supply highly-detailed technical information to RF experts. While essential for Wireless ISPs, manufacturers, and other organizations with on-staff radio engineers, they can be less approachable for consumers seeking simple tools to provide RF visibility at a level not available from 802.11 traffic analyzers.

Mobile Wi-Fi spectrum analyzers

Recently, three new products have emerged to help satisfy this need, delivering Wi-Fi spectrum analysis from off-the-shelf laptops and PDAs:

BumbleBee is an addition to BV's extensive wireless test equipment portfolio; related products include BV's Yellowjacket handheld 802.11 analyzer. ASA complements AirMagnet's well-known suite of WLAN analysis and intrusion prevention products; for example, see this SearchMobileComputing.com review of AirMagnet Laptop and Handheld. ASA uses Cognio's Mobile PC card, which Cognio recently added to its Intelligent Spectrum Management System, an enterprise-class distributed RF management platform.

This products differ from traditional spectrum analyzers in form factor and target user. While small businesses may still find prices a bit stiff, mid-to-large companies with serious Wi-Fi applications should take a look at these tools, and others that seem certain to follow.

What can you expect from a mobile Wi-Fi spectrum analyzer?

  • You'll be able to monitor transmissions in the radio frequency bands used by 802.11b/g (2.4 GHz) and 802.11a (5 GHz) and others wireless applications (900 MHz RFID on BumbleBee, 4.9 GHz Public Safety on Cognio/AirMagnet).

     

  • You'll see real-time graphic display of spectral usage, letting you visualize how other transmission sources are competing for Wi-Fi channels.

     

  • You'll be able to concretely measure "air quality." For example, ASA plots average RF power, maximum RF power, interferer power, and duty cycles (percentage of time signal was present). This information can be useful for both site planning and performance trouble-shooting.

     

  • You'll be able to identify sources of interference (e.g., Bluetooth, ovens), and be armed with a mobile tool that can help you physically track down that source.

What do you need to support these tools? Cognio/AirMagnet combines Windows XP software with a Type II PC card and a clip-on 2.4/5 GHz omni antenna, so you'll need an ordinary laptop with an open Cardbus slot. BumbleBee combines Pocket PC 2003 software with a "jacket" that slides onto the back of an iPAQ 4700 or 2200. The jacket's wireless receiver connects to the iPAQ's CF slot and a 900 MHz, 2.4 GHz, or 5 GHz omni antenna (included). Both products can be coupled with optional directional antennas, which are useful when trying to locate an interference source.

Eliminating interference

Wi-Fi spectrum analyzers can be used in many ways that can't be fully appreciated by those of us who are not RF engineers. But you don't have to be an RF engineer to make productive use of these tools. Consider one common task: identifying and avoiding interference between your WLAN and devices competing for the same frequencies.

If you suspect RF interference, turn off the affected AP or station, then use one of these tools to measure other nearby radio transmissions. By eyeballing real-time graphics, you'll be able to see whether any device is transmitting within a given frequency range -- with ASA, consult Spectrum Plots and Channel Summary views; with BumbleBee, consult the Spectrum Waveform display.

Over time, WLAN administrators may become familiar with RF patterns employed by other technologies (i.e., what microwave interference looks like, what Bluetooth looks like.) The Cognio/AirMagnet tool simplifies this task, first by enumerating non-802.11 transmission sources (Interferers List), then by matching those transmissions against signatures to guess ("fingerprint") the type of device.

Having confirmed that interference exists, you'll want to take steps to eliminate it. One possibility is to reconfigure your WLAN to use another band or channel(s) that don't overlap with known interference. If this is not possible or practical, then try to remove or shield the interference source.

For example, use the ASA Device Finder tool to watch a continuous plot of signal strength for a given Interferer, moving in the direction of increasing power until you find the device. This is not always as easy as it sounds when interference is intermittent or narrowband, but a systematic search can help you achieve your goal. AirMagnet recommends dividing a search area into quadrants, measuring power in each quadrant. Repeat that process in the quadrant with the strongest signal, iterating until the device is found. Omni antennas can be used for this task, but this is where direction-finding antennas can help to speed your search.

Conclusion

To learn more about Wi-Fi Spectrum Analyzers and how they can help companies faced with trouble-shooting and optimizing business WLAN installations, consult product documentation at the URLs given above, this Base Station Earth Station article (PDF), and this ASA review by Tom's Networking. Happy Hunting...

 


Read about Lisa

About the author: Lisa Phifer is vice president of Core Competence Inc., a consulting firm specializing in network security and management technology. Phifer has been involved in the design, implementation, and evaluation of data communications, internetworking, security, and network management products for nearly 20 years. She teaches about wireless LANs and virtual private networking at industry conferences and has written extensively about network infrastructure and security technologies for numerous publications. She is also a site expert to SearchMobileComputing.com and SearchNetworking.com.

This was last published in July 2005

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