How to use an RF site survey for WLAN installations

Wireless LAN installation at client sites is tricky. You can't visualize the wireless medium as you can with wired networks. The construction of a facility and silent sources of radio frequency (RF) interference impact the propagation of radio waves, often in odd ways. This hinders your ability to plan the location of access points for your clients.

How do you avoid these drawbacks? Perform an RF site survey using appropriate tools that help you plan access point locations for adequate coverage and resiliency to potential RF interference. Let's look at the types of tools you have at your disposal.

Basic tools for an RF site survey

The traditional method of performing an RF site survey includes a laptop equipped with an applicable 802.11 PC card (802.11a, 802.11g or 802.11n) and site survey software supplied at no additional cost from the radio card vendor. The software features vary greatly by vendor, but they all display the strength and quality of the signal emanating from the access point. This helps determine effective operating range (i.e., coverage area) between end users and access points.

For example, after "best guessing" the potential position of access points for adequate coverage and overlap, you verify your thoughts by placing an access point at each location, and then walk around with the laptop while monitoring your ability to connect to the network and utilize intended applications. The goal is to verify the maximum distances that will maintain adequate usage of the applications. If the predetermined location of an access point doesn't provide the coverage you had in mind, then reposition or include additional access points and repeat the testing.

This relatively inexpensive site survey method has some drawbacks. For one, it's physically demanding to lug a laptop around a building all day when doing...

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the testing. You can ease this problem, though, by using an 802.11 CompactFlash card along with a handheld PC device. This reduces the physical demands of performing the tests, but you'll be lacking a significant capability: the detection of RF interference between access points and from other RF sources, such as Bluetooth devices, microwave ovens and wireless phones.

For one-time installations, especially smaller facilities, you could also get by with the free vendor-supplied software for performing the signal coverage testing. You can relocate the limited number of access points easily enough until everything works adequately.

Advanced tools for an RF site survey

Advanced 802.11 site survey tools, such as those from AirMagnet and Ekahau, provide the "eyes" and "ears" that let you understand the effects of the environment on the transmission of 802.11 signals. For example, the tools have built-in spectrum analyzers that graphically illustrate the amplitude of all signals falling within a chosen frequency band. This enables you to distinguish 802.11 signals from other RF sources that may cause interference, making it possible to locate and eliminate the source of interference or use additional access points to resolve the problem. In addition, the tools allow you to produce signal coverage maps that graphically portray where users should be able to successfully utilize various applications. AirMagnet Survey, for example, allows you to perform the RF site survey and then later simulate making changes to the positioning of access points to see the corresponding changes in signal coverage.

Because of the higher cost (ranging from several hundred to several thousand dollars) of advanced tools, you should only consider them if installing multiple wireless LANs or if the wireless LAN environment is complex. Warehouses with lots of high metal racks and manufacturing plants full of machinery will wreak havoc on radio waves. In these cases, you'll find it easy to justify the additional cost of using advanced tools.

About the author
Jim Geier is principal consultant of Wireless-Nets Ltd. and assists companies with the design, implementation and testing of wireless LANs. Jim is author of over a dozen books, including Deploying Voice over Wireless LANs (Cisco Press) and Implementing 802.1X Security Solutions (Wiley).