By Jim Geier
January 11, 2002
Proper site survey tools can be crucial when deploying anything other than the smallest WLAN. Knowing what those tools might include can save time and money, especially in 802.11b networks prone to radio frequency (RF) interference from microwaves, wireless phones, and Bluetooth.
Wireless LAN installation is tricky. Unlike wired networks, you can’t visualize the wireless medium. The construction of a facility and silent sources of RF interference impact the propagation of radio waves, often in odd ways. This hinders your ability to plan the location of access points.
How do you avoid these drawbacks? Perform an RF site survey using appropriate site survey 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 everyone
The traditional method for performing an RF site survey includes a laptop equipped with an 802.11 PC Card and site survey software supplied at no additional cost from the radio card vendor. The software features vary greatly by vendor, but a common function among them all displays 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 and noting signal levels. The goal is to verify the maximum distances that will maintain adequate signal levels, generally the value that continues to enable operation at the planned data rate (e.g., 11 Mbps). 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 tool has some drawbacks. For one, it’s physically demanding to lug a laptop around a building all day when doing the testing. You can ease this problem, though, by using one of the recently released 802.11 CompactFlash cards along with a pocket PC device, such as the Compaq iPAQ, Casio Cassiopeia, or HP Jornada. 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.
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For one-time installations, especially smaller facilities, you should get by with the free vendor-supplied software. You can relocate the limited number of access points easy enough until everything works okay.
Advanced tools let you “see” wireless LANs
Advanced 802.11 site survey tools include spectrum analysis, providing the “eyes” and “ears” that let you understand the affects of the environment on the transmission of 802.11 signals. For example, an 802.11b spectrum analyzer graphically illustrates the amplitude of all signals falling within a chosen 22 MHz channel. 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.
Another key spectrum analysis feature is the monitoring of channel usage and overlap. 802.11b limits up to three access points to operate in the same general area without interference and corresponding performance impacts, causing difficulties when planning the location and assignment of channels in large networks. Spectrum analysis displays these channels, enabling you to make better decisions on locating and assigning channels to access points.
A handful of test equipment companies are currently developing advanced site survey tools – Berkeley Varitronics Systems (Metuchen, New Jersey) and Softbit (Oulu, Finland) now have products on the market. Softbit’s TriCycle. is software that installs on a laptop equipped with a radio card and provides a very useful display of nearby access points, association status, signal levels, and has the ability to display coverage areas. This product doesn’t get you away from lugging a laptop around, but it certainly has features that decrease the time and increase the accuracy of performing site surveys. Berkeley Varitronics Systems’ Grasshopper. has fewer graphical features, but it is a small handheld device weighing only 3 lbs.
Because of the higher cost (up to several thousand dollars) of advanced tools, you should only consider them if installing multiple wireless LANs or 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 warrant the additional costs of using advanced tools.
Stay tuned, next time we’ll focus on network analyzers that assist with troubleshooting 802.11 wireless LANs.
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