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Deploying Point-to-Point Wireless Links: Page 2 of 8

• Effective isotropic radiated power (EIRP): Defines the gain of an antenna referenced to an ideal isotropic antenna, which would radiate equally in all directions.

• Fresnel zones: For a radio beam, the elliptical area immediately surrounding the visual line of sight. The zone's thickness varies depending on the length of the path and the frequency of the signal.

In multipoint systems, standards are critical because they drive cost points lower and allow multivendor equipment interoperability. But in the point-to-point segment, standards aren't quite so important because you'll typically deploy turnkey systems. And though vendors' offerings may be proprietary, the industry has enough competition to encourage innovation and aggressive pricing.

In today's market, purchasing a point-to-point system based on 802.11b may provide the lowest up-front cost, but you'll take a performance hit for the overhead that comes with adapting a LAN standard to the point-to-point market. Further, the technical features of these products may complicate your efforts to deliver rock-solid reliability.

Technical Points

The first point-to-point wireless systems we tested in our Real-World Labs® in 1995 (see "The Bridges of Wireless County,") gave us a feel for the challenges associated with field deployment of links requiring line of sight over distances of 3 miles. Much has changed in the past seven years, and redesigned systems are now a lot easier to install, though the requirement for line of sight between antennas remains. While you may enjoy initial success in deploying point-to-point wireless even if you don't understand the technical issues, you'll sacrifice long-term reliability. For example, we've seen point-to-point systems deployed using inappropriate antennas, and while they work on Day 1, their susceptibility to interference may cause them to fail later, probably when the installer is cruising the Caribbean.

So what are the core technical issues involved in engineering links that have very high reliability? First, you need to understand some of the basic physics associated with analog radio systems. Second, you need to be aware of the governmental regulations designed to facilitate the shared use of radio spectrum, particularly in the unlicensed 2.4-GHz and 5-GHz bands. And finally, you need to recognize the design trade-offs that vendors make when developing systems and the consequences for specific instal- lation scenarios.