The point-to-point market shares many technical challenges with multipoint, but the economics are much different--the aim is to save thousands of dollars per month on service charges as opposed to competing with $79 DSL. And unlike multipoint systems, point-to-point systems make radio interference issues manageable because you don't have hundreds or thousands of links communicating to the same base station.

Glossary • 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.

That Old Math Magic

For several years, we have referred to the "magic of physics" associated with wireless systems. Obviously, there's nothing magical about it, but the underlying complexity of radio engineering makes it difficult for nonexperts to grasp how hundreds of megabits of data can be pushed down an invisible pipe every second. Regardless, you'll need to understand some basic principles to install one of these systems.

First, there's frequency. Radio-wave behavior varies with changes in frequency. As you move to higher frequencies, the wavelengths become shorter, affecting their propagation characteristics. For example, low frequency AM radio waves (500 KHz to 1,500 KHz) can propagate for hundreds or thousands of miles, and adverse weather and physical obstructions have little effect. But at frequencies greater than 40 GHz, radio waves are readily absorbed by rain and fog. Most point-to-point systems are deployed at microwave frequencies, with operations in the unlicensed 2.4- and 5-GHz bands most common.

Closely related to frequency is the amount of bandwidth available for a specific link. This is a product of both the size of the RF channel (for example, 10 MHz) and the analog-to-digital conversion techniques employed. One key issue facing system designers is spectral efficiency, or the amount of data that can be packed into an RF channel. Since available spectrum is limited, it's obviously beneficial to use advanced signal modulation techniques to pack as many bits into a channel as possible. However, as spectral efficiency increases, so too does the cost of underlying components and the possibility of errors. It's a delicate balancing act. Most point-to-point systems can operate at multiple alternate channels within a specific frequency band, which can be important in mitigating interference.