The Bridges of Wireless County

Crystal Radios And Rabbit Ears?

by Dave Molta

Is installing a radio-based bridge a do-it-yourself operation? Are these systems secure? Can you depend on these systems to provide reliable transmission day in and day out? These are questions we had before our testing and, while we can't answer all of them authoritatively, we can provide some insights.

All of the products we tested were based on spread-spectrum radio technology operating in the 900-MHz, 2.4-GHz and 5.7-GHz frequency spectrums designated by the Federal Communications Commission for unlicensed operation. In essence, you can install and operate these units without help from anyone.

Deciding whether you should do that, however, will depend on your willingness to take some risks. All of the vendors provide ample warnings of the potential dangers associated with installing antennas. Yet, while installing such a system is a bit more complicated than programming your VCR, many organizations will find it to be relatively easy.

The two major components you'll be dealing with are the bridges and antennas. When the distance between buildings is measured in hundreds of feet, you may be able to get away with low-cost omnidirectional antennas. You also may be able to install antennas indoors, perhaps inside an office pointing out a window.

One of the key decision points relates to the placement of the bridge. Ideally, you'll want the device in an environmentally appropriate location and you'll need to be able to get to it with an Ethernet connection. On ce you configure the bridge, you shouldn't need frequent physical access to it; all of the units support some form of in-band management. Cabling distance between the antenna and the bridge may be an issue, since long antenna-cable runs typically require thick coaxial cable. All of the vendors should be able to help you calculate maximum cable lengths, which vary depending on the product and the distance between the antennas.

Originally developed by the defense department for battlefield applications, spread-spectrum radio technology is both reliable and secure. Unlike narrow-band microwave systems, which are currently available but are expensive, spread-spectrum radio systems are easier to design and manufacture and are less likely to cause interference problems, even if improperly installed.

Spread-spectrum systems transmit at very low power levels, typically less than one watt, and spread the signals over a wide frequency spectrum.

All of the systems we tested use a data-encoding technique known as direct sequencing. By using a unique "chipping code" known only to the other radio, a pair of bridges can communicate in a relatively secure manner. Since the signal levels are so low, multiple systems can be used in a given geographic area without much danger of interference. For those who are particularly paranoid, encryption options also are available on several of these units.

As appealing as this technology is, it has potential problems. First, to span significant distances, specialized directional antennas must be employed. In general, there is a relationship between performance, distance and the size of an antenna. We were able to connect our Persoft bridges using a 12-inch Yagi-style antenna.

The CYLINK Corp. radio, however, which has a rated distance of 20 miles, used a parabolic antenna that was much bulkier (CYLINK has smaller antennas available for short distances). WinData's antennas look more like standard microwave dishes, probably not the kind of system an amateur would i nstall.

Although point-to-point configurations are probably the most popular use of these devices, all of these units can be configured for multipoint operation. Be forewarned, however, that overall bandwidth may be limited, particularly on 900-MHz radios, so proper antenna aiming is important. Although the antennas are directional, interference from adjacent units is possible. If you are contemplating the installation of multiple wireless links in close proximity to one another, you'd be well advised to discuss your specific site requirements with an expert.

Finally, concerning overall reliability of these systems, we can offer only limited guidance since we tested the long-distance connection over a relatively short period of time. We have, however, been running a 900-MHz bridge in production at Syracuse University for several years without a single notable disruption in service. Adverse weather does not have a significant effect on reliability, but interference from other sources of radio waves, including long-distance cordless phones operating at 900 MHz and microwave ovens running at 2.4 GHz, may present more serious threats to system reliability.

October 15, 1995