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The Point-to-Point of Wireless Bridging
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November 15, 2002
By Dave Molta and Cornell Robinson III
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Wireless microwave communications systems have been used to transmit voice and data traffic for many years, but their entry in the data-internetworking market is more recent, spurred largely by the government's decision to allocate valuable portions of the electromagnetic spectrum for use with unlicensed devices. Of course, the appeal of unlicensed products is clear: Need to link two sites 5 miles apart by the end of business tomorrow? No problem.
Over the years, we've tested wireless point-to-point Ethernet bridges that operate in the 2.4-GHz frequency band. These products are still popular, offering reasonable performance at low cost. We've also reviewed 5-GHz point-to-point systems designed to replace T1 local loops. But this is the first time we've focused on 5-GHz Ethernet bridges, a market that is heating up and may eventually render 2.4-GHz products obsolete.
Why 5 GHz? First, these devices are less vulnerable to interference than those in the 2.4-GHz band, where wireless LANs, cordless phones and microwave ovens spew radio signals in all directions. Second, and closely related, 5 GHz offers more than three times as much bandwidth as 2.4 GHz. That extra bandwidth not only lets products run at higher data rates, it means added flexibility to deploy multiple systems at a single location and to move to alternate channels should interference occur. The downside? You may have to sacrifice a little range at 5 GHz, and products tend to be more expensive (see "A Spectrum of Choice").
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We cast a wide net for our tests, inviting a dozen vendors of wireless point-to-point systems to send their wares to our Syracuse (N.Y.) University Real-World Labs®. Some do not have 5-GHz units at all, while some did not have devices ready for test, but we got products from six big names: Airaya Corp., BitRage, Proxim, RadioLAN, Wi-LAN and Young Design.
Several of the products we tested had just been released. And, as is typical of many emerging technologies, the performance, range and overall capabilities varied considerably, making head-to-head comparisons challenging. If product A offers twice the performance of product B but at half the range and double the price, does that make product A better? The answer, of course, depends on your needs.
The Basics
the products we tested work as wireless Layer 2 bridges. In an age where Layer 3 switches and routers dominate the market, you may wonder why these products are still running at Layer 2. The positive spin is that with a point-to-point system, you don't really need the sophisticated traffic-management capabilities of Layer 3 devices. In addition, many point-to-point systems will be linked to backbone routers anyway. Nonetheless, we would like more Layer 3 control--say, to filter certain protocols to optimize performance.
Although all the systems we tested are unlicensed and therefore must adhere to rules defined by the FCC and other international regulatory bodies, the underlying radios and modulation systems varied considerably from product to product in our tests. In most cases, it's a trade-off between price and performance. Packing more bits into each clock cycle requires more sophisticated radio technology--and you'll pay for that luxury. Because regulations vary by sub-band at 5 GHz, you'll get longer range from products that operate in the 5.8-GHz UNII-3 sub-band. The chart, "FCC Regulations for Select Unlicensed Radio Bands", shows some of the key regulations in the most commonly used unlicensed bands. We expect to see increased availability of low-cost chipsets that operate in the UNII-3 band, a development that will push the price-performance ratio lower.
An interesting trend visible in some of these products involves integrating the antenna and modem into a single weatherproof enclosure. This makes installation easier and increases effective system range compared with older designs by eliminating RF cabling, which is a source of significant signal loss. An alternative yielding similar benefits is to collocate the modem and antenna on a single antenna mast; this strategy allows flexibility of antenna selection--and even more range--but is more complex.
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