Gain in point-to-point systems is a product both of radio output power and of an antenna's ability to focus that power. All other things being equal, higher gain systems are more difficult to build. Because all RF signals experience predictable loss over distance, you might logically assume those that operate over longer distances will need more gain. However, transmission is only one side of the coin. A system's receive sensitivity is equally critical. And as you probably guessed, more sensitive receivers are most costly to build.
In deploying a point-to-point system, you'll often be working with a loss budget. Loss in RF systems begins at the output of the radio transmitter and continues through to input of the receiver. Free space loss, also known as path loss, occurs as signals pass through the air, mainly because radio signals spread over distance, somewhat like water exiting a garden hose. As frequency increases, so too does path loss, meaning a 2.4-GHz system will have a greater range than that of a 5-GHz system of equal power output. For a frame of reference, note that a 2.4-GHz radio signal will experience a free space path loss of about 120 dB over a distance of 5 miles. (For a free space loss calculator, see "Free Space Loss.")
Loss also occurs in other areas, most notably in the cabling that connects the radio transmitter to the antenna. By integrating the antenna and radio into a single weatherproof outdoor unit, some vendors eliminate this loss.
As noted earlier, antennas provide gain in a radio system, but the way they do this is different from the way radios deliver gain. Radio-system engineers often refer to an isotropic antenna, a theoretical concept that shows the spherical nature of how signals would radiate from a single point in space. You can't build an isotropic antenna, but some designs come close. Omnidirectional dipole antennas (rabbit ears), for example, radiate in a pattern that looks like a doughnut with a very small hole at the center. Omnidirectional antennas generally provide modest gain and are inappropriate for point-to-point links.
Directional antennas provide additional gain by focusing the radio energy in a directional beam. They don't add any power to the radio signal, but they concentrate it, thereby increasing range relative to the theoretical isotropic antenna. The gain of an antenna relative to isotropic is expressed in dBi. There are many different directional antenna designs, but the most common alternatives used in point-to-point links are patch antennas, multi-element Yagi antennas and parabolic dish antennas.
Two examples: A 2.4-GHz, 15-element Cushcraft Corp. Yagi antenna has a beam width of 30 degrees and a gain of 14 dBi, while a 3-foot Cushcraft parabolic antenna has a beam width of 10 degrees and a gain of approximately 24 dBi.
Now the key measures are in place. You start with output power, say 20 dBm, add antenna gain and then subtract loss from cables and free space. If the resulting number still exceeds the minimum for receive sensitivity, the signal gets through. To provide some margin for error, installers will typically define a fade margin of perhaps 20 dB.
While point-to-point wireless systems require line of sight between antennas, getting it is not quite as simple as aiming the antennas by using a pair of binoculars. In some cases, you might have direct visual line of sight, but not true RF line of sight. Likewise, minor foliage might inhibit visual line of sight, but under some circumstances, a point-to-point wireless link may be able to tolerate leaves and tree branches.
Playing it Safe
Security is always a significant concern with wireless systems, but since most well-designed point-to-point links are highly directional, intercepting signals is difficult. Nonetheless, most vendors include hardware-based encryption features that do not hurt system performance. And because you are concerned about securing a single point-to-point link, you can employ simple shared keys.
Many enterprises struggle to decide whether to contract with a wireless integrator/installer or do it themselves. The main factors to consider in making this decision are the complexity of the link and your risk tolerance. Connecting two buildings separated by a public thoroughfare at 10 Mbps is a lot easier than connecting two sites separated by 20 miles at 100 Mbps. Experienced radio system installers not only have a reliable gut feel for what will work but also understand some of the subtle complexities of RF link engineering, including antenna polarization, Fresnel zones and multipath interference, and they have the tools to measure RF signals, including potential interference. They also have a good understanding of government regulations, including those related to acceptable EIRP (effective isotropic radiated power), a measure of total system gain in specific frequency bands.
In addition, professional installers have experience with antenna and lightning arrestor installation.
In selecting specific products for installation, consider your throughput and reliability requirements and the RF characteristics of available systems. Think about output power, receiver sensitivity, management and security capabilities, mean time between component failure, and cost. Many fixed wireless systems are adapted from WLAN technology, while others are designed specifically for point-to-point applications. The latter generally offer greater reliability to meet the standards imposed on them by service providers, which are major customers. You can spend $10,000 for a system or 10 times that amount, but except for typically modest ongoing maintenance costs for hardware and software, monthly service charges paid to your local telco will be a thing of the past.
Dave Molta is a senior technology editor of Network Computing. He is also an assistant professor in the School of Information Studies at Syracuse University and director of the Center for Emerging Network Technologies. Molta's experience includes 15 years in IT and network management. Send your comments on this articles to him at dmolta@nwc.com.
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