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Fixed-Wireless Applications
Fixed-wireless systems can be used for almost anything that a cable is used for, whether the cable is a T1 circuit, a cable television cable, an Ethernet cable or a fiber optic cable. Fixed-wireless systems are designed so that they emulate cable connections, and they use the same type of interfaces and protocols, such as T1, frame relay, Ethernet and ATM. For this chapter, we¹ll assume that your application is data, and we emphasize wireless systems designed for data communications. Keep in mind that fixed-wireless systems are also used for voice communications as well as for carrying television programming. But most new development in fixed-wireless systems is data-centric, such as for Internet access, or is flexible in supporting both voice and data communications. Fixed-wireless systems match cable-based systems for all important parameters, including delay, bit-error rate (1 in 100 million or better) and throughput (1 Mbps to 155 Mbps). Consequently any application that operates over a cable should be able to operate over a fixed-wireless system. The only exception is communication involving geosynchronous satellites where delays can exceed a quarter of a second.

Wireline versus Wireless
In some cases, a fixed-wireless system is the only wireless option. So you must decide if a fixed-wireless connection is practical and if it is competitive with available wireline connections. Today fewer than 10 percent of buildings have fiber to them, and only about 50 percent are close enough to a central office (12,000 feet or 3.5 km) to take advantage of DSL technology. Thus in many cases, a wireless connection could be the only option for high-speed communications. This is especially true in more remote areas. In some areas, the only option for communications will be by satellite.

When both wireless and wireline options exist, the potential reasons to consider wireless include lower costs, faster deployment, greater flexibility and better reliability. Unfortunately costs and deployment have to be evaluated on a case-by-case basis and actual costs will depend on the particular circumstances. In developed countries, a wireless system won't be more reliable than wireline options, but in developing countries wireless communications may be much more reliable.

Private versus Carrier
A fundamental distinction is whether you deploy your own wireless connection or whether it is supplied by a wireless carrier. In the past, most fixed-wireless connections were private, but with new spectrum licenses a number of companies are deploying or planning to deploy networks in most major metropolitan areas. Don't think of these companies as "wireless" companies, though. Think of them as CLECs or ISPs that happen to be using wireless technology. Instead of running fiber to a building, they are using wireless links that may be less expensive to deploy than fiber.

Once one of these companies, Winstar (http://www.winstar.com) and Advanced Radio Telecom (http://www.art-net.net) are two examples, has a wireless connection to a building, often to the roof, they can then market their communications services to the tenants of the building. These services look like standard networking services, and interfaces include frame relay, ATM, T1, and Ethernet using the same connectors, such as BNC and RJ-45. These services also can include value-added services, such as network management, Internet access, Web hosting and e-mail. The fact that these carriers use wireless technology will be transparent. (See the section How To Use a Wireless Carrier for a list of some tough questions you can ask to ensure dependable service. For instance, terrestrial wireless carriers only have regional licenses so you should ask whether the carrier can provide service to all of your locations.)

Figure: wireless broadband providing last mile of connectivity to buildings not having fiber connections.

Satellites are another form of carrier service, offering broadband services using geosynchronous satellites today and higher capacity LEO satellites in the near future.

Unique Aspects of Wireless
Fortunately, you do not need to be a radio engineer to take advantage of fixed-wireless communications. But it is helpful to understand what makes wireless different from other forms of communication, particularly when qualifying and specifying service parameters.

Wireless communications offers tremendous flexibility and ever-improving performance, but it does have some limitations. First and foremost, wireless uses radio spectrum, a finite resource. This limits the number of wireless users and the amount of spectrum available to any user at any moment in time. The amount of spectrum available equates almost directly to data bandwidth, with 1 Hz of spectrum typically yielding between 1 bps and 4 bps of throughput depending on various factors, such as the type of modulation used and environmental factors. The amount of spectrum actually available varies from radio band to radio band, but suffice it to say that fiber optic cable offers far greater overall capacity. Despite this capacity limitation, wireless offers more than sufficient bandwidth for many applications. But it is important to know the capacity of a particular wireless system in order to understand how it can satisfy your requirements if they should expand in the future. Another limitation is that fixed-wireless systems operate at frequencies that almost always require line of sight and that are restricted to distances that vary from a few miles to tens of miles. It is no mystery why microwave dishes are located at tops of towers, hills and buildings. Unlike cellular and other mobile wireless systems, fixed-wireless systems use fixed antennas with narrowly focused beams. A 3 degree to 4 degree beam is not uncommon. And unlike cellular systems, in which base stations communicate with dozens of mobile stations, broadband systems usually operate in a point-to-point manner, though a number of point-to-multipoint systems are in development.

Very few standards exist for fixed wireless systems, and you will need to purchase equipment from the same vendor for both sides of the connection to ensure interoperability.

Radio Spectrum
Fixed-wireless systems use frequencies allocated for such use from about 900 MHz to 40 GHz. The number of different bands can be bewildering, with multiple frequency bands assigned for private use and multiple bands assigned for carrier use. In addition, some bands are designated for licensed use while others can be used without a license.

Should you care what frequency you use? Yes, but only in a general sense. Higher frequencies have some advantages over lower frequencies, but also suffer some drawbacks. The principle advantage of higher frequencies is that there is more spectrum available for broadband applications. Most higher bandwidth systems use frequencies above 10 GHz. Antennas at these frequencies are smaller due to the smaller wavelengths, making systems easier to deploy. But with higher frequency, components demand more sophisticated technology, so systems cost more. Also, propagation distance for reliable communications decreases and the signal is more susceptible to weather conditions like rain and fog. Higher frequency systems, those above about 30 GHz, are sometimes referred to as millimeter wave because the wavelength of these signals is on the order of 1 millimeter.

Both private and carrier systems have a choice of using licensed or unlicensed spectrum. The main advantage of unlicensed spectrum is being able to deploy a system without applying for a license from the FCC (or equivalent body in other countries). The disadvantage is that you could experience or cause interference, though the type of technology used in these frequencies minimizes this possibility.

The principal frequencies of interest in this chapter are:

· 900 Hz, 2.4 GHz and 5.8 GHz: unlicensed systems using spread-spectrum techniques
· 2.5 GHz: licensed to carriers for MMDS (Multichannel Multipoint Distribution System)
· 5 GHz: new unlicensed band referred to as UNII (Unlicensed National Information Infrastructure) band
· 23 GHz: commonly used for microwave LAN systems
· 28 GHz: licensed to carriers for LMDS (Local Multipoint Distribution Service)
· 38/39 GHz: licensed to carriers for general purpose communications services

A good resource for additional information about frequency allocations is a book called "The Spectrum Guide" by Bennett Kobb. See http://www.newsignals.com for details.