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Extended LANs (Wireless Bridging)
Wireless technology can connect LANs in two different buildings into an extended LAN. This capability is, of course, also available through other technologies (such as a T1 line or a leased line from a telephone provider), but depending on the conditions, a wireless solution is sometimes more cost-effective. A wireless connection between two buildings also provides a solution to the ground potential problem described in a note earlier in this chapter.

A wireless bridge acts as a network bridge, merging two local LANs over a wireless connection (see Chapter 2, “Networking Standards,” and Chapter 6, “Connectivity Devices,” for more information on bridges). Wireless bridges typically use spread-spectrum radio technology to transmit data for up to three miles. (Antennae at each end of the bridge should be placed in an appropriate location, such as a rooftop.) A device called a long-range wireless bridge has a range of up to 25 miles.

The Radio Frequency Spectrum

The radio portion of the electromag-netic spectrum extends from 10 KHz to 1 GHz. Within this range are numerous bands, or ranges, of frequencies that are designated for specific purposes. You are probably familiar with the following frequency bands:
Within the United States, the Federal Communications Commission (FCC) controls the use of radio frequencies. The majority of frequency allocations are licensed; an organization is granted an exclusive license to use a particular range of frequencies within a certain limited geographic area. Thus, you can have only one television Channel 5 within a given area, and Channel 5 allocations are spread out so that they don’t interfere with each other. A licensed frequency allocation guarantees the license owner a clear, low interference communication channel.

A few frequency ranges are unlicensed, which means that they can be used freely for the purpose specified for those frequencies. The FCC has designated three unlicensed frequency bands: 902-928 MHz, 2.4 GHz, and 5.72-5.85 GHz. The 902 MHz range has been available the longest and has been used for everything from cordless telephones to model airplane remote control. Because the 902 MHz range is quite crowded, many vendors are pushing development of devices for the less crowded 2.4 GHz band. Equipment for the 5.72 GHz remains expensive and is used infrequently.

Use of an unlicensed frequency occurs at the user’s risk, and a clear communication channel is not guaranteed. Equipment used in these frequency bands, however, must operate at a regulated power level to limit range and reduce the potential for interference.