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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
Energy is transmitted in three ways: electromagnetic radiation, conduction, and convection (heat transfer). Because electromagnetic waves do not necessarily need a material medium for transmission, they are used for a wide range of communication, including communication over copper and fiber-optic cable, as well as through water, air, and the vacuum of space.
Electron movement causes electromagnetic radiation. The frequency is the number of oscillations per second of the resulting wave, also called Hz (hertz). The wavelength is the distance between crests in the wave. The higher the frequency, the shorter the wavelength. As outlined in Figure E-2, the length of a wave may be larger than the earth's diameter and smaller than an electron.
ANCHOR HERE: Figure 2 (see book)
Wireless systems use specific bands in the radio, microwave, and infrared range, as shown in Figure E-2. The spectrum is allocated by governments and international organizations. In the United States, the FCC allocates the spectrum and sells it at auctions to companies that want to operate communication services in designated markets. Spectrum allocation is designed to prevent overlapping signals and interference. In fact, interference still occurs. Devices like microwave ovens, wireless LANs, and cellular phones operate in the same frequency. Some common frequency allocations are listed in the following table.
Note that this topic continues in "The Encyclopedia of Networking and Telecommunications."
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.