equipment and systemsmetal wire, terrestrial and satellite radio, and optical fibreemployed in the transmission of electromagnetic signals. Every telecommunications system involves the transmission of an information-bearing electromagnetic signal through a physical medium that separates the transmitter from the receiver. All transmitted signals are to some extent degraded by the environment through which they propagate. Signal degradation can take many forms, but generally it falls into three types: noise, distortion, and attenuation. Noise is the presence of random, unpredictable, and undesirable electromagnetic emissions that can mask the intended information signal. Distortion is any undesired change in the amplitude or phase of any component of an information signal that causes a change in the overall waveform of the signal. Both noise and distortion are commonly introduced by all transmission media, and they both result in errors in reception. The relative impact of these factors on reliable communication depends on the rate of information transmission, on the desired fidelity upon reception, and on whether communication must occur in real timei.e., as in two-way voice telephony and video teleconferencing. Various modulating and encoding schemes have been devised to provide protection against the errors caused by channel distortion and channel noise. These techniques are described in telecommunication: Analog-to-digital conversion, Channel encoding, and Modulation. In addition to these signal-processing techniques, protection against reception errors can be provided by boosting the power of the transmitter, thus increasing the signal-to-noise ratio (the ratio of signal power to noise power). However, even powerful signals suffer some degree of attenuation, or reduction in power, as they pass through the transmission medium. The principal cause of power loss is dissipation, the conversion of part of the electromagnetic energy to another form of energy such as heat. In communications media, channel attenuation is typically expressed in decibels (dB) per unit distance. Attenuation of zero decibels means that the signal is passed without loss; three decibels means that the power of the signal decreases by one-half. The plot of channel attenuation as the signal frequency is varied is known as the attenuation spectrum, while the average attenuation over the entire frequency range of a transmitted signal is defined as the attenuation coefficient. Channel attenuation is an important factor in the use of each transmission medium. Additional reading An interesting historical perspective on the development of telecommunications media and the International Telecommunications Union (ITU) is discussed in a special issue of IEEE Communications Magazine, vol. 22, no. 5 (May 1984). Sybil P. Parker (ed.), Communications Source Book (1988), is an elementary overview of many aspects of communication systems and media. Roger L. Freeman, Reference Manual for Telecommunications Engineering, 2nd ed. (1993), comprehensively discusses all types of telecommunications media and standards.The physics of wave propagation and scattering is discussed in Leopold B. Felsen and Nathan Marcuvitz, Radiation and Scattering of Waves (1972, reissued 1994). George Jacobs and Theodore J. Cohen, The Shortwave Propagation Handbook, 2nd ed. (1982), is a nontechnical discussion of high-frequency radio propagation. Lucien Boithias, Radio Wave Propagation, rev. and updated ed. (1987; originally published in French, 2nd ed., 1983), discusses the general principles underlying the propagation of waves in the radio spectrum from VLF to EHF (very low frequency to extremely high frequency). H.L. Bertoni et al., UHF Propagation Prediction for Wireless Personal Communications, Proceedings of the IEEE, 82:133359 (September 1994), is an in-depth discussion of channel characteristics for mobile radio. Louis J. Ippolito, Jr., Radiowave Propagation in Satellite Communications (1986), deals with microwave propagation relevant to satellite communications.Historical coverage of satellite communications systems and equipment is provided in J.R. Pierce, The Beginnings of Satellite Communications (1968). Heather E. Hudson, Communication Satellites: Their Development and Impact (1990), is an excellent nontechnical history. Walter L. Morgan and Gary D. Gordon, Communications Satellite Handbook (1989), focuses on technology. David W.E. Rees, Satellite Communications: The First Quarter Century of Service (1990), emphasizes early international telecommunications.John Gowar, Optical Communications Systems, 2nd ed. (1993), thoroughly discusses optical communications media, including free space and fibre. Richard D. Hudson, Jr., Infrared System Engineering (1969), discusses the applications of free space optical channels, including infrared communication systems. Raymond M. Measures (ed.), Laser Remote Sensing: Fundamentals and Applications (1984, reissued 1992), characterizes free space optical channels in the context of remote sensing. A unified treatment of microwave and optical communications can be found in A. David Olver, Microwave and Optical Transmission (1992). Govind P. Agrawal, Fiber-Optic Communication Systems (1992), is an excellent presentation of optical fibre communications, including erbium-doped optical amplifiers. A thorough treatment of erbium-doped amplifiers for repeaterless long-distance optical communications is Emmanuel Desurvire, Erbium-Doped Amplifiers: Principles and Applications (1994). Alfred O. Hero III

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