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 (reduction in power). 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 time”—i.e., as in telephone conversations and video teleconferencing.
