The transmission system provides circuits and path between two subscribers. The transmission circuits are capable of receiving the electrical signals at one point and delivering them to a destination with good quality. If necessary, more transmission system can be setup for long distant transmission for good quality. The transmission path, which is also referred as telephone channel or transmission media is designed to provide voice-grade communication. It has evolved into a worldwide network that encompasses a variety of transmission media and switching systems. Thus the transmission system behaves as an excellent candidate for a data communication over long distances.
The transmission system involves analog transmission (voice communication) and digital transmission. The analog signals are characterized by frequency, amplitude and phase. In analog transmission system, signals propagate through the medium as continuously varying electromagnetic waves. The medium for an analog transmission may be twisted pair cable, coaxial cable, optical-fiber cable, microwave radio and satellites. The analog signals are subjected to deterioration due to attenuation and noise addition in the channel. Hence amplifiers, filters and necessary circuits are added in transmission system to upgrade the analog signal. Fig. 3.8 shows the voice signal and digital signals.
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