<P> Data transmission (also data communication or digital communications) is the transfer of data (a digital bitstream or a digitized analog signal) over a point - to - point or point - to - multipoint communication channel . Examples of such channels are copper wires, optical fibers, wireless communication channels, storage media and computer buses . The data are represented as an electromagnetic signal, such as an electrical voltage, radiowave, microwave, or infrared signal . </P> <P> Analog or analogue transmission is a transmission method of conveying voice, data, image, signal or video information using a continuous signal which varies in amplitude, phase, or some other property in proportion to that of a variable . The messages are either represented by a sequence of pulses by means of a line code (baseband transmission), or by a limited set of continuously varying wave forms (passband transmission), using a digital modulation method . The passband modulation and corresponding demodulation (also known as detection) is carried out by modem equipment . According to the most common definition of digital signal, both baseband and passband signals representing bit - streams are considered as digital transmission, while an alternative definition only considers the baseband signal as digital, and passband transmission of digital data as a form of digital - to - analog conversion . </P> <P> Data transmitted may be digital messages originating from a data source, for example a computer or a keyboard . It may also be an analog signal such as a phone call or a video signal, digitized into a bit - stream, for example, using pulse - code modulation (PCM) or more advanced source coding (analog - to - digital conversion and data compression) schemes . This source coding and decoding is carried out by codec equipment . </P>

A sequence of data is transmitted to a set of
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