Claude E. Shannon

The fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point. Frequently the messages have meaning; that is they refer to or are correlated according to some system with certain physical or conceptual entities. These semantic aspects of communication are irrelevant to the engineering problem. The significant aspect is that the actual message is one selected from a set of possible messages. The system must be designed to operate for each possible selection, not just the one which will actually be chosen since this is unknown at the time of design.
By a communication system we will mean a system of the type indicated schematically in Fig. 1.  It　consists of essentially five parts:

1. An information source which produces a message or sequence of messages (letters, functions of time and space, etc.) to be communicated to the receiving terminal.
2. A transmitter which operates on the message in some way (change, encode, sample, compress, quantize, interleave, construct, etc.) to produce a signal suitable for transmission.
3. The channel is merely the medium used to transmit the signal from transmitter to receiver (a pair of wires, a coaxial cable, a band of radio frequencies, a beam of light, etc.).
4. The receiver ordinarily performs the inverse operation of that done by the transmitter, reconstructing　the message from the signal.
5. The destination is the person (or thing) for whom the message is intended.

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