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Claude E. Shannon: Communications as a Statistical Process, 1955

Communication Systems

In any given communication system (take a telephone conversation, for example), there is an information source, or sender, who is attempting to convey a message (like, "Happy Birthday!") to a receiver. That message is changed into an electronic signal by a transmitter (like a telephone mouthpiece), and the signal is then sent over a communication channel (like a telephone line) to reach the receiver. Any communication channel necessarily has a certain capacity; that is, a certain limit on the amount of signal information it can carry. If a signal exceeds the capacity of a communication channel, then that signal cannot be transmitted. In "The Mathematical Theory of Communication," Shannon used this formula to measure the capacity of a communication channel:

In this formula, n(T) represents the number of signals of duration (T) which the communication channel is able to carry as (T) approaches infinity.