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Information theory and its more specialized branch, communication theory, are based on the statistical definition for the information content of messages. Statistics deals with discrete elements and is therefore directly applicable to pulse systems. The `system engineerÂ¿ has to use results of this theory for designing o communication systems. Four classes of systems are distinguished: communicationÂ¿data transmissionÂ¿locationÂ¿computers. The main sections in each system are: sourceÂ¿message encodingÂ¿signal formationÂ¿multiplexing (train formation)Â¿wave formation (carrier modulation)Â¿transmission over the mediumÂ¿reception Some sections may be omitted in particular systems. Information is defined and its transmission over all the sections of a system is discussed in the light of modern theory. Message encoding has to transform the multi-dimensional message space into the three-dimensional signal space at the same time allowing for sufficient reduction of redundancy. Signal encoding will reshape the signal space for matching a given channel. Authenticity examination at the receiving end will assure reliable service by error detection. Error correction methods may be employed to reduce equivocation (error expectancy).