Transmitting information between two entities, commonly referred to as the “source of information” for the first and the “recipient” or “user” for the second, assumes that this information is represented in a common and understandable form by both parties. It is also necessary that the amount of information exchanged does not exceed the transmission capacity of the channel. Lastly, it is desirable that the disturbances (modifications of all kinds) in the transmission channel have, if possible, little or, ideally, no effect on the information delivered to the recipient or user.

All this is studied, analyzed and treated in what is called the “theory of information”, the theoretical basis of different techniques of communication.

In a schematic way, the problems raised and treated by this are the following:

• – quantitative measurement of information;
• – representation of common information between sender and recipient: code;
• – source coding: adapting the source rate to the capacity of the transmission channel;
• – channel coding: protecting against transmission errors.

The block diagram of Figure I1.1 shows at what levels of the communications chain these problems are, both on the side of the emission of the information (transmitter) as on the side of the reception of the information (receiver).

We have divided this first part, dedicated to the theory of information, into three chapters following a first chapter of general introduction to telecommunications (Chapter 1). Each of these three chapters is devoted to each of the three issues raised previously:

• – measurement of information (Chapter 2);
• – source coding for the transmission on channels without disturbances (Chapter 3);
• – channel coding for transmission on channels with disturbances (Chapter 4).