Digital Communications 1. Safwan El Assad

channel coding for transmission on channels with disturbances (Chapter 4).

      1

      Introduction to Telecommunications

      1.1. Role of a communication system

      A communication system aims to convey as faithful and reliable messages as possible between a sender and a recipient, at any distance, with reasonable costs. Messages are information entities and their routing requires the existence of a communication channel to convey them (Figure 1.1).

      Figure 1.1. Simple communication system. For a color version of this figure, see www.iste.co.uk/assad/digital1.zip

      The particular features of a communication system are described below:

       – the transportation of information (words, images, texts, data, etc.) and not the transportation of the information medium (paper, disk, etc.);

       – the users of a telecommunications system want the messages they transmit to be delivered without loss or modification. This implies a high fidelity of the system (quality), in spite of the inevitable imperfections and disturbances to which it is subjected;

       – the communication service must be available in all circumstances, irrespective of unforeseeable and unavoidable partial failures which can reach part of the system. This corresponds to a high reliability of the service (availability);

       – the transportation of information at various distances, up to thousands of kilometers (or more) requires the use of specific transmission means (copper or optical cables, radio channels, satellites). This is a transmission problem.

1.1.1. Types of services offered by communication systems

      We can characterize these in several ways by distinguishing (see Figure 1.2):

       – the type of information transmitted;

       – the number of partners;

       – the role played by the partners: monologue, dialogue, conference.

      Figure 1.2. Modes of communication:

source;

recipient

      When a certain number of users benefit from the same service, all the transmission links constitute a network. This corresponds to an information broadcast network, if one is dealing with the unilateral transmission of a source to several recipients, or to an information collection network, if the transmission is unilateral from several sources to only one recipient.

      When there is no permanent transmission path between two points of a network, it is called a switched transmission: the link is established only after receipt and execution of connection orders from the partners. Generally, the switched network is very common, that is to say, the elements of the system relating to switching and transmission are common resources, made available to potential users who have access to this network by a means of individual transmission.

      Figure 1.3 shows the three types of networks.

      Figure 1.3. Type of networks

      1.1.2. Examples of telecommunications services

      We give some examples of telecommunications services in Table 1.1.

      Table 1.1. Examples of telecommunications services

Information type	Communication mode	Network	Service
Speech	Bilateral Multilateral	Switched communication	Phone Conference call
Video	Unilateral	Fixed broadcast point to point	Video conference Television

      The situation is not steady. In fact, the services offered and their diversity have considerably increased, both for long-distance telecommunications systems (in particular, for text, data, images and video-type information) and the considerable development of local business networks, which make it possible to connect computer and production equipment of a heterogeneous nature. Regarding telecommunications, almost all digital communications can integrate the transmission of different types of information in a single ISDN (Integrated Services Digital Network). In addition, communications over the Internet have been developing for almost a couple of decades.

1.2. Principle of communication

      In a broad sense, the communication is a transfer of information (messages) between a sender and a recipient, through a medium called a channel. There are basically three main functions corresponding to this principle, as shown in Figure 1.4.

      Figure 1.4. Principle of communication. For a color version of this figure, see www.iste.co.uk/assad/digital1.zip

      A message has been considered as any means of modifying the state or evolution of the addressee. The signal is only an auxiliary supporting the message. It technically makes it possible to transmit this message.

      The message has meaning only for the sender and the recipient. On the other hand, for the messenger, the meaning of the messages does not matter. Only the quantitative aspect interests them (to make money for the service means the transmission of a quantity of information). It is this approach that is used to design communication systems.

      For communications, a message will be defined as a sequence of elements taken from a finite set of symbols: an alphabet. The transmission of a message consists of making a choice (according to a law that remains to be defined) among all the possible messages. The recipient is supposed to know, a priori, all possible messages. In addition, they must have a decision rule, allowing them to decide which message was sent. A more detailed diagram of a digital communication system makes it possible to highlight the various problems encountered in effectively achieving communication between the sender and the recipient of a message, because the message must satisfy the following conditions:

       – the source and the recipient must agree on the symbolic representation used in the information transmitted (definition and use of a message representation code);

       – the transmission