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).
The particular features of a communication system are described below:
We can characterize these in several ways by distinguishing (see Figure 1.2):
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.
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.
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.
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:
This diagram corresponds to a digital information transmission system. The following elements are distinguished in Figure 1.5.
The role of the different blocks is as follows:
In this system, the transmitted messages undergo various types of degradations:
All this leads to a very important role played by the signal processing at different levels of the communication system, especially at the receiver which regenerates the transmitted signal, before degradation in the transmission channel.
Optimizing the performance of communication systems has led to a diversification of methods and techniques used:
Previously, transmission systems used analog representation and transmission techniques almost exclusively. A fairly clear evolution has emerged since the 1970s for the introduction and use of digital techniques in representation and transmission, regardless of the nature of the information at the origin: digital (data) or analog (phone, images, and videos). In the latter case, this supposes the use of a process of digitization (sampling and quantization) of the signal.
This use of digital techniques in transmission is motivated by the following four reasons:
This digitization was first applied to cable transmission systems and then to radio transmission systems: radio-relay systems and satellite telecommunications. The current trend is in the design and use of fully digital transmission systems (integrated transmission and switching) and the integration of services in the sense where in the same system, and therefore on the same transmission medium, a whole set of different services coexist.
However, a digital transmission a priori requires a larger bandwidth than an analog transmission if one use methods of compression and representation to no more than two levels (M-ary representation, with M> 2).
Indeed, an analog telephone channel conventionally occupies a bandwidth of 3.1 KHz, the same channel in digital form requires a bitrate of 64 kbit/s, so in binary transmission, a bandwidth of about 64 × 0.8 ≅ 51 KHz (according to the extended Nyquist criterion), that is, about 16 times more than analog transmission.
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