We are surrounded by communication networks, they are part of our life. If asked, we can easily enumerate examples of them: the fixed or mobile telephone network, the Internet, or someone's Ethernet home network would probably be the most popular answers. However, difficulties appear when we try to define the concept “communication network” more formally, without mentioning any specific network technology, looking for a definition applicable to any of them. We will start this book addressing precisely this basic question.
There are two basic elements on which networks are constructed: telecommunication systems and switching systems. A telecommunication system or “link” consists of a transmitter and one or more receivers connected through a medium that propagates the involved electromagnetic signals. Applying this definition, two telephones A and B directly connected through a bidirectional cable pair (Fig. 1.1) contain two telecommunication systems: (i) one composed of the transmitter at A, the medium A B, and the receptor at B, and (ii) another system formed by the transmitter at B, the medium B A and the receiver at A.
Telecommunication systems are the basis for assembling ...
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B, and the receptor at B, and (ii) another system formed by the transmitter at B, the medium B 
A and the receiver at A.