 formal way the layering technique, since it is a task requiring a dedicated book. The scope of this section is to give an idea of the layer- ing technique and a few details of the graphic language associated to it SO as to be able to understand the layered structure of telecommunication networks. Analyzing the layered structure, two interactions among functions can be individuated: interactions among functions in the same layer and interactions among functions in dif- ferent layers. The first interaction type is needed to coordinate the work of the functions belonging to a layer; the second type of interactions is needed to realize the client-server relationship on which the model is based. Functions of the same layer communicate via messages coded using specific rules that are called layer protocols. Thus, every layer has a set of specific layer protocols. Of course, the fact that the functions have to exchange messages means that they also have to accept control signals as input. Moreover, the functions output will be constituted by processed input data plus a set of control signals. The communication of management information between different functions is carried out via a service communication network that can be either embedded in the system under design or completely separated. As far as the interlayer communication is concerned, the first point is that while func- tions belonging to one layer generally exploit a logical layer network to communicate, this does not exist if functions belonging to different layers have to be connected. To realize interlayer connectivity there are the so-called access points that are simply points in which the output of a function of the upper layer can be connected with the input of a function of the lower layer or vice versa, operation that cannot be done without the presence of this particular interface. Telecommunication Networks Protocols level 2 Func. 1 Func. 1 Func. 2 Func. 2 Adaptation function Access points Func. A Func. A Protocols level 1 FIGURE 3.18 Graphical def