inition of protocols, adaptation functions, and access points. Moreover, if interlayer communications have to exist, a communication interface has to be foreseen, which solves the problem that functions in different layers will have com- pletely different protocols. In order to realize the interlayer data exchange, a special function is needed, which translates the signals produced by the source layer in a form that the destination layer can process. This is the so-called adaptation function and in general it is represented between two layers. The scheme of intralayer and interlayer communication is shown in Figure 3.18 where layer protocols, access points, and adaptation functions are represented in the layering graphical language. Using the concepts of access points and adaptation functions, we can go further in detail in the layered structure of a telecommunication network. In general, a layer receives an input in correspondence to an access point from the output of an adaptation function and, since we are dealing with communication networks, its role is to deliver the output of its processing carried out from the layer functions to another access point that transmits it to the upper layer via another adaptation function. The layer operation can be represented like a path through a logical layer network that connects a suitable number of functions SO that the output of the first is the input of the second and SO on. The layer network is simply the virtual network constituted by the layer functions located in correspondence to the locations of the physical network nodes. As will be clear describing specific examples, not all the network nodes are equipped to implement all the functions belonging to a layer. There are network nodes whose equip- ment implements only a subset of the functions of a certain level SO that in the correspond- ing location only those functions are available. On the other hand, every network element has its own layered structure, thus providing functions at several layers. The path throu