gh the network layer connecting the input access point to the output access point is called "trail" (or "link" in Internet engineering task force [IETF] documents) and in order to mark the trail start and the trail end the "trail termination points" (TTP) are introduced. TTPs are not only markers of trail start and end, but are functions in their own right. As a matter of fact, at the trail ends a set of operations have to be performed, generally related to layer monitoring, and those operations are carried out by the TTPs. Networks Fundamentals and Present Architectures Trail Network connection Connection FIGURE 3.19 Graphical representation of a trail and its components within a single layer. AF, adaptation functions; AP, access points; TTP, trail termination points; TCP, trail connection points; CP, connection points. TTPs are connected to the first and to the last layer function by the "terminal connec- tion points" (TCPs)* while the layer path itself is created by connecting the functions via the "connection points" (CPs). The role of TCPs and CPs is somehow similar to the role of the access points. They represent the points performing every adaptation that is needed to allow different functions to communicate. The structure of the trail through a generic network layer is represented in Figure 3.19 where all the trail elements are evidenced. In practice different communication network models exist, built over the abstract method of layering and using different layers to model and design the network. In order to adopt a common language, the layers are generally numbered following as much as possible the OSI definitions. This is an informal way to proceed, since there is no complete correspondence between the layer functions as coded in the OSI model and the layer functions as coded in the model used in practice (see, e.g., the Internet model or the MPLS technology) and care has to be adopted not to create confusion and errors due to the application of OSI characteristics that are not reflected in the model un