se to put all functions of a particular network system in a logical network of its own. This all depends on the extent of the network system definition. For example, to allocate all functions of the PSTN/ISDN-network system in a single logical network would not do any good, considering the enormous evolution these network systems have already been exposed to for many years. Basically, without an understanding of how services in a network rely on each other, it is difficult to define adequate logical networks. The industry therefore tries to define a service hierarchy for networks, SO far without any essential result. Nev- ertheless, since the days of the OSI RM, the industry seems to agree at least on one basic paradigm for separating functionality in networks, which is in a structure of connectivity layers and connectivity strata ("connectivity structures" in AMLn). The OSI RM layer structure relies completely on this paradigm, as we showed in Chap. 2, although it defines neither physical nor logical network concepts Let's now look more thoroughly at how logical networks and logical-network structures are modeled. Figure 3.7 shows the logical network nw1 in its graphical representation and in tabular form, as well as in the context of its hosting networks nw01, nw02 and nw03 (the tables are used by a management system when configu- rating the logical network as a part of another network). A logical network model shows a number of logical nodes interconnected over some types of connectivity relations. Due to the large variations in structure and properties of connectivity relations, we use the generic name route for all types of such relations (route specializations are discussed in Sect. 3.3). All logical networks rely on one or several hosting networks for transporting messages between logical nodes of the same logical network. The (logical) net- work identifier (NWi) for the hosting network that realizes a route is annotated on the route. The logical-network structure that is implied by these hosting networks 