aces and pro- tocols were already known, and we used that knowledge to analyse the internal structure of types of terminating nodes. We will in the discussion in this chapter, take more of a network-design approach. This implies that we have to look at com- plete networks, including many individual nodes, some of them of the same type. This view on networks is a topological view. The topological view allows us to discuss addressing and routing in networks, which are functions that are closely related to the SAPs of logical nodes. 1 Transport network (as e.g. SDH, SONET, PDH) is the term used to denote a network that pro- vides reliable, high-capacity semipermanent or permanent connections between physical nodes of telecom networks. Connectivity provisioning is controlled by transport network operators, not by the operators of the telecom network or by the users of telecom networks. The ITU-T architecture for transport networks is described in recommendation ITU-T G.803. 3.2 The Communication Service Provider The Communication Service Provider 3.2.1 Introduction As an introduction to a more formal way to model logical networks, this section presents a logical network model for ISDN², without using formalized network service interface symbols, see Fig. 3.1. Section 3.2.2 gives some OSI RM back- ground, while Sects. 3.2.3-3.2.5 provides the semantics and symbols for different types of network service interfaces. The ISDN-CSP service control Terminal Q.931 Process Controller Distrib. SAPI 0 Network TE-SAPs SAPI 16 Process Controll. User-Data Termination controller Teleservice protocol A- or u-law - X.25 Operator User-Data Termination controller Distrib. Network Terminal Process Process Controll. Controller Fig. 3.1 A telecom system is a communication service provider that offers teleservices The structure of logical nodes in a physical network can be partitioned into logi- cal networks, as we discussed in Sect. 2.3.6. A logical node in such a network depends on the connectivity services that are available from logica