sible physical structures on the terminal side The key to this structure is the functional separation between network termina- tion and TEC and TEU functions, since the former must collaborate with and be known by the operator network, while the latter need not. It therefore does not mat- ter to the operator network what the physical structure looks like on the terminal side. The network termination function and TEC&TEU functions may be inte- grated in a single physical node (as a mobile phone is with respect to voice and short message services), or they could be separated into a special network termina- tion and a number of terminals. HLR is a database node in mobile, cellular networks that e.g. is used to track the movements of mobile terminals. SSF and SCF are parts of the "intelligent network" concept (IN) in public net- works. 2 Foundations of Sysnet Modelling The physical structure of a telecom network is created by combining the physi- cal structures for terminal systems, access networks and core network, as shown in Fig. 2.9. Core Network Teleservice Access Terminal Systems Protocol TE-TPC Access term. TE-UDT Networks Resource Teleservice Control Protocol relay Protocol Teleservice TE-TPC Distribution TE-UDT term. Protocol TE-TPC TE-TPC term. TE-UDT relay TE-TPC TE-UDT Fig. 2.9 A physical network for a telecom system The NT1-NT2 solution is the ISDN solution for PABXs. The model is denoted with the principle protocol types. The physical nodes of the network are normally given names associated with their composition of logical nodes. Considering all possible combinations of logical nodes in physical nodes, one realizes that it is more important (from a modelling point of view) to understand and define the smaller set of logical node types, than to know all physical node types that are defined in various network concepts. An important conclusion from this discussion is that the functional structure of a telecom system, which can be described as one or a number of related logical net- works built by logical 