ates the distinc- tion between the ISDN-CSP and an ISDN physical network: a CSP is a logical sys- tem that looks the same for a number of physical network solutions. The physical network view according to Fig. 3.1 shows that the interface between the ISDN- CSP and its users is defined by TE-SAPs, not by how these are physically imple- mented. The physical structure of the operator network part of the ISDN physical network is hidden inside the CSP and does not affect the specification of the ISDN-CSP. Thus, if we exclude the physical structure, we arrive at the model in Fig. 3.2 of the ISDN-CSP and its internal structure of logical units. Terminal Q.931 service Process control Controller Distrib Network Process Controll User-Data The ISDN Termination controller Communication TE-SAPs Service Provider Operator (ISDN-CSP) User-Data controller (Termination) Distrib. Network Terminal Process Process Controll Controller. Fig. 3.2 The ISDN-CSP and its logical node structure It is important to remember that a CSP is a logical system, that in no way pre- scribes how its interfaces are to be mapped on network interfaces (i.e. physical interfaces). Mapping the interface between the CSP and the users on a physical model falls either under Case A or Case B in Fig. 3.3. Case A makes the terminal a part of the CSP (as, e.g. an ordinary telephone); while in Case B, it is not (there exist special physical nodes for network temination functions, as in the ISDN phys- ical network). 3 Logical Network Modelling Communication Service Provider (CSP) service Terminal Case A control Network termination functions TE-SAP Access Operator Network Network Interfaces Terminal Network Termination Case B Network termination functions Fig. 3.3 Mapping a CSP on a physical network model The distinction between a CSP and a physical network is a very important aspect in network modelling, although seldom explicitly identified in descriptions and standards for existing network concepts. One reason is that the CSP is a stable specification object, compa