red to physical networks. The CSP interfaces towards teleservice users define the properties of SAPs and of service control interfaces, without dealing with implementation of these interfaces. The physical structure of the CSP implementation can be modified without affecting the specification of the CSP. Probably most important is that the services that are provided to CSP users can be described independently of the access network interfaces. These interfaces exhibit rather complicated protocols and physical structures. Modifications to these interfaces are done continuously, in order to provide more reliable and efficient access networks. As long as such modifications do not include CSP service aspects, they cannot affect the specification of the CSP. Figure 3.2 shows that a CSP can be built by a number of supporting CSPs (in this case, a number of ATNs and an NTN) and a number of logical nodes that belong to either the process control system or the resource system. The model of the structure of the ISDN-CSP was shown in a topological view, i.e. showing all individual CSPs and logical nodes of the CSP, comprising both the C- and the D- dimensions. We can show this model in the C-dimension only as the ISDN logical network structure, if we regard the resource system as one logical network, the process con- trol system as another, and only show the types of SAPs and control service inter- faces, see Fig. 3.4 (compare to Fig. 2.32). This model also shows the major logical nodes in the process control system and the major resources in the Resource Sys- tem. This information is extracted from different logical network fragments and is not actually part of the logical network structure. It is shown here only to facilitate understanding of how to interpret a logical network structure. 3.2 The Communication Service Provider Unrest. Access to User-to-user messages Operator Telephony packet service In-call Outside call CSP control TEC Audio 64-128K 16K Process Control System Terminal Process Distrib. Operation Resource Sys