own in Fig. 3.5. Only a selection of resource controllers is shown in the model. The model also exemplifies the differ- ence between local and remote resource controllers: local resource controllers exist as part of the same physical node as a distributed network process controller, while remote resource controllers are accessed by a distributed network process control- ler over some resource control protocol. Operator CSP control TE user service control {ATNS} User-to-user messages In-call Outside call NTD route {NTDS} manager Distributed Service Switch 64K service Terminal Q.931 Network {SWC} controller switch Process Controller Process Controller Local A-u converter SAPI 0 A/D encoder (DNCP) TR controller Remote TR controller Remote Resource resource Control Protocol SAPI 0 SCCP MTP Fig. 3.5 The ISDN process control system The model of the ISDN resource system depends on the type of teleservice that is supported. Figure 3.6 shows the resource model fragment for telephony services. Note that only those resource that are used for this service are shown. 3 Logical Network Modelling Switch controller Terminal Process {SWC} Telephony Controller controller controller 64K CS A- or u-law converter Additional converter types of B-channel {ATNS} 64 kbps (64 kbps NTD-channels Fig. 3.6 The ISDN resource system for telephony services The ATN, NTD channels and 64K CSs are integrating mechanisms for the com- plete resource system model. The complete model is designed step by step: First a resource system model is created for each type of teleservice, then all these models are integrated in a common resource system model. 3.2.2 The OSI Service Provider An OSI service provider is a logical system that, when seen as a black box, offers a number of service access points (SAP) to users. Users who connect to these points have access to a data communication service. Let's make the following terminolog- ically substitutions: Substitute the SM&AML term network service interface with the OSI term service interface. Substitute the SM&A