o offer SAPs. If we 3 Logical Network Modelling describe the logical system as consisting of logical nodes that cooperate over pro- tocols, we have a logical network. The internal structure of a logical network is, therefore, a distributed system of logical nodes that is defined in the DC-plane. Individual SAPs (and CSIs) are handled by the logical nodes. To describe the complete structure of logical nodes in a CSP as a single logical network is possible (and in no way prohibited by AML), but seldom very wise, considering the functional complexity of most CSPs of interest. That's why the C- dimension is defined. It allows you to partition the complete picture into a number of logical networks that can be further specified independently from each other (since network service interfaces are simple and quite stable, from a functional point of view). Figure 3.87 is an example of a logical network in ISDN, extracted from the model in Fig. 3.39. This is a part of the logical network of the process con- trol system in ISDN. user-user messages Q.761-766 (ISUP) TE-TPC Q.931 TE-DNPC SAPIO SAPI63 SAPIO Teleservice resources 64K CS SAPI63 64K CS service switch Teleservice resources Fig. 3.87 Part of the logical network model for the process control system in ISDN The model qualifies as a logical network, since the logical network depends on a number of SAPs (CCS and LAPD) and offers a CSIs to users (the TEC). The model identifies two types of logical nodes (TPC and DNCP), as well as which protocols are used in communication between those nodes. The separation between the DNCP and serving units to the right is based on the generic units we have defined. This separation does not affect the structure of the logical network in the D-dimen- sion, since the serving units do not depend on SAPs and do not terminate any pro- tocols (however, be aware of the possibility of remote resource control interfaces). Note that a process control system does normally not provide any SAPs. The SAPs for user-to-user message communication in ISDN 