l nodes of other logical networks. A SAP-using and -providing logical network has, however, oth- erwise no interest in the structure of other logical networks. This led the developers Note that we use ISDN as an example, not because it is an important network concept today, but because it is well-known, and because it represents an unusually complex logical system. 3 Logical Network Modelling of OSI RM developing group to the definition of the SAP concept and the concept "service provider", which in SM&AML is modified and renamed communication service provider (CSP). The OSI service provider concept is defined in recommen- dation ITU-TX.210. Although OSI RM defined SAPs only for data communication services, the gen- eral view of a CSP can be applied to any type of network. For example, if we spe- cialize the model discussed in Sect. 2.2.1 to a telecom system that provides ISDN teleservices, we can identify that system as a teleservice CSP (TE-CSP), see the topological model in Fig. 3.1 (only two terminal systems are shown, and the details of the physical structure on the terminal side are not covered). Figure 3.1 is explained as follows: An ATN of an ISDN system provides three important SAPs to a terminal sys- tem. Two of the SAPs, called SAPI 0 and SAPI 16, are used by the terminal proc- ess controller. The third, called B in the model, is used by the user-data termination. These SAPs correspond to the A-SAPs we discussed in Sect. 2.2.1. The B-SAP offers two separate 64-kbps user-data flows, called the B-channels, that in the operator network are relayed over telecom resources (TR), some of them which are circuit switches. These switches handle 64-kbps channels, and we will therefore call them 64K CS switches. These switches are the key tele- com resources in an ISDN system and parts of the ISDN resource system in the operator network. An ATN also supports a 16-kbps data flow between the terminal side and the network side. This flow is logically divided in up to 64 different SAPs. All these flows are multiplexed