physical network structures can be created. Thus, the most stable part of a sysnet model is not its physical network fragment but its structure of logical network fragments. The number of basic concepts and integrating functions (e.g. discrimination, asso- ciation handling, message handling, etc.) used in creating logical network models is also far less than the number of physical node types, considering all network concepts that are used to describe today's public and private networks. It is safe to say that without an understanding of the logical network structure of telecom sys- tems in general, you can still become an expert on one or a few network concepts, but you will not easily become a (good) network designer. In reality, every new telecom system concept that becomes standardized intro- duces new modelling concepts for physical networks and nodes (which is to be expected), but also often introduces new concepts for logical networks and nodes to an extent that is not always necessary. Nearly two decades ago, the ITU-T real- ized the need for such a set of concepts, which became the OSI RM. The telecom- munication industry did not take much notice, however, primarily because the model was said to be only applicable on data networks, not networks for teleph- ony-their primary business. Early attempts to use the ideas formulated in the OSI RM for other types of services were not very successful. For example, when ITU-T standardized ISDN, it became obvious that the OSI service access point concept could not support the user-control plane distinction in ISDN. This is basically the distinction we made in Sect. 2.2.1 between the ATD and the ATC in the access transport network, and between the NTD and the NTC in the network transport network (in ISDN denoted "common channel sig- nalling"). Another attempt to use the concepts of OSI RM was made in 1993 when ITU-T tried to provide an architecture for SDH (Synchronous Digital Hierarchy) trans- T. Muth, Modeling Telecom Networks and Systems Architecture © Springer-Ve