rlag Berlin Heidelberg 2001 3 Logical Network Modelling port networks1. The OSI RM concepts could not support that effort, primarily because of lack of concepts for describing switching and relaying. Real networks have also become more and more stratified, which basically means a number of networks are used to create other networks, similar to the structure of the Internet. Some attempts have been made to enhance the OSI RM for supporting stratification. The question still remains how the OSI transport layer (layer 4, which is not the same as transport networks in public networks) fits into that picture. Networks that were built according to the seven-layer structure of the OSI RM have not been very successful, for several reasons. One reason is that the layers (and sublayers) confuse the C-dimension with integration mechanisms like encoding and remote operations. In SM, only the logical nodes of the C-dimen- sion (i.e. in the message discrimination structure, see Fig. 2.25) define SAPs, while, e.g. encoding and dialogue control are functions that exist in every logi- cal node. In OSI RM there are also SAPs for accessing the OSI presentation layer, which is an encoding mechanism, and the session layer, which is a dia- logue-controlling mechanism. The more successful Internet architecture did not make these mistakes. Internet applications must define session control and encoding as part of the application, a view that is in alignment with SM. In this chapter, we will show that a rather small set of basic concepts (with spe- cialization) can be used for modelling any kind of logical network and structures of logical networks in the telecom domain. Although these concepts are very much influenced by the OSI RM, they have been considerably improved and aligned with telecom terminology, in order to make them applicable to a larger domain of com- munication system types. When we discussed the integration hierarchy in Sect. 2.3.7, we were only inter- ested in analysing the types of nodes. We also assumed that all interf