des information flows over TE-SAPs and exchanges control information with users of the telecom system. The boundary between a telecom system and its users may exist inside terminal systems, or between terminal sys- tems and special network termination equipment. Thus, the boundary may exist as a physical boundary, but it may as well be just a logical boundary inside some physical equipment. A more detailed presentation of the CSP concept is found in Sect. 3.2.3. 2.2.3 Physical Networks and Physical Nodes Every (concrete) network element in a telecom network is connected to at least one other network element over a physical telecom medium. In telecom standardi- zation, the electromagnetic and the mechanical aspects of such a medium are nor- mally kept apart. This separation is motivated by the fact that the electromagnetic 2.2 Focus on Telecom Systems and Networks aspects affect the functional properties of the network elements. The mechanical aspects have no influence on the functionality, but they affect the costs of network elements and of the network as a whole. An example of separation of electromagnetic and mechanical aspects are recom- mendations ITU-T V.10, V.11, and the standards ISO 2110. Recommendations V.10 and V.11 describe electrical aspects of physical media connection endpoints that are used by modems in the voice band. ISO 2110 describes the mechanical aspects of the connector used to connect terminals to modems. This separation of aspects of physical media should also be reflected in different types of system models, as shown in Fig. 2.4. Concrete network Physical Physical node. Physical Physical Spatial network Physical network Concrete Physical Physical Physical system Connector Physical physical media conn. end-point Cable, etc. Mechanical/ Mechanical & spatial Site manual physical media connection properties) cross-connect (electromag. properties) Fig. 2.4 Concrete networks versus spatial and physical networks Concrete networks include a number of components that presently cannot be defined b