these are the interfaces between an operator network and a number of terminal systems that are connected (often remotely) to that network. The boundary of a telecom network is defined by such points in space where a network element is connected to terminal systems by physical media (means for information transfer). There are two aspects of physical media to consider in telecom network modelling: The electromagnetic properties of the points where a network element accesses a physical medium. Such points are called physical media connection endpoints. The electromagnetic properties determine the lowest level of communication between network elements-the transmission protocols. In public networks there is normally a separate, common transport network (PDH, SDH, SONET, etc.) that serves a number of other physical networks. Transport networks are often operated by separate network operators, who sell transport services to other net- work operators. The mechanical/spatial properties of media and network elements. These prop- erties define the mechanical/spatial structure of network elements, sites and interconnections between sites (by cables, radio links, etc.). A site can be a means for concentrating physical media and radio channels, and may therefore comprise nodes that belong to different physical networks. 2.2 Focus on Telecom Systems and Networks Section 2.2.3 discusses the concepts physical network and spatial network, and defines the context for sysnet modelling as physical networks. Section 2.2.4 dis- cusses the relation between logical and physical nodes in a telecom network. Finally, in Sect. 2.2.5, sysnets are defined as physical parts of physical networks. 2.2.1 The Logical Network View on Telecom Systems The model in Fig. 2.1 is a specialized version of the model in Fig. 1.8. The names of units, interfaces and protocols are defined in AML and used in logical network modelling of telecom systems. The symbols for interfaces are intuitive. AML interface symbols are introduced in Chap. 3. Telecom System term