nsport network are CSPs. 2.2 Focus on Telecom Systems and Networks The structure of this model can be further refined into a more detailed structure of physical nodes. We will look at typical solutions for public networks, separating the discussion into: The structure between terminal side and the network side. The structure of the network side. The structure of the terminal side. Possible physical nodes that operate over the interface between terminal sys- tems and operator networks depend on the functions and structure of the access transport network. The two functional parts of the access transport network are nor- mally integrated over a common transport link over the terminal-network interface, SO that both user-data flows and the control-data flows are terminated in a common physical node in the operator network. This node is an access node, see Fig. 2.6. Telecom System Terminal Systems Operator Network Access Core Network Network TE-TPC Teleservice Access Protocol Access Node term. term. relay TE-UDT Teleservice Protocol term. Fig. 2.6 A physical structure of the interface between terminal side and network side The functions of an access node are termination of both types of flows, integrat- ing the flows on the terminals side and separating them on the network side. Con- sidering that such a node will connect many terminal systems, and that the operator network may consist of many access nodes, an access node is part of a physical net- work called access network. The remaining part of the operator network is regarded as the core network. The model of Fig. 2.6 shows the physical structure in the net- work-terminal interface, according to these assumptions. The logical nodes of the access transport network are ATN-relaying nodes and ATN-, ATC- and ATD-termi- nations in terminal systems and in the core network. An ATN-relaying node is physically allocated in an access node. Examples of access nodes are the base sta- 2 Foundations of Sysnet Modelling tions used in cellular networks, and concentrators in some 