nodes, can be used to create physical networks in many dif- 2.2 Focus on Telecom Systems and Networks ferent shapes by defining physical nodes with different logical node compositions. The terminology that is used to describe the physical network of a telecom system must therefore be well-separated from the terminology that is used to describe its structure of logical networks. The terminology that is used for describing physical parts of a telecom network is well established in the telecom industry. Such parts are referred to as terminal systems (or terminal networks), operator network, access network and core net- work. Physical nodes within those networks are referred to as terminal, network termination, access node, switching node, HLR and SSP (an IN-node). The same level of maturity does not exist with respect to terminology for logical networks and logical nodes. These concepts are increasingly applied in telecom standardiza- tion, but in a rather ad-hoc fashion. 2.2.5 Sysnet Models In the previous section, we modelled physical nodes without regarding physical media connection endpoints, which means we did not specify the physical nodes completely. For a physical node to be completely specified, one also has to define its physical media connection endpoints to the environment. Figure 2.10 shows a physical network with its logical networks and physical media connections. Fig. 2.10 A physical network The design of this network means that each logical node must support both a control information flow and a data flow. The logical node B2 must also relay the data flow that supports the control information flow between A1 and A2. There are two logical networks, A1/A2 and B1/B2, which seem to have nothing more in com- mon than that they both rely on the physical node in the middle and that A1/A2 relies on B2's relaying capabilities for data flows. This is an obvious case for separating data flow handling from control informa- tion handling. The solution is normally to separate the logical nodes in different parts 