ect to its electrical characteristics. Within each class, some media are point-to-point, others are multiple-acces media. The multiple-access radio media can be used both to denote cellular access and to denote point-to-multipoint access between a single radio access station and mobile systems that cannot move outside the stations access area. This is because whether or not terminal mobility over multiple cells (i.e. in a cellular network) is 4 Physical Network Modelling supported is not defined by the physical media connection. It depends on whether mobility control functions are implemented in the physical nodes or not. Physical media specializations: Wireline, point-to-point Wireline, multiple access Wireless, multiple access (cellular). Carrier/time-slot. Wireless, point-to-point. Carrier/time-slot Fig. 4.2 Symbols for physical media connections A physical network model that defines a connection structure based on physical media connections only is a valuable model for planning concrete networks, but otherwise is not of great help. We have analysed a number of models in Chap. 3, which showed that there are many communication relations between physical nodes that are not directly connected by physical media connections. Since a phys- ical network model should show all communication relations between physical nodes, we need the definition of network interface as well: a network interface comprises all communication relations between two physical nodes. In order to understand what this means, we must look inside physical nodes and separate them into the node platform (NPL) and the node network function (NNF), see Fig. 4.3. In the upper model of Fig. 4.3, the functions of physical nodes are divided into NPL and NNF (this separation of concerns was first discussed in Sect. 2.3.6). An NNF handles all communication relations that are visible on the outside of a physical node, including the termination of physical media connections. NNFs communicate with each other over protocols. Some NNFs communicate with each othe