ibing physical structures from wide area levels down to the smallest phys- ical structure of components in physical nodes. That requires, however, that the physical network model of a sysnet is developed by the substructuring principle (this principle was discussed in Sect. 2.3.4). By applying substructuring, a physical network of a sysnet will consist of a hierarchical structure of physical subnet- works. This structure develops over the lifetime of a sysnet. It must be carefully documented and managed. 4 Physical Network Modelling Physical Network Structures 4.3.1 Physical Substructures The principle for substructuring was described in Sect. 2.3.4. The figures in the present section show, in four steps, how a physical substructure can be created and described. The example is based on the ISDN access interface. Fig. 4.4 Step 1: separate the network interface into its information and data flows Figure 4.4 shows an elementary physical network, consisting of two nodes, X and Y. They are connected over a network interface, NI, that is based on a physical media connection. The first disintegration step is to isolate the node platform (NPL) from the node network function (NNF) in each physical node. The reason is, of course, that the NI, and therefore how the NNFs cooperate, is of concern to both X and Y, while the NPLs are not. Secondly, we apply a first interface refinement step (see Sect. 2.3.3), which separates the information exchange between X and Y from the data flow. As long as we have not disintegrated the NNF, the data flow is repre- sented by the physical media connection. The information exchange is represented by the protocol that is implemented by the NNFs. The next step is to analyse the logical structure of the NNFs. This structure can be expected to be quite similar in X and Y. If we assume that X is an ISDN terminal system and Y an ISDN operator network, the logical node structure of one of the NNFs can be expected to look as shown in Fig. 4.5, although slightly simplified. This NNF supports packet d