stem, the first fully specified network interface is the network access interface. In this context, only two types of physical nodes are identified: terminal systems and the operator network as a whole. Next, the specification of the network access interface is used to develop an internal view of that interface for the operator network (based on a logical net- work structure), using a wide area perspective. This results in a generic structure of major physical nodes and fully specified network interfaces of the Operator Network. System developers create their own solutions to a physical substructure for each major physical node that was specified in the previous activity. These solutions 4.3 Physical Network Structures show major physical subsystems, and which parts of the external network inter- face must be handled by each subsystem. Within the development organization, different groups develop their own solu- tions to each physical subsystem. The first two activities are normally performed by standardization bodies, and the results are documented as some type of architecture description (a sysnet model would be appropriate), in the form of a number of protocol specifications. The third activity is performed by system architects in the development organization (and should be described as a sysnet model). The last activity is performed by the design departments within that organization. Whether the subsystem solution is described as a sysnet model or not, depends on to what degree such a model will support code generation for software and hardware implementation. This is feasible if AML is fully used, but other considerations might result in the use of other languages and methods for describing subsystem solutions. A structure of physical substructure models within a sysnet can be managed by identifying physical substructure, physical nodes and network interfaces with respect to physical network levels. This is shown in Fig. 4.9 for the ISDN case, using a dotted identification system. This system also includes t