ions that belong to different (physical) substructures will soon be mixed, and functional spaces that originally were orthogonal become blurred, which is devas- tating for the network architecture. Due to the functional complexity of physical networks and nodes, they make very poor specification objects. They are, however, natural object types for prod- uct definitions, for network design, for network dimensioning and for physical net- work management. Some proprietary modelling methods regard physical network models as primary specification objects. This is in no way encouraged by SM&AML. This approach creates a static, inflexible structure of specifications on higher abstraction levels, which most likely cannot be related to any practical logi- cal network structure. However, this approach can still be supported by SM&AML. By analysing the external interfaces of physical nodes, using the substructuring techniques that are described in Sect. 2.3.4, a set of models on lower integration levels can be created (this approach is a method for reverse engineering). As a matter of fact, reverse engineering has, in the past, been one of the most successful applications of SM&AML. Section 4.2 introduces the concept physical network and subnetwork. Section 4.3 describes how subnetworks are constructed and introduces the concepts physi- cal network level, subnetwork variants and external and internal views on network interfaces. An example of reverse engineering with SM&AML is given in Sect. 4.4. Section 4.5 describes how network service interfaces are mapped on network interfaces in subnetworks. Properties of Physical Networks In this section we will look at the basic definition of what a physical network model is, and is not. The first definition is: a physical network model shows physical nodes, interconnected over physical media connections. Figure 4.1 shows an example. 4.2 Properties of Physical Networks Functional space Physical node, Physical node, not part of the model part of the model Physical media connection Fig