odes. The N dimension only defines which logical nodes exist and how they are con- nected. The nodes that exist in a network, however, are both connected to other nodes and run a number of layer elements that are interconnected over layer inter- faces. To support mapping an AMLn model on a model that defines node objects in real networks, AMLn also defines how to model network structures of derived nodes, as shown in Fig. 1.11. Derived node Network interface Fig. 1.11 Derived nodes and network structures Derived nodes are entities that exist both in the L and N dimensions. The rela- tion between derived nodes are network interfaces that comprise a route as well as one or several protocols. The term "derived" indicates that this kind of model is not specified separately. It can be extracted by a modeling tool out of the layer and node structures upon a user's request, e.g., when an AMLn model is to be trans- lated to a UML model. 1 Introduction to Network System Modeling 1.4.1.2 Management Plane and Managed Plane A network system concept such as PSTN/ISDN or ATM, actually comprises two separate planes, a managed plane (we prefer "traffic plane") and a management plane, since no network can survive without some degree of control from manage- ment functions. Each of these planes describe its own L and N dimensions, which implies that the two planes define different layers and layer interfaces as well as nodes and routes. The relations between these planes is therefore of a special kind that we define in a separate dimension called the management dimension (M), conceptually depicted in Fig. 1.12. manages Managed Management plane functions plane functions serves Fig. 1.12 The management dimension (M). LT-NT=managed plane; LM-NM=management plane; T=traffic (plane), another term for managed (plane) The management dimension consists of two separate relations, called MR and Mc in the figure: The management plane consists of functions that manage resources existing in the traffic plane. These resources are commonly accesse