 the L dimension Associated to this model are all layer-interface specifications and protocol specifications, since these are independent of the actual structure of interconnected nodes in a network of the kind. Figure 1.10 shows the network system of Fig. 1.9 in the N dimension. Fig. 1.10 The network system in the N dimension 1.4 Modeling Network Systems in AMLn This is obviously a model of one of many specific networks that can be built from the definition of the network system in the L dimension. This indicates the primary reason for keeping layer structures and node structures apart. Defining and maintaining the L- and N-dimensional views of a network system are two very dif- ferent activities, regarding the incitements for modifying these structures, when in time modifications are made, and who manages the changes. Layer structures are modified as a result of functional requirements, while node structures are modified as a result of nodes being introduced or removed in a network and/or routes being changed, which is not necessarily related to any modifications that affect the L-dimensional view. The L-dimensional view is modified as a result of functional requirements defined by implementors or their customers. These are relatively rare events, which implies that the L-dimensional model is rather stable. Furthermore, modi- fications to the L-dimensional view may in due time result in modifications of some networks, but not necessarily in all existing networks. The N-dimensional view is modified by a network operator, mainly to respond to needs of increasing capacity and for dealing with reliability problems in his network. The N-dimensional view therefore changes more often than the L- dimensional view. Figure 1.8 showed that a network interface includes both routes and (one or several) protocols. When we show the system in the L dimension only, we do not know or say anything about routes. When we show the system in the N dimension only we do not know or say anything about the contents of layer elements in n