finement of the OSI RM view on this subject) consists of a structure of layers that communicate over layer interfaces. This structure may be simplified to'a structure of strata by aggregating layers into strata. A layer consist of layer elements that communicate according to protocols. Communi- cation over layer interfaces and according to protocols represent the behavioral aspect of a network system. A node structure consists of a structure of nodes, interconnected over routes. "Route" is a general concept used for all types of connections (single or multiple channels, synchronous or asynchronous, connection-oriented or connection- less). When nodes of an AMLn model are mapped on elements of a common-value model (e.g., a UML model) they also exist in a processing view, which means that they have access to processing platform functions. Such functions are 1 Introduction to Network System Modeling defined first in common-value models, and are therefore of no concern to an AMLn model. Layer structure and node structure are two views of the same thing. By defining how these views are related, we create a complete model of a network. This rela- tion is simply that a functional element must be allocated to a node since other- wise it has no access to connectivity (i.e., it cannot communicate with other elements), nor can it have access to processing functions (i.e., it cannot execute). We can depict the relation between the two views as that a network system exists in a plane defined by two logical dimensions, called the layer dimension (L) and the node dimension (N), se Fig. 1.8. Protocol Layer element Layer Layer interfaces Network interface Route Fig. 1.8 A network system in a plane defined by the layer and node dimensions Each view is maintained separately in a network system model. For example, the network system in the L dimension is depicted by eliminating the N dimension (see right side of Fig. 1.9). Described by: Protocol specification Described by: Layer interface specification Fig. 1.9 The network system in