he actor-agent separation Fig. 1.43 Logical dimensions defined by AML 1.4.7 Views A view is, by definition, whatever part of an AMLn model to which a user has access, and that the modeling tool can display. The modeling dimensions and the structures defined through substructuring (such as stratum structures and the struc- ture of network levels) are often enough for view selection. Views may be restricted to read and write accesses, however, and to which attributes a particular user has access. Furthermore, it should also be possible to select views that are smaller fragments of the network system model than those defined by modeling dimensions and substructures. For example, a user may want to analyze the behavior with only some of the nodes of a particular logical network involved (typical for when SO called "protocol architectures" are designed). The smallest possible view is a single attribute of a single logical node, an agent, or an actor (e.g., the state diagram describing the behavior of a controlling actor). Which views are actually supported is, however, a matter for tool designers to consider. 2 Layer Structures Concepts Based on the OSI RM 2.1.1 Layers and Layer Structures A common appreciation of the concept "network" is that it consists of nodes (machines, such as switches and terminals) that are interconnected by physical media (wires, fibres, radio channels, etc.). Inside a node exists a lot of hardware (i.e., electronic circuits) and software elements, the latter performing most of the functions of the network when executed by the node's processing system. Some elements communicate between nodes by means of message exchange. When this exchange is described, we normally call that description (message-) protocol spec- ification (the bulk of network system standards consists of these types of docu- ment). Such a model of a network is very implementation-oriented, however. It depends on the type of processing system and the technologies that are used to implement the hardware and the software. Since 