sk, since it assumes that such resources exist locally in nodes. This view has never reflected the architecture of real network systems. Resources in terms of data are often pooled in separate data base nodes in a network. With the event of both the intelligent network concept (IN) and the Internet and UMTS, other resources (such as switches and diverse media devices) are pooled separately as well. Furthermore, resources that exist locally in layer elements (diverse tables and data) must be man- ageable from remote management nodes. In AMLn, therefore, all resources are made visible in a layer-structure model, with some as local resources, others as resource layers (see Fig. 1.16). When you look at a conventional message-protocol specification, the behavior of an actor is normally described in terms of states only. For example, one state may be "service request is accepted." However, this "state" represents that certain resources on the performing actor's site are used (e.g., a subscriber data record) and 1 Introduction to Network System Modeling other resources are dedicated to the service request (e.g., a path through a switch). Which these resources are is of no interest to the requesting actor. Therefore, these resources must be defined separately in an extra refinement step, according to Fig. 1.16. Such resources may be "local," i.e., they exist in the actor's node, or "remote," i.e., they exist in a remote node, or are distributed to the actor's node and remote nodes. Remote and distributed resources are parts of a resource layer. Resource handler Local Resource resource layer Resource handler Local Resource resource Remote agent resource Fig. 1.16 Resource identification and modelling. rCP=resource control point A resource layer may further depend on other resource layers, i.e., part of the layer structure of a network system may be a layer structure of resource layers. Thus, resource layers appear as any other layers in a layer structure, which is why we define a special control point symbol, the resource