is the only resource this agent needs for its operation. The actor interfaces all other types of (local and remote) resources that are needed for call handling. By using rCPs, the call-handling actor is (in principle) completely independent of if resources exist locally or remote, provided that rCPs are standardized and pre- served. The rCP notation supports modeling both local and remote access. For example, the B-channel manager is drawn as a single functional element that has no associated intralayer structure because we define it as local resource, i.e., it must exist in the same node as the call-handling actor. We may later add an intralayer structure to this element which (in principle) is transparent to the this actor. The 2 Layer Structures other resources are drawn as parts of resource layers, however. Such layers will show up in the layer structure of the network system as layers separated from (in this case) the ISDN call-handling layer. Let's now turn to the behavior and functions of the actors. As we have seen, the agents perform a generic functions that is independent of what the actors do, i.e., of the semantics of the actor-actor communication. The function of actors in a control layer can generally be described as controlling the state of the layer, which is related to the resources that are implied. For example, before the SETUP arrived, there was no call established through the network. This state was represented by the fact that the corresponding B channels managed by the B-channel manager were idle, and that no paths existed through the switching actor for the call requested by the SETUP. As a result of SETUP reception, the call-handling actor will change this situation: a B channel is now made busy and a path through the switching actor is established. This is interpreted as a new layer state that was real- ized by, and will be supervised by, the actor of the call-handling layer element. A state machine is a way of describing a behavior. 15 We therefore say that an actor of a control layer h