gent actor agent manager Fig. 2.19 One method for providing remote resource access to actors in control layers 14 These are channels that carry user data between a terminal and its LE, once a path is established between the called and calling parties. 2.3 Agents and Actors Note that this is an example where a layer may define several protocols for dif- ferent purposes. The depicted actor is now more than just a "DSS1 L3 actor." It is an actor of the call-handling layer, which is a layer that comprises DSS1 L3 agents, resource agents, call-handling actors, and a remote subscriber-data manager. This emphasizes the importance of separating actors from agents. There are several disadvantages of making resources integrated parts of a layer as in Fig. 2.19. Among others, the using actor must know if a particular resource is available locally or remotely. Resource configuration also becomes an issue for the very layer since a resource using actor must know addresses to remote resources. In addition, if the actor-agent interface is not standardized, every modification of the access protocol will affect the call-handling layer element. A more general solution to resource access is therefore to provide local inter- faces to all kinds of resources. We model such interfaces as layer interfaces of the type resource control point (rCP). In Fig. 2.20, the previous model is redrawn with all resources that the actor needs accessible over rCPs. Call-handling layer element in an LE DSS1 L3 SETUP Call-handling actor agent Subscr. Resource B-channel access agent agent manager manager actor Local resource Separate resource layers Switching actor Switching layer Fig. 2.20 Transparent resource modeling using resource control points (rCP) The protocol for accessing subscriber data needs an asynchronous connectivity service that is accessible in all nodes of an ISDN. We have therefore assumed that TCAP is used (over SCCP). The DSS1 L3 agent is the part that interfaces connec- tivity layer interfaces (to different LAPD layers here), which 