D strata Call-handling-actor layer and DSS1 L3-agent layers: Control Connectivity structure structure (actors only) (agents and Call-handl. connectivity actor in TE layers only) DSS1 L3 Call-handl. agent DSS1 L3- actor in TE layer Call-handling agent actor layer layer Call-handl. Call-handling- actor in LE actor layer Fig. 2.28 The actor layer and agent layer representation of DSSI L3. TU=terminal user The three DSS1 L3 agents together constitute a DSS L3 agent layer in this case. The set of actors belong to the actor layer. To the lower right a model that separates the call-handling layer into its control structure and connectivity structures. This model hides the distribution of agents and actors completely, and is therefore valid for any node structure of an ISDN. 2 Layer Structures The benefit of the agent layer notation is that all LPM aspects (i.e., message pro- tocol) are confined to an agent layer, and all LSM aspects to an actor layer. The relation between the actor layer and the agent layers on which it relies are fully defined by defining AAIs as SAGs. As we know, this implies that modifications of the agent protocol, as well as other changes of the layer structure on the connectiv- ity-structure side are transparent to an actor layer, as long as the SAG remains the same. The separation of actor layers from agent layers shows a property for control layers that cannot be made visible in a layer structure that rely on the OSI RM layer concept: the distinction between a control structure and a connectivity structure model. We can define the control structure that is realized by an actor layer by extracting the LSM out of the call-handling-actor layer, as we have done in Fig. 2.29. A DSS1 L3-actor layer: The corresponding LSM: Call-handl. Call-handling Call-handling actor in TE ESM in TE ESM in TE Call-handl. DSS1 L3 actor in TE resources abstract protocol resources Call-handl. Call-handling Call-handling- ESM in LE actor in LE actor layer LE resources Fig. 2.29 The actor layer and its LSM representation f