another (the TCAP, part of SS7) is used for some PSTN/ISDN control layers only (e.g., INAP, MAP, and OMAP), and again another (the GIOP) is used for distrib- uted object systems on CORBA platforms. ROSE, TCAP, and GIOP are not com- patible, however. Thus, there exist no standardized definition of SAG primitives that can be used for all network systems. As we understand by now, a prerequisite for such a standard is that all control layers define the same set of parameters for LSM events (which of course implies that they specify the LSM of the layer in the first place). ROSE and TCAP define a kind of invokes that contain similar parameters as the AMLn r-invOrd and I-invRep, with one important exception, however: the AMLn parameter primitive type is not a parameter in the invokes, but used to create a specific invoke for every possible ROSE/TCAP-defined primitive type (as a matter of fact, ROSE defines nine SAG primitives, TCAP defines 17 through its rather strange construction, and AMLn 2.3 Agents and Actors copes with two). This is because ROSE/TCAP confuses LSM primitives with SAG primitives, thereby creating an unfortunate and unnecessary tight coupling between an LSM and the LPM on which it relies (in addition to unnecessarily complex SAGs). 2.3.2.2 Agent Layers and Actor Layers The distinction between the LSM and the LPM representation of a control layer offers an excellent possibility to separate control structures from connectivity structures in layer structures. The key to this modeling approach is to focus on the actor-agent interfaces instead of on layer interfaces. This allows us to define agent layers separately from actor layers, as depicted in Fig. 2.28 for the DSS1 L3 layer. We define an agent layer as a system of agents that communicate by a message pro- tocol, and interface one or several connectivity strata (in this case LAPD strata). A small DSS1 L3-agent layer: DSS1 L3 Call-handl. agent in TE actor in TE Call-handl. DSS1 L3 actor in LE agent in LE DSS1 L3 Call-handl. agent in TE actor in TE LAP