y the circuit-switching symbol defines the layer as a circuit- switching layer. Actors support connectivity service points (actors in control layers support con- trol points only). The fact that all actors support a resource control point (in control layers, only terminating layer elements do). The most important difference between control layers and connectivity layers is, however, that there exists no layer state machine (LSM) representation for the lat- ter. This is because these actors do not interpret data, they only relay and/or termi- nate data, Consequently, there are no states to be controlled, no abstract protocol to be handled, and therefore also no need for any meta-primitives (i.e., invokes). 2.3 Agents and Actors We must therefore find another way to describe the relation between actors of such layers than by means of abstract protocol relation. The only relation between actors in a connectivity layer is that they are interconnected over some type of "connections." These are, in the circuit-switching case, realized by synchronous connectivity layers, and in the X.25 case by (asynchronous) LAPB layers. In AMLn we use the generic term route for all types of connections (read more about this subject in Chap. 3). Suffice here to say that a "route" terminates in logi- cal nodes of a logical network. By including an actor of a connectivity layer, all its agents, and all its users as functional parts in a logical node, we can let routes abstract how switching nodes are interconnected, and thereby create a simple logi- cal network model. Figure 2.42 exemplifies this approach for the ISDN circuit- switching network. The LPM representation of an ISDN switching layer: A node in the switching network BRA or BRA or PRA transmission agent layer layer transmission SDH transmission BRA or PRA BRA or PRA transmission transmission The logical-network transmission representation of the layer Fig. 2.42 The switching layer of an ISDN Note that logical node symbols are chosen SO that logical nodes cannot be con- fused wi