Figure 1.25 shows the LPM of a typical PSTN/ISDN switching layer, which relies on many different types of agent layers. BRA or PRA PDH agent layer agent layer route BRA or PRA transmission transmission BRA or PRA transmission agent layer layer transmission BRA or PRA transmission routes SDH transmission TR LE Fig. 1.25 The LPM of an ISDN switching layer (common actors and agent layers). CS=control store; T=terminal The technology for connectivity layers, and thereby also for agent layers, is in constant evolution. It is therefore important for actors that switch and relay to not depend too much on changes in transport technology. The actor-agent interfaces (i.e., SACs) between a common actor and agents is therefore defined by the actors and is uniform over the network. In most switching networks, a small set of differ- ent SACs are defined in order to connect to different types of agent layers (e.g., one or a few SACs for access and one or a few for network internal transport). 1.4.3 Modeling Node Structures The rationale for separating layer structures from node structures was discussed already in Sect. 1.4.1.1. Basically, for all network systems and networks, the things that are depicted in these structures are handled by different activities, at different points in time and with different frequencies of occurrence. While layer structures 1.4 Modeling Network Systems in AMLn describe how layers are related over functional interfaces called "layer interfaces," a node structure shows some kind of nodes, interconnected by some kind of "routes." Packaging layer elements of a layer structure (thereby producing node defini- tions), can be made in many ways however. Furthermore, there are not only many types of connections around, we also know that one type of connection can be a bearer of other types of connections, such as 64 kbps bitstream connection that car- ries a voice connection, or a route in a switching network that defines a number of channels. We discussed in Sect. 1.4.1 the node concept that is defined in