The LPM of an ISDN call-handling layer (state-machine handlers and agent layers) 1.4.2.6 Common Agent Layers Agent layers are just transport mechanisms. An agent layer can therefore be designed to support many actors concurrently. If it does we call it common agent layer (a well known example is TCAP). This principle is also valid for both control layers and connectivity layers, but normally only used for control layers. Figure 1.24 shows an example of a common agent layer that supports two actor layers, {11..13} and {21,22}. The messages of a common agent layer must include parameters for identifying actor layers (ALi) and actors (As). A common agent layer may also support associations between actors (read more about this in Sect. 2.3.2.3). Note that a common agent layer must provide identical actor-agent inter- faces (SAGs) over the whole network in order for actors to be allocated freely in the network. Thus, the SAG for a common agent layer is defined by the agents. Common Common agent ALIA, agent layer Layer element {ALi,As) Actor ALi=1 Fig. 1.24 A common agent layer When the common agent layer principle is applied, the OSI RM layer concept breaks down completely. The actor layers perform separate and independent func- tions in the network. If the actors perform control functions, each actor layer is a control layer of its own, defined by one or several abstract protocols. 1 Introduction to Network System Modeling It is also conceivable that a common agent layer serves a number of independent actor layers that are separate relaying or switching layers. This principle is applied by the PPP layer in the Internet, for example, where PPP supports the IP switching network, as well as a number of other packet-switching networks. 1.4.2.7 Common Actors A common actor is a switching or relaying element. While a common agent is used by several actors, a common actor relies on more than one agent. These agent layers may be of the same or different kinds, depending on the underlying connec- tivity layers that are used. 