 its LE. The ISUP standard gives the corresponding view of this actor in the network interface between the LE/SSP and another LE or transit node (TR), as is the general rule in standards. The INAP and TCAP standards are the exceptions of the rule, however. TCAP is a protocol that defines a common agent layer. INAP is an abstract proto- col that defines an LSM, called "the intelligent network" (IN) in standards. The depicted INAP actor in Fig. 2.35 handles the SSP function defined in IN, and is an 2.3 Agents and Actors added part of the ISDN call-handling layer. INAP is the abstract protocol that this actor handles in communication with other IN actors of the SCP type. If we analyse the relations between the three parts (DSS1 L3, ISUP, and INAP/ TCAP) of the layer element, we will find that a message that is received by anyone of the agents frequently results in a message sent out by one or several others of the agents. Thus the DSS1 L3, ISUP, and INAP actors are not isolated from each other. On the contrary, they are all fictitious parts of one and the same call-handling actor that takes all the decisions in the layer element that regards the state of the layer. The difference between these parts is only that the call-handling actor uses a stand- ardized SAG (TCAP invokes, etc.) when communicating with the TCAP agent, but means that are defined by the implementor when communicating with the other two agents. If we refine every layer element of Fig. 2.34 into actors and agents, we can draw the model of the call-handling actor layer in two ways, as discussed before: as a layer protocol machine (LPM) and as a layer state machine (LSM). Figure 2.36 shows the LPM. TCAP common agent layer DSS1 L3 T-ROs DSS1 L3- agent layer ISUP agent layer T-ROs SCCP MTP L3 T-ROs LE/SSP TE TE Fig. 2.36 The call-handling LPM in an ISDN In this model, all agent layers are drawn as functional elements of their own, interconnecting actors over actor-agent interfaces. Only TCAP does define a standardized SAG, however. The LSM representation