or a small DSS1 L3 layer Note that DSS1 L3 defines a single abstract protocol. In general, however, an LSM model may include several abstract protocols. As a matter of fact, the LSM model in Fig. 2.29 is just a small part of the complete LSM model for call handling in ISDN. A more complete model is discussed in Sect. 2.3.2.4 (see Fig. 2.37 in par- ticular). 2.3.2.3 Common Agent Layers The DSS1 L3 standard does not define a SAG for the AAI between its actors and agent layer, since the agent layer is not designed for being used by anything other than a single DSS1 L3-actor layer. In case we want to design an agent layer that can support multiple actor layers, we call it common agent layer (which is a system of common agents). One prerequisite for such a layer is that it specifies a SAG, as for ROSE, TCAP, and GIOP. Another is that actors can identify each other over the network in which the common agent layer runs. In the DSS1 L3 case, this was not an issue (see the upper model in Fig. 2.28): every TE is connected semi-perma- nently over its own LAPD stratum to the LE and therefore need not use any kind of 2.3 Agents and Actors identifier. An LE just chooses the LAPD stratum that also serves the actual TE. The same goes for all kinds of link layers. For a common agent layer, actor identification is an issue, however. This is due 1. Common agents will most likely run on switching networks, which requires that an agent must know the network address of the agent that handles the AAI to the wanted actor. If one wants to avoid making actors dependent on where they are allocated in a network, an actor identification system is needed. 2. Most likely we want a common agent layer to be able to support many inde- pendent actor layers simultaneously. In that case we do not want an actor to communicate with an actor that belongs to another actor layer. Obviously, this requires actors to be identified with respect to which actor layer they belong. 3. A common agent layer may be used for simple request-reply types of interac- ti