ters for negotiation about the abstract protocol version to be used, authentication, and charging of used services. Release primitives may include parameters that explain the reason for releasing the association. The number and the actual definition of association operations in standards dif- fer a lot, depending on the context. In open network systems, such as the Internet, authentication and charging may be important issues while it is not in closed sys- tems (such as the use of TCAP in ISDN). 2.3.2.4 Modeling the Actor Layers of an OSI Layer Since almost no network system standard applies the actor-agent separation, we will show how actors and actor layers can be identified out of an OSI-control-layer standard. Let's therefore return to the intralayer structure of the ISDN call-handling layer, modelled as an OSI layer (see Fig. 2.34 that is identical to Fig. 2.6). REQUEST, OFFER, etc., are names of AMLs operation classes. An operation class defines a par- ticular set of primitive types. For example, REQUEST defines Request, Confirm, and Refuse. Read more about this in Appendix D. 2 Layer Structures INAP in TCAP Call- DSS11 L3 handling layer handling DSS1 L3 MTPL3 TE TE LE/SSP TE TE Fig. 2.34 The call-handling layer in an ISDN Let's now see how we can identify the actor of the layer element in a combined LE/SSP. This layer element handles three different protocols (more than that in reality). By separating the agent part from the actor part for each of them, we can identify the call-handling actor of an LE/SSP (see Fig. 2.35). Call-handling-layer element in an LE/SSP INAP events carried Call handling INAP AC by TCAP messages actor DSS1 L3 (SSP) messages DSSIL3 DSS1 L3 messages Resources SCCP MTP L3 handling DSS1 L3 actor T-ROs Resources SCCP MTP L3 An ISDN LE/SSP Fig. 2.35 The call-handling actor in an ISDN LE/SSP. AC=actor; AG=agent; T-ROs=routing table for a switching layer The DSS1 L3 standard describes the DSS1 L3 agent and the call-handling actor as seen in the network interface between a terminal (TE) and