Ei=y element Discrimination CEi,data AMLn CON stratum by CEi VC control CEi,data actor network OSI-CON network {Ci,data} {Ci,data} termination layer layer element element Agent CON terminating actor Link and transmission Link and transmission layer element layer element Fig. 2.47 The OSI-CON service modelled as an OSI layer element (to the left) and as an AMLn layer element (to the right) 2.3 Agents and Actors We may recall that, in order to specify where and how discrimination is per- formed in the OSI RM definition of the CON service, we had to define a functional element in an OSI transport-layer element that did not deal with any message-han- dling aspect of this protocol. Such an element violates the OSI RM layer-interface concept (as well as the one in AMLn) since a functional element that connects to a lower layer (over layer interface) must handle a protocol (or a part of it). The actor-agent separation principle solves this contradiction (see the model to the right in Fig. 2.47). The discrimination function is in reality performed by termi- nating actors of a CON switching layer. Multiple users are supported by adding a layer-interface identifier to the T-CO table. It is the responsibility of terminating actors to see to that connection endpoints for each layer interface are unique. Note that the OSI RM model for CON services is identical to X.25 VC, except for how parameters are named (Ci and CEi instead of VCi, etc.). Thus, back in 1984, the OSI RM obviously thought of X.25 as the solution for packet-switching net- works in general. However, we have seen many other CON technologies (FR, ATM, and TCP/IP) appearing since then, which cannot be fitted in the OSI RM CON model. By re-modeling this service using the actor-agent separation, we remove the ambiguity regarding the layer concept and arrive at a more precise and generally applicable definition of the layer interface that is offered by a CON switching layer, without changing anything in external interfaces of X.25 nodes (i.e., network interfaces and