perations that support PDU communication Fig. 5.22 The message-handling model for logical nodes The model shows the four conceptual levels of communication (not to be con- fused with discrimination levels) that exist in all protocol specifications, whether they are described separately or not. The levels are defined by the four functions 5 Protocol Network Modelling OPH, FOH, ENC and TAH. The type of integrator, and consequently also the type of service element, is defined by how many of these levels are implemented by the integrator. For example, if the integrator implements all four levels, service elements are concrete objects. The interface between the objects and the integrator is, in this case, a concrete language-defined object interface (using operation calls in the case of object-oriented languages). AML provides no symbol for such inter- faces, since they are specific for each concrete language. If the integrator implements the three lowest levels, the service element is an event handler. The interface between the service element and the integrator is, in this case, an implementation of the detectedEventC_REPORT (devC_REP) and generateEventORDER (gevORD) operations we discussed before. These opera- tions support ASP communication between FOH users. AML provides a spe- cific symbol for such interfaces, see Fig. 5.22. If the integrator covers only the two lowest levels, the service element is an OSI application. The interface between the service element and the integrator is, in this case, an implementation of two other AML-defined operations: detectedMessageC_REP (demC_REP) and generateMessageORD (gemORD). These are operations that support communication in terms of abstract data between ENC users. AML provides a specific symbol for such interfaces, see Fig. 5.22. If the integrator covers only the lowest levels, the service element is a message- handling element. The interface between the service element and the integrator is, in this case, an implementation of some type of send and receive operations, i.