e. operations that support PDU communication between TAH users. The con- nection/association endpoint symbol is used for such interfaces, see Fig. 5.22. In the following, we will describe each function by following how a remote pro- cedure call invocation, that is executed by the concrete object, is handled by these functions until it can be sent as SDUs over the transport-CSP. 5.4.2 Operation Handling The operation handler, OPH, is part of an event handler. The OPH interfaces concrete objects that communicate by language-dependent remote procedure calls. A concrete object is an implementation, written in a suitable design language. The OPH maps language specific procedure calls on ASPs that are defined by operations of the abstract protocol. An abstract protocol is describe by some abstract (interface description) language. Examples of such languages are AML and IDL (the interface description language used in CORBA). The OSI ROSE standard (see ITU-T X.219) also defines a syntax for specifying kinds of ASPs. 5.4 Message Handling The OPH hides concrete object interfaces and makes abstract protocols visible. Thus, the most important purpose of an OPH is to create language transparency, regarding communication between concrete objects. We will demonstrate the functions of an AML-operation handler by the scenario in Fig. 5.23. In the upper part of the figure, two service objects communicate (about service object symbols, see Sect. 7.2). The provider service object manages a personnel register. One of the operations that the user service object can request from the provider object is to change address parameters. This operation has the name chAddr ("change address") and is a REQUEST-class operation. According to the AML operation classification system, a REQUEST-class operation implies that the user service object initiates a Request primitive, and that the provider object must respond to that request after having performed the operation. Two alternative responses exist: the provider object responds with a Confirm pri