encoding function for supporting application protocols is defined as a separate layer (the presentation layer, based on Basic Encoding Rules, BER, see recommendation ITU-T X.209). The AML message-handling model is more generic, since it can be used to specify all types of protocols, not just OSI applica- tions. The actual type of encoding will, of course, vary between different types of logical networks. For example, it is not likely that BER will be used to encode transport protocols, or protocols that use low-capacity transport services. Such cases are, e.g. the ISDN service access protocols, service access protocols over nar- row-band radio interfaces and wherever processing capacity is scare. The interface between FOH and the encoder (ENC) is defined only because it represents the OSI presentation layer interface to the OSI application layer. Besides the OSI architecture, I know of no other architecture that separates format handling from encoding, particularly not as a layer interface. In the CORBA architecture, the FOH and ENC functions are integrated in the GIOP, as they are in TCAP. 5.4 Message Handling The purpose of the OSI presentation layer is to provide a general encoding serv- ice for application layer units. The service translates data in local representation to data that are described as ASN.1 data types, and that are encoded by some set of encoding algorithms. The application layer in OSI includes the FOH function (OSI ROSE). FOH delivers data to the ENC in local representation and provides a data type specification in ASN. for that data. The data specification is called by OSI an abstract syntax. For operation-oriented application units, it is constructed from typed operation specifications and from the message specification. The conceptual operations for interactions between FOH-ENC interfaces are generateMessageOR- DER (gemORD) and detectedMessageC_REPORT (demC_REP), see Fig. 5.27. Tuead Tuned oporation Typed operation reference specification Message specification gemOrd(ASPid, localValueSet) 