ocol: Most existing protocols define communication by messages, i.e. PDUs. A PDU is a data element with a defined bit-representa- tion. The PDU normally includes a message type identifier (MTid) that refers to 5.4 Message Handling a description in the protocol specification of how the bits of the PDU shall be interpreted. The only message-handling support for this type of protocol that might be needed in the integrator is adaption to the transport properties of the service that is used by the logical node. If, for example, the transport service supports a small and fixed-length SDU, the adaption function might have to fragment the PDU, and format the fragment to fit into fixed-length SDUs. Message protocols are transparent to transport service constraints, provided that the integrator implements any necessary adaption function. The integrator provides a single (communication) service in its interface to its users, which is why the symbol for service relation is used. The type of service depends on the type of protocol that is supported. To make models more expres- sive, three different symbols are defined for this interface. In order to understand the differences between these interfaces, some understanding of the processing steps in translating a remote operation call to an SDU that can be sent over a trans- port-CSP is needed. These steps are shown in the SM&AML message handling model in Fig. 5.22. This model is an example of interface refinement. It shows how a protocol is refined to different abstractions of the protocol. Event handler Different types Concrete procedure of service elements Concrete object Abstract protocol procedure OSI application protocol Message application protocol Different types of message handlers (MEH) ENC Encoding FOH Format handler IDU Interface data unit Adaption protocol OBH Object handler OPH Operation handler TAH Transport service adaption handler Defines operations that support ASP communication transport IDU Defines operations that support abstract data communication Defines o