Two directions were initially pursued in the development of the standards. Firstly, the provision of application-oriented basic services for standard applications such as file transfer, electronic mail and file sharing. The other was to provide basic functions within the application layer that could be used by specific applications. Therefore, there are two classes of service elements within the application layer. Commonly usable application service elements( ASE), the Common Application Service Elements( CASE), and application-specific elements, the Specific Application Service Elements( SASE). Service elements of SASE include File Transfer Access and Management( FTAM) and Virtual Terminal Service (VTS). An example of an ITU-T recommendation is the Message Handling System( MHS).
When structuring the application layer, however, it has become apparent that the separation between CASE and SASE is not always practical, since each application-oriented basic service not only implies its own data structure for layer 6, but also, for example, a specific directory and management structure. For this reason, the separation is dispensed with and we speak here generally of application service elements, but distinguish the application control service element( ACSE) and the user element in these. The Single Association Controlling Function (SACF) monitors the interaction of the Application Control Service Element (ACSE) and the local Application Service Elements, while the Multiple Application Control Function (MACF) is responsible for cross-systeminteraction. Other components relate to network management and system management.
The DIN specification for the application layer is as follows: In the application layer, as the highest layer of the reference model, the information processing function manifests itself in a communication. Communicating application instances form distributed applications.