ABSTRACT
The application layer is the highest layer of OSI-RM and provides an interface between OSI-RM and the application processes which use OSI-RM to exchange the data. The application process uses application entities and application protocols for exchange of information with OSI-RM. It receives services from its lower layer, the presentation layer, which are also used during information exchange. The presentation layer is the only layer which provides services directly to the application processes. The interface points between the top three layers (application, presentation, and session) are different from those of the lower layers. The lower layers do not offer services and functions independent of each other; i.e., the services and functions are supported by many layers. In the case of higher levels, in many applications, the presentation layer may not provide any services to the dialogue unit primitives between application and session layers. For example, the token primitives P-TOKEN-PLEASE and S-TOKEN-PLEASE are used between application and session layers, and when these are passed through presentation entities, the presentation layer simply passes them to the session layer (this is due to the fact that most of the service primitives of the presentation layer are similar to those of the session layer, as discussed in previous chapters).
