An Implementation of the OSI Upper Layers and Applications

1 Abstract Above the transport layer, the open systems interconnection (OSI) basic reference model describes several application standards supported by a common upper layer protocol stack. Digital's high-performance implementation of the upper layers of the protocol stack concentrates on maximizing data throughput while minimizing connection establishment delay. An additional benefit derived from the implementation is that, for normal data exchanges, the delivery delay is also minimized. The implementation features of Digital's two OSI applications-file transfer, access, and management (FTAM) and virtual terminal (VT)-include the use of common code to facilitate portability and efficient buffer management to improve performance. The open systems interconnection (OSI) basic reference model defined in the International Organization for Standardization standard ISO 7498-1 specifies a layered protocol model consisting of seven layers.[1] By convention, the first four layers-physical, data link, network, and transport-are referred to as the lower layers.[2] These layers provide a basic communication service by reliably transferring unstructured user data through one or more networks. The remaining layers-session, presentation, and application-build on the lower layers to provide services that structure data exchanges and maintain information in data exchanges to support distributed applications. These three layers are known collectively as the upper layers. This paper first gives an overview of the OSI upper layers and of two application standards-file transfer, access, and management (FTAM) and virtual terminal (VT). The discussion that follows concentrates on the features of Digital's implementation of the upper layers and the two applications, with emphasis on novel implementation approaches. 2 Summary of OSI Upper Layer Standards The application-independent parts of the OSI upper layers are defined in the following standards: