Web and XML Glossary

OSI is a reference model for computer communications. It was created as a competitor of the Internet Protocols. While OSI as a whole has not been very successful, parts of it are widely used, and many concepts pioneered in OSI are slowly finding their way into Internet technologies. OSI is defined in terms of different "layers". OSI-based applications belong to the "application layer" (layer 7) of the OSI "Basic Reference Model". The application layer resides above the "presentation layer" (layer 6), which identifies alternative encodings, and the "session layer" (layer 5), providing dialogue control. Collectively, these three layers provide application services, and are commonly referred to as the "upper layers". The "lower layers" of the OSI stack are "transport" (layer 4), "network" (layer 3), "link" (layer 2) and "physical" (layer 1).