Abstract :
In the uppermost layer of the OSI 7-layer model are applications. These are processes which make use of the underlying communications architecture and services. Their purpose is to handle data on behalf of users: for storage, manipulation and display, as well as for transfer. This application layer is itself layered. In particular, the applications processes manipulate information which is encoded in a defined structure, which has specific semantics. The encoding often represents numeric and alphabetic characters. To successfully achieve the open interchange of such information, it is necessary to have an agreed international standard representation. There is a range of character encoding schemes (from IS 646 to DIS 10646); the author briefly describes one prime candidate for document encoding, and compares it with three other structural mechanisms for imparting semantics upon data streams. The document encoding scheme ODA, is ASN.1 (Abstract Syntax Notation 1), EDIFACT and SGML (standard generalized markup language)
