sion paths or switched services on a demand basis. technology are now providing comprehensive capabilities for the Switched paths interconnecting communicating users can be on a establishment and evolution of distributed information systems fully-reserved basis or on a demand basis using various switching fully interconnected and integrated to serve as the foundation of technologies. business around the world. An illustrative distributed information system is shown in Figure These distributed information systems have diverse operational 12.1. The many different types of systems shown contain Applica- requirements and will be supported by continually advancing tech- tion Processes (APs) that may need to communicate. APs can be nologies through a vast world-wide multivendor market-place of manual, computerised, or physical. For example: products. The question that immediately arises is: "How can com- patibility among the variety of systems, designs, technologies, and A manual process could be a person operating a "point of sale" manufactures be realised without constraining innovation, perform- terminal entering data or receiving an output. ance, and ongoing evolution?" A computerised process could be an operational program in a In 1978, the International Organization for Standardization (ISO) "host" computer performing its task, such as an accounting established a massive standards development programme called program processing a payroll. Open Systems Interconnection to establish an architecture and fam- 3. A physical process could be the operation of a "robot" in a ily of standards that will serve as the generic basis for compatibility manufacturing plant. among systems for information interchange. This chapter presents Communications are often required in order for APs to perform the perspective, concepts, and functions of the Reference Model for Open Systems Interconnection (OSI), defined in International their designated tasks. The data that an AP requires may need to be retrieved from a remotely located data