Open Systems Interconnection

Open Systems Interconnection. The Open Systems Interconnection is a conceptual model created by the International Organization for Standardization to standardize the functions of a telecommunication or computing system. It partitions communication flows into seven abstract layers, from physical hardware to software applications, to enable interoperability between diverse products and software. The model's formal definition is provided by the ITU-T Recommendation X.200, and it has served as a fundamental teaching tool and reference point in computer networking.

Overview

The development of the Open Systems Interconnection framework began in the late 1970s, driven by the need for multivendor interoperability as networks like ARPANET expanded. Pioneering work by experts such as Hubert Zimmermann and contributions from organizations like the Comité Consultatif International Téléphonique et Télégraphique culminated in its formal publication. The model was a direct competitor to the emerging protocol suite developed for the United States Department of Defense, which later evolved into the Internet protocol suite. A key goal was to create a universal standard that would allow systems from manufacturers like IBM and Digital Equipment Corporation to communicate seamlessly, moving beyond proprietary architectures such as IBM Systems Network Architecture.

OSI model layers

The model is defined by seven distinct layers, each serving a specific function and providing services to the layer above it. The Physical Layer involves the electrical and physical specifications, dealing with hardware elements like Ethernet cabling and network interface cards. The Data Link Layer provides node-to-node data transfer and error correction, with protocols like IEEE 802.3 and technologies such as MAC addresses. The Network Layer handles the routing of packets across different networks using logical addressing, a function performed by protocols like the Connectionless Network Protocol and devices like routers. The Transport Layer ensures complete data transfer, with services ranging from the connection-oriented Transport Protocol Class 4 to simpler datagram services. The Session Layer manages dialogues and synchronization between applications, while the Presentation Layer translates data between network services and applications, handling tasks like encryption. The Application Layer, the topmost layer, provides interfaces for software applications such as File Transfer Protocol and X.400 message handling.

Protocol suite and standards

Alongside the reference model, a full suite of protocols was developed to implement its specifications. These were standardized in documents such as the ITU-T X.200 series and ISO/IEC 7498. Notable protocols include the Connectionless Network Protocol for the network layer and the X.25 packet-switching standard. For the transport layer, the Transport Protocol Class 4 was a major offering. At the application layer, standards like X.400 for email and the File Transfer, Access and Management protocol were created. These specifications were advanced through international meetings and working groups, often held in cities like Geneva and involving bodies like the European Computer Manufacturers Association.

Comparison with TCP/IP model

The Open Systems Interconnection model is frequently contrasted with the practical Internet protocol suite, often called the TCP/IP model. While the Open Systems Interconnection model is a rigorously defined, seven-layer theoretical framework, the TCP/IP model is a four-layer description of the protocols that actually power the Internet, such as the Transmission Control Protocol and the Internet Protocol. The Internet Engineering Task Force, led by figures like Vint Cerf and Bob Kahn, focused on the pragmatic development and deployment of TCP/IP, which was championed by institutions like the Defense Advanced Research Projects Agency. This divergence led to the so-called "protocol wars" of the 1980s and 1990s, where the widespread adoption of TCP/IP by networks like the National Science Foundation Network ultimately limited the commercial implementation of the full Open Systems Interconnection protocol suite.

Implementation and adoption

Full implementation of the entire Open Systems Interconnection protocol suite was limited, though certain components saw significant use. The X.400 messaging and X.500 directory standards were adopted in some corporate and governmental environments, particularly in Europe. Networking equipment from companies like Cisco Systems and Nortel often used Open Systems Interconnection terminology in their architecture. However, the explosive growth of the Internet, based on TCP/IP, cemented that suite's dominance. The model's primary legacy is as an invaluable pedagogical tool for teaching network architecture in academic institutions worldwide and as a reference for standardizing other technologies, influencing later work in areas like Asynchronous Transfer Mode and the IEEE 802.11 standard for wireless networking.

Category:Network protocols Category:ISO standards Category:Computer networking