OSI model

OSI model. The Open Systems Interconnection model is a conceptual framework that standardizes the functions of a telecommunication or computing system into seven distinct layers. Developed by the International Organization for Standardization during the late 1970s, it was formally published in 1984 as standard ISO/IEC 7498-1. The model aims to facilitate interoperability between diverse network systems and equipment from different vendors, serving as a foundational reference for understanding network protocol design and communication processes.

Overview

The creation of the OSI model was driven by the need for a universal standard amidst the proliferation of incompatible computer network architectures from companies like IBM and Digital Equipment Corporation. A key contributor to its development was Charles Bachman of Honeywell, who presented the initial layered concept. The model's architecture was significantly influenced by earlier work such as the ARPANET network and the CYCLADES project. Its formal ratification involved collaboration between the International Organization for Standardization and the International Telecommunication Union, specifically its Telecommunication Standardization Sector. The model's primary goal is to provide a common basis for the coordination of standards development for system interconnection, while allowing existing standards to be placed into perspective within the overall architecture.

Layer architecture

The model partitions communication functions into seven abstraction layers, often grouped into the upper layers (5-7) and lower layers (1-4). Layer 1, the physical layer, defines electrical, mechanical, and functional specifications for activating and maintaining the physical link, dealing with elements like cable types and modulation. Layer 2, the data link layer, provides node-to-node data transfer and handles error correction from the physical layer, with protocols like Ethernet and Point-to-Point Protocol operating here. Layer 3, the network layer, manages the routing and forwarding of data packets across different networks, a function central to the Internet Protocol. Layer 4, the transport layer, ensures complete data transfer, with protocols like Transmission Control Protocol providing reliable, connection-oriented service. Layer 5, the session layer, controls the dialogues (sessions) between computers, establishing, managing, and terminating connections. Layer 6, the presentation layer, translates data between the application layer and the network, handling encryption and compression. Layer 7, the application layer, is the interface for user applications to access network services, such as File Transfer Protocol for file transfers.

Protocol examples

While the OSI model itself is a theoretical framework, specific protocols are often mapped to its layers to illustrate its concepts. At the physical and data link layers, standards like IEEE 802.3 for Ethernet and IEEE 802.11 for Wi-Fi are prime examples. The X.25 protocol suite, developed by the International Telecommunication Union, aligns closely with the lower three layers. For the network layer, the Connectionless Network Protocol was developed as an OSI counterpart to the Internet Protocol. The transport layer features the OSI transport protocols, which include classes like TP0 and TP4. In the upper layers, protocols such as X.400 for message handling and X.500 for directory services were developed within the OSI suite. The File Transfer, Access and Management protocol was an OSI application layer protocol for file manipulation.

Comparison with TCP/IP model

The OSI model is frequently compared to the practical Internet protocol suite, often called the TCP/IP model, which originated from the work on ARPANET funded by the Defense Advanced Research Projects Agency. The TCP/IP model consolidates the OSI's application, presentation, and session layers into a single application layer. Its transport layer correlates directly, featuring Transmission Control Protocol and User Datagram Protocol. The TCP/IP internet layer is equivalent to the OSI network layer, and its link layer combines the OSI's data link and physical layers. This simpler, four-layer architecture was championed by proponents like Vint Cerf and Bob Kahn, and its protocols, particularly the Internet Protocol, became the foundation of the global Internet, leading to the OSI model's relegation primarily to a teaching and design tool.

Implementations and adoption

Full implementations of the OSI protocol suite, sometimes called the OSI protocol stack, were developed by various organizations and governments. In Europe, initiatives like the European Strategic Programme on Research in Information Technology promoted its use. The United States Government initially mandated OSI protocols through the Government Open Systems Interconnection Profile, and it saw use in certain telecom networks. Companies like Digital Equipment Corporation implemented OSI protocols in their DECnet Phase V architecture. However, widespread commercial adoption was eclipsed by the explosive growth of the Internet, which relied on the simpler TCP/IP suite. Consequently, the OSI model's primary legacy is as an invaluable pedagogical tool for teaching network architecture, and its terminology and layer concepts remain universally used in network engineering and protocol analysis discussions. Category:Network architecture Category:ISO standards Category:Telecommunications standards