Open Systems Interconnection

The Open Systems Interconnection (usually abbreviated to OSI) was an effort to standardize networking that was started in 1982 by the International Organization for Standardization (ISO), along with the ITU-T.Prior to OSI, according to its proponents, networking was largely vendor-developed and proprietary, with protocol standards such as SNA, Appletalk, NetWare and DECnet. OSI was an industry effort, attempting to get everyone to agree to common network standards to provide multi-vendor interoperability. It was common for large networks to support multiple network protocol suites, with many devices unable to talk to other devices because of a lack of common protocols between them. However while OSI developed its networking standards, TCP/IP came into widespread use on multivendor networks, while below the network layer, both Ethernet and token ring played much the same role.
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An abstract description of the digital communications between application processes running in distinct systems. The model employs a hierarchical structure of seven layers. Each layer performs value-added service at the request of the adjacent higher layer and, in turn, requests more basic services from the adjacent lower layer:
Physical Layer : Layer 1, the lowest of seven hierarchical layers. The Physical layer performs services requested by the Data Link Layer . The major functions and services performed by the physical layer are: (a) establishment and termination of a connection to a communications medium ; (b) participation in the process whereby the communication resources are effectively shared among multiple users, e.g., contention resolution and flow control; and, (c) conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel .
Data Link Layer : Layer 2. This layer responds to service requests from the Network Layer and issues service requests to the Physical Layer . The Data Link Layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Note: Examples of data link protocols are HDLC and ADCCP for point-to-point or packet -switched networks and LLC for local area networks.

Network Layer : Layer 3. This layer responds to service requests from the Transport Layer and issues service requests to the Data Link Layer. The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport Layer. The Network Layer performs network routing , flow control, segmentation/desegmentation, and error control functions.

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Pertaining to the logical structure for communications networks standardized by the International Organization for Standardization (ISO). Note: Adherence to the standard enables any OSI-compliant system to communicate with any other OSI-compliant system for a meaningful exchange of information.

Communications system architecture that adheres to the set of ISO standards relating to open systems architecture .

The lowest level of abstraction within the OSI standards scheme. Note: Each OSI--Protocol Specification operates at a single layer . Each defines the primitive operations and permissible responses required to exchange information between peer processes in communicating systems to carry out all or a subset of the services defined within the OSI--Service Definitions for that layer.

The next lower level of abstraction below that of the OSI--Reference Model. The OSI--Service Definitions for each layer define the layer's abstract interface and the facilities provided to the user of the service independent of the mechanism used to accomplish the service.

In the Application Layer of the OSI--Reference Model (OSI--RM), the set of functions related to the management and status of various resources identified in all layers of the OSI--RM.

<networking> (OSI-RM, OSI Reference Model, seven layer model) A model of network architecture and a suite of protocols (a protocol stack) to implement it, developed by ISO in 1978 as a framework for international standards in heterogeneous computer network architecture.
The OSI architecture is split between seven layers, from lowest to highest: 1 physical layer, 2 data link layer, 3 network layer, 4 transport layer, 5 session layer, 6 presentation layer, 7 application layer.
Each layer uses the layer immediately below it and provides a service to the layer above. In some implementations a layer may itself be composed of sub-layers.
OSI is the umbrella name for a series of non-proprietary protocols and specifications, comprising, among others, the OSI Reference Model, ASN.1 (Abstract Syntax Notation 1), BER (Basic Encoding Rules), CMIP and CMIS (Common Management Information Protocol and Services), X.400 (Message Handling System, or MHS), X.500 (Directory Service), Z39.50 (search and retrieval protocol used by WAIS), and many others. Apart from its actual application to real protocols, it also serves as a useful teaching model.
(2004-02-13)