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OSI Model
The OSI model for Open Systems Interconnection.

In 1978, the International Standards Organisation (ISO) created a universal standard for exchanging information between and within networks and across geographical boundaries. This standard for network architecture was the seven-layer model for Open Systems Interconnection (OSI). The OSI model has encouraged conformity in designing communications networks and controlling distributed processing.

Manufacturers are developing intelligent computers and equipment, and numerous private- and public-data networks have been created to connect this equipment. But communication among these distributed systems and networks requires a standard approach to network design, one that defines the relationships and intersections between network services and functions via common interfaces and protocols.

The layered approach to network architecture stems from the operating system design. Because of their complexity, most computer operating systems are developed in sections, each of which has a particular function. This makes it simpler to refine each section to meet its functional goal. Ultimately, all sections are integrated to provide complete capabilities and services with a smooth-running operating system.

The same is true in designing networking systems. A network architecture specifies a hierarchy of independent layers that contain modules for performing defined functions.

This translates into a set of rules that defines the way participating network nodes must interact to communicate and exchange information. The OSI model defines standard relationships between the hardware and software in today's complex computer systems.

Each layer of the OSI model (provides specific services that contribute to the overall network functions.

  • The Physical Layer defines the electrical and mechanical aspects of interfacing to a physical medium for transmitting data, as well as setup, maintenance, and disconnection of physical links. When implemented, this layer includes the software driver for each communications device, plus the hardware itself-interface devices, modems, and communications lines.

  • The Data-Link Layer establishes an error-free communications path between network nodes over the physical channel, frames messages for transmission, checks the integrity of received messages, manages access to and use of the channel, and ensures the sequence of transmitted data.

  • The Network-Control Layer addresses messages, sets up the path between communicating nodes, routes messages across intervening nodes to their destination, and controls the flow of messages between nodes.

  • The Transport Layer provides end-to-end control of a communication session once the path has been established, which enables the reliable and sequential exchange of data independent of the systems that are communicating as well as their location in the network.

  • The Session Layer establishes and controls system-dependent aspects of communication sessions provided by the Transport Layer and the logical functions running under the operating system in a participating node.
  • The Presentation Layer translates and converts transmitted encoded data into formats that can be understood and manipulated by users.

  • The Application/User Layer supports user and application tasks and overall system management, including resource-sharing, file transfers, remote file servers, and database and network management.

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