Open System Interconnection (OSI) Reference Model

The Open System Interconnection Reference Model or OSI Reference Model or OSI Model is for the most part a conceptual description for layered communications and computer network protocol design. It was created as part of the Open Systems Interconnection or OSI initiative. In its most simplistic form, it segments network architecture into seven distinct layers: the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. Because of this it is frequently referred to as the OSI Seven Layer Model.

A layer is a collection of ideally similar functions that permit services to the layer directly above it and gets service from the layer beneath it. On each single layer an instance provides services to the instances at the layer above and requests service from the layer beneath. An example of this relates to a layer that provides error-free communications across a network which then provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that comprise the contents of the path.

Work on a layered model of network architecture commenced in the year 1977. Simultaneously the International Organization for Standardization or ISO started to develop its OSI framework architecture. OSI has two major constituents: an abstract model of networking, called the Basic Reference Model or seven-layer model and a set of specific protocols. It is vital to note that the standard documents that describe the OSI model can be freely downloaded from the ITU-T as the X.200-series of recommendations.

A number of the protocol specifications can also be accessed as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model are available from ISO, however not all of these are available free. Every area of the OSI design evolved from experiences with the CYCLADES network, which also had a great influence on the design of the internet. The latest design has been meticulously documented in ISO 7498 and several of its addenda. In this latest model, a networking system is separated into layers. Within each layer, one or more entities establish its functionality. Each entity will interact directly only with the layer just below it, and enables facilities for use by the layer above it. Protocols will allow an entity in one host to interact with a corresponding entity at the same layer in another host.
Service definitions ideally describe the functionality provided to an (N)-layer by an (N-1) layer, where N is one of the seven layers of protocols operating in the local host. Neither the OSI Reference Model nor OSI protocols designate any programming interfaces, other than a purposely abstract service specifications. Protocol specifications exactly define the interfaces between multiple computers, but the software interfaces within computers are implementation-specific. Take for example, Microsoft Windows’ Winsock, and Unix’s Berkeley sockets and System V Transport Layer Interface, are interfaces between applications (Layer 5 and above) and the transport (Layer 4). NDIS and ODI are interfaces between the media in Layer 2 and the network protocol in Layer 3. Interface standards, with the exception of the Physical Layer to media, are estimated implementations of OSI Service Specifications.

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