OSI Model - Networking Concepts

[VelMurugan]

OSI Model

What is the OSI model?

The OSI (Open System Interconnection) model is a standard developed by the ISO (International Standards Organization) to describe the flow of data on a computer network. The model takes into account the flow of data from the physical connections up to the end user application.

The OSI model defines a "layered" architecture in the form of a protocol stack. There are specific, discrete functions that take place at each layer of the protocol stack with lower level layers providing services to upper layers. When two systems communicate on the network, information is sent down through the protocol stack of one system, over the cable and then up through the protocol stack to the appropriate layer on the other system.

Data flow under the OSI model is organized into the following seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

What is the purpose of the OSI model?

The OSI model (as well as IEEE 802 and NDIS) were established to standardize the design and construction of computer networks for developers and hardware manufacturers. Standards allow hardware and software components from a variety of different vendors to operate together. Without standards, everything would be vendor-specific and interoperability would suffer.

Almost all vendors implement standards in slightly different ways. This is normally done to take advantage of unique functionality in their product that just won't fit neatly into the model. However, if a vendor strays too far from the standards, they risk creating a 'closed' or proprietary product that is not supported by the rest of the industry.

IBM mainframes use Systems Network Architecture (SNA) that is a set of layered protocols like the OSI model. However, the SNA layers are not directly comparable to the OSI model layers. This makes interoperability between PC-based networks and IBM mainframes more difficult.

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