Home Electronic Projects

The Open Systems Interconnection Model

A communication framework which has had a tremendous impact on the design of LANs is the Open Systems Interconnection (or OSI) model. The objective of this model is to allow existing and developing standards to be placed in a common framework to ensure open connectivity between different systems.

The International Standards Organisation define the purpose of the OSI model: '...to provide a common basis for the co-ordination of standards development for the purpose of systems interconnection, while allowing existing standards to be placed into perspective within the overall Reference Model.'

It should be realized at the outset that the OSI Reference Model is not a protocol or set of rules for how a protocol should be written but rather an overall framework in which to define protocols.

A summary of the seven different layers of the OSI model is given below.

1. Application Layer - File transfer, message exchange.
2. Presentation Layer - Data format or representation.
3. Session Layer - Organisation and synchronisation of the data exchange.
4. Transport Layer -  Channel for transfer of messages from one application process to another.
5. Network Layer - Optimum routing of messages from one network to another.
6. Data Link Layer - Framing and error correction format of data.
7. Physical Layer -  Electrical and mechanical definition of the physical system.