ABSTRACT

The Open Systems Interconnection (OSI) Reference Model was formulated by the International Standards Organization (ISO) in the early 1980s with computer networks as its target. The usefulness of such a model was soon recognized by communications engineers and its application extended to other complex communications networks. The model simplifies the design of complex networks by means of the use of a modular and structured approach in which network functions are identified that can be implemented independently of one another. In the OSI Reference Model, network operability is partitioned into seven layers and protocols implement the functionality assigned to each layer. Each layer provides services to the layer above it and uses the services from the layer below it. At the transmitting side, each layer adds its own header to the message received from the layer above it and delivers the composite message to the layer below it. On the receiving side, each layer removes the corresponding header from the message and delivers it to the layer above it.