OSI (Open Systems Interconnection) Reference Model

The OSI model is based on a proposal developed by the International Standards Organization as a first step towards international standardization of the protocols used in the various layers. The model is called the ISO - OSI (International Standard Organization - Open Systems Interconnection) Reference Model because it deals with connecting open systems — that is, systems that follow the standard are open for communication with other systems, irrespective of manufacturer.

Its main objectives were to:

• Allow manufacturers of different systems to interconnect equipment through standard interfaces.
• Allow software and hardware to integrate well and be portable on different systems.

The OSI model has seven layers shown in Figure below. The principles that were applied to arrive at the seven layers are as follows:

• Each layer should perform a well-defined function.
• The function of each layer should be chosen with an eye toward defining internationally standardized protocols.
• The layer boundaries should be chosen to minimize the information flow across the interfaces.
• The set of rules for communication between entities in a layer is called protocol for that layer.

The seven layers of ISO OSI reference model are:

1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer