Consists of Protocol stack which defines interfaces and services of the protocol at each layer

The OSI Model has seven layers:

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

The OSI model has several layers as shown in the figure. The principles that were applied to arrive at the seven layers can be briefly summarized as follows:

1.        The layer boundaries should be chosen to minimize the information flow across the interfaces.
2.        Each layer should perform a well-defined function.
3.        The function of each layer should be chosen with an eye toward defining internationally standardized protocols.
4.        A layer should be created where a different abstraction is needed.
5.        The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwieldy.

Note: OSI model itself is not network architecture because it does not specify the exact services and protocols to be used in each layer. It just tells what each layer should do.