Each layer of the OSI model exists as an independent module. In theory, you can substitute one protocol for another at any given layer without affecting the operation of layers above or below.
The design of the OSI model is based on the following principles:
A layer should be created only when an additional level of abstraction is required.
Each layer should perform a well-defined function.
The function of each layer should be chosen with the goal of defining internationally standardized protocols.
The layer boundaries should be chosen to minimize the information flow across the interfaces.
The number of layers should be large enough to enable distinct functions to be separated, but few enough to keep the architecture from becoming unwieldy.
Figure A.1 shows the layers in the OSI model, beginning with the physical layer, which is closest to the network media.
Figure A.1 Layers of the OSI Model