The OSI reference model

                             IN THE OSI REFERENCE MODEL

In the OSI reference model, the communications and related-connection functions are organized as a series of LAYERS OR LEVELS with well-defined interfaces, and with each layer built on its predecessor. In particular, each layer performs a related subset of primitive functions, and it relies on the next lower layer to perform additional primitive functions. Moreover, each layer offers certain services to the next higher layer and shields the latter from the implementation details of those services. Between each pair of layers, there is an interface. It is the interface that defines the services offered by the lower layer to the upper layer.

The OSI model is composed of seven layers; this figure also includes a description of the functions of the individual layers of the model. Layer k on system A, say, communicates with layer k on some other system layer k protocol, where k=1,2,....7.(The term "protocol" has been borrowed from common usage, describing conventional social behavior between human beings.) The entities that comprise the corresponding layers on different systems are referred to as peer processes. 

In other words, communication is achieved by having the peer processes in two different systems communicate via a protocol, with the protocol itself being defined by a set of rules of procedure. Physical communication between peer processes exists only at layer 1. On the other hand, layers 2 through 7 are in virtual communication with their distant peers. However, each of these six layers can exchange data and control information with its neighboring layers below and above through layer-to-layer interfaces.

The major principles involved in arriving at seven layers of the OSI reference model are as follows:

·          Each layer performs well-defined functions.

·          A boundary is created at a point where the description of services offered is small and the number of interactions across the boundary is the minimum possible.

·          A layer is created from easily localized functions, so that the architechure of the model may permit modifications to the layer protocol to reflect changes in technology without affecting the other layers.

·          A boundary is created at some point with an eye toward standardization of the associated interface.

·          A layer is created only when a different level of abstraction is needed to handle the data.

·          The number of layers employed should be large enough to assign distinct functions to different layers, and yet small enough to maintain a manageable architecture for the model.

The OSI reference model is not a network architecture; rather, it is an international standard for computer communications, which just tells what each layer should do.

here...Alas Mrose...original contents by www.sensualityface.com or www.fairyage.com / describe with help of TEACHER