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What is OSI (Open System Interconnections) model ?

What is OSI model ?

             The OSI model is based on a proposal developed by the International Standards Organization (ISO) as a first step towards international standardization of the protocols used in the various layer, the model is called the ISO OSI reference model because its deals with connecting open systems that is, systems that are open for communication with other system.
     Principles follows by different layer are :
  • A layer should be created where a different level of abstraction is needed.
  • The function of each layer should be chosen with an eye toward defining internationally standardized protocols.
  • Each layer should perform a well defined function.
  • The layers boundaries should be chosen to minimize the information flow across the interfaces.
  • The number of layer 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.
OSI model itself is not a network because it does not specify the exact services and protocols to be used in each layer.  It just tells what each layer should do.
aThe OSI Refefence Model

The physical layer :  This layer is concerned with transmitting raw bits over a communication channel the design issues have to do with making sure that when one side sends a bit, it is received by the other side as a 1 bit, not as a 0 bit.

The data link layer : The main task of this layer is to take a raw transmission facility and transform it into a line that appears free of undetected transmission errors to the network layers.  It accomplishes this task by having the sender break the input data up into data frames, transmit the frames sequentially, and process the acknowledgement frames sent back by the receiver.

The network layer : The network layer is concerned with controlling the operation of the sub-net. It provides the route to the frames.  If there are too many packets in some route it remove the 'bottleneck' from that route.

The transport layer : The basic function of transport layer is to accept data from the session layer, split it up into smaller units if need to be, pass these to the network layer and ensure that the pieces all arrive correctly at the other end.

The session layer : The session layer allows users on different machines to establish sessions between them.  One of the services of the session layer is to manage dialogue control.  A related session service is token management. Session can allow traffic to go in both direction at the same time or in one direction at a time.  Another session service is synchronization.  The session layer provides a way to insert checkpoints into the data stream, so that after a crash, only the data transfer after the last checkpoint have to be repeated.

The presentation layer : It performs certain functions that are requested sufficiently often to warrant finding a general solution for them, rather then letting each user solve the problems.  It is concerned with the syntax and semantics of the information transmitted.

The application layer :  It contains a variety of protocols that are commonly needed.  Application layer function is file transfer.  Different file system have different file naming convention, different ways of representing test lines, and so on.  Transferring a file between two different systems requires handling these and other incompatibilities.

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