The OSI Reference Model

The OSI Reference model is based on a proposal developed by the international standard Organization (ISO) as a first step towards international standardization of the protocols used in the various layers. It was revised in 1995. The model is called the ISO (open system interconnection) reference model because it deals with connecting open systems-that is, systems that are open for communication with other systems. We will just call it the OSI model for short.

We will just call it the OSI model for short.

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

1.    The layer should be created where a different abstraction is needed.

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.    The layer boundaries should be chosen to minimize the information flow across the interfaces.

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.

Now we will study each layer of the model in turn starting at the bottom layer. Note that he OSI model itself is not a network architecture because it does not specify he exact services and protocols to be used in each layer. It just tells what each layer should do. However, ISO has also produced standards for all the layers, although these are not the reference model itself. Each one has been published as a separate international standard.

The physical layer:

The physical layer is concerned with transmitting raw bits over a communication channel. The design issue has to do with making sure that when one side sends a 1 bit, it is received by the other side as 1 bit not as 0 bit. Typical questions here are how many volts should be used to represent a 1 bit and how many for a 0, how many nanoseconds a bit lasts, whether transmission may proceed simultaneously in both directions, how the initial connection is established and how it is torn down when both sides are finished, and how many pins the network connector has and what each pin is used for. The design issues here largely deal with mechanical, electrical, and timing interfaces, and the physical transmission medium, which lies below the physical layer.

The Data Link Layer:

The main task of the data link layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors to the network layer. It accomplishes this task by having the sender break up the input data into data frames and transmit the frames sequentially. If the service is reliable, he receiver confirms correct receipt of each frame by sending back an acknowledgement frame.

Broadcast network have an additional issue in the data link layer: how to control access to the shared channel. A special sublayer of the data link layer, the medium access control sublayer, deals with this problem.

The Network Layer:

The network layer contains the operation of the subnet. A key design issue is determining how packets are routed from source to destination. routes can be based on static tables that are "wired into" the network and rarely changed. They can also be determined at the start of each conversation, for example, a terminal session. 

In broadcast network networks, the routing problem is simple, so the network layer is often or even nonexistent.

The Transport Layer:

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

The transport layer also determines what type of service to provide to the session layer, and, ultimately, to the users of the network.

The Session Layer:

The session layer allows users on different machines to establish sessions between them. Sessions offer various services, including dialog control, token management , and synchronization.

The Presentation Layer:

Unlike lower layers, which are mostly concerned with moving bits around, the presentation layer is concerned with the syntax and semantics of the information transmitted. In order to make it possible for computer with different data representations to communicate, the data structures to be exchanged can be defined in an abstract way, along with a standard encoding to be used "on the wire".

The Application Layer:

The application layer contains a variety of protocols that are commonly needed bu users. One widely-used application protocol is HTTP , which is the basis for the world wide web. When a browser wants a web page , it sends the name of the page it wants to the server using HTTP. The server then sends the page back, Other application protocols are used for file transfer.

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