OSI Model

OSI Model.

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Introduction

If you are in the field of networking or you are curious to learn about networking you have heard the term OSI(Open Systems Interconnection) model. So the OSI model is a reference model which explains how a network works dividing it by seven layers, which we are going to study now.

The seven layers of the OSI model.

Seven layers of the OSI model[/caption]

1. Physical layer.

In the physical layer, there are all the physical connections between the networking devices. Usually, the major problems in networking come here like damaged cables, unplug switches, and many other physical problems.

2. Data link layer.

The data link layer is the protocol layer in a program that handles the moving of data into and out of a physical link in a network.

3. Network layer.

The network layer is responsible for packet forwarding including routing through intermediate routers. Data is transferred in the form of packets via logical network paths in an ordered format controlled by the network layer.

4. Transport layer.

It provides logical communication between application processes running on different hosts within a layered architecture of protocols and other network components.

5. Session layer.

The session layer provides the mechanism for opening, closing and managing a session between end-user application processes.

6. Presentation layer.

The presentation layer is sometimes called the syntax layer. The presentation layer is responsible for the formatting and delivery of information to the application layer for further processing or display.

7. Application layer.

In this layer, the data which had come from the previous layers are being processed and display. This layer operates on the host side from the application which they are using.

The advantages of the OSI model are

1. It is a generic model and acts as a guidance tool to develop any network model.

2. It is a layered model. Changes are one layer do not affect other layers, provided that the interfaces between the layers do not change drastically.

3.It distinctly separates services, interfaces, and protocols. Hence, it is flexible in nature. Protocols in each layer can be replaced very conveniently depending upon the nature of the network.

4. It supports both connection-oriented services and connectionless services.

The disadvantages of the OSI model are

1.    It is purely a theoretical model that does not consider the availability of appropriate technology. This restricts its practical implementation.

2.    The launching timing of this model was inappropriate. When OSI appeared, the TCP/IP protocols were already implemented. So, the companies were initially reluctant to use it.

3.    The OSI model is very complex. The initial implementation was cumbersome, slow and costly.

4.    Though there are many layers, some of the layers like the session layer and presentation layer have very little functionality when practically deployed.

5.    There is a duplication of services in various layers. Services like addressing, flow control and error control are offered by multiple layers.

6.    The standards of the OSI model are theoretical and do not offer adequate solutions for practical network implementation.

7.    After being launched, the OSI model did not meet the practical needs as well as the TCP/IP model. So it was labeled as inferior quality.

8.    TCP/IP model was very much preferred by academia. It was believed that OSI was a product of the European communities and the US government, who were trying to force an inferior model to researchers and programmers. Hence, there was considerable resistance in adopting it.