Thursday, December 11, 2008

What Is ISO OSI Model

OSI stands for Open System Interconnection. The OSI standards were laid down by the International Organization for Standardization in 1984. The OSI model gives seven different layers through which the data must travel when it is transferred from one computer to another in a network.

These seven layers can be distinguished into two groups:
  • Transport set and
  • Application set
Application layer
This layer provides network service to the end users by directly interacting with the operating system or the application software. This layer interacts when you perform any activity on the network like-mail, request websites and transfer files.

Presentation layer
The data from the application layer moves to the presentation layer so that it can be into the standard format in which it can be presented to other layers so that they understand it and process it further. This is the sixth layer. The encryption and decryption of data takes place in this layer.

Session layer
This is the fifth layer. This layer controls the link between the users. The layer establishes and ends the communication between receiving and sending devices in the network.

Transport layer
This is the fourth layer in the model. Transmission control protocol and user datagram protocol are there in this layer. Transmission control protocol establishes the link between two devices in the network.

This protocol also keeps a track of the packets that need to be resend and delivered. This layer checks the errors in transmission. It also integrates the data from different application. This layer thus ensures transparent transfer of data to the upper layers.

Network layer
This layer decides how the data will be sent to the other computer in the network. Routing of the data packets to particular IP address, take place in this layer. This is the third layer in the model. Internet protocol is a network layer

Data layer
This is the second layer. The packets are sequenced in this layer. The data is divided or broken down into small size of certain bytes. These small sized data are called as the packets. All the information that you exchange with your friends or the web pages that you request move in packets till they are delivered to the recipient. These packets have a head, payload and footer.

The header contains the information about the length, synchronization, number, recipient address and the origination address of the packet. The payload contains the main information that is to be delivered to the destination address. The footer contains the bits that check the errors in delivery.

Physical layer
This is the first layer in the OSI model. This layer defines the physical characteristics of the network defining the mechanical characteristics and voltage levels of the network.
Different protocol stacks are formed by combining the different OSI layers into one layer. This model is considered as the standard model for network communication.

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