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Monday, March 25, 2013

Working of the 7 Layers of OSI Network Model


working of the 7 layers of OSI network model















In the previous article ,we talked about the two network models which we consider,  whenever we talks about computer networking, for effective communication between the network. The two network models are OSI and TCP/IP network models. These models are used to connect the sender with the receiver in a network. These are the layered network models in which the data has to pass through different layers of these models on both sides of the sender and the receiver.
Today, we will talk about the working of each layer of the OSI model. As we know that OSI model is a
7 layer network model and the TCP/IP is a 4 layer network model. OSI model is a reference  model whereas TCP/IP model is its implementation.

working of 7 layers of OSI Network model

 So we will just talk about the working of the OSI network model from where the working of the 4 layers of the TCP/IP model can be understood easily.
Different layers of OSI model:- The different layers of OSI network model are :-

1. Application Layer
2. Presentation Layer
3. Session Layer
4. Transport Layer
5. Network Layer
6. Data Link Layer
7. Physical Layer

Let’s talk about the function of each layer one by one.

1) Application Layer:-  The application layer is the closest layer of the OSI model to the user which means that both the OSI application layer and the user can interact directly with the software application. This layer is responsible to provide services to the user. It serves as the window for users to access the network services. The data provided by the user is then forwarded to the next layer, Presentation layer. The example of the application layer is a browser like google chrome, mozilla firefox etc.

2) Presentation Layer:-  This layer adds its own protocols in the form of header to the data received from the application layer. Presentation layer is responsible for the translation, Encryption (to encrypt the data for security purposes) of data, Decryption ( to decrypt the encrypted data) of data, Compressing( to reduce the no. of bits) . So, to convert the data to the format required by the sender or the receiver, to provide the privacy by encryption and to compress the data is the main task of this layer. The data from presentation layer is then transmitted to the next layer, Session layer.

3) Session Layer:- This layer adds its own protocols ,in the form of header, to the data received from the Presentation Layer. This layer is for the session establishment, maintainence and termination of the session. Session layer is responsible for the dialog control and synchronization. In dialog control, this layer allows the communication either in Half-Duplex or in Full-Duplex mode. It also adds the checkpoints to the stream of data. The data of the session layer is then moved to the next layer, Transport layer.

4) Transport Layer:- The transport layer adds its own protocols in the form of headers to the data received from the session layer. Transport Layer is responsible for the segmentation of the data, arrange them in sequence with no loss and duplication and provide the acknowledgement of the successful data transmission. It only sends the next data if no error occurred in the previous data transmission. The message is divided into segments, each segments having sequence number which helps to reassemble the message correctly in the receiver side. The data of transport layer is transmitted to the next layer, Network layer.

5) Network Layer:- This layer adds its own protocols in the form of headers to the message received from the transport layer. The header of the network layer consists of the logical address of the sender and the receiver. Logical address is different from the MAC address (physical address). Logical address is the address given to each computer on a network for the complete transmission of the message from the sender to its receiver. Network layer is required where the two systems are connected to different networks having connecting devices between them. The data from the Network layer then sends to the next layer called Data Link Layer.

6) Data Link Layer:- This layer adds its own protocols in the form of headers to the data received from the network layer. Data link layer is responsible for the speed, security, physical addressing, error free transmission of data frames from one node to another node over the physical layer, framing etc. Data link layer divides the message into manageable data units called frames. This layer adds the header to each frames to define the sender and the receiver over a network. Data from the data link layer is transferred to the next layer called Physical Layer.

7) Physical Layer:- This layer is the costliest layer of all. The purpose of this layer is to carry the data from the sender to the receiver. Physical layer can be a wired media (guided media) or wireless media (unguided media).In a wired media, physical layer can be twisted pair cables, coaxial cable or optical fibres. It can be of different network topologies like Mesh topology, Bus topology, Star topology and Ring topology. In a wireless media, physical layers can be radio waves or microwaves. The physical layer is responsible for the transmission of bits from one node to the next node.

Credits:- Yuvraj Salaria

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