OSI MODEL: OSI MODEL : 7 Layers

Open System Interconnection Model (OSI Model)

It is a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to its underlying internal structure and technology. It has been developed by ISO – ‘International Organization of Standardization‘, in the year 1984. It is a 7 layer architecture with each layer having specific functionality to perform. All these 7 layers work collaboratively to transmit the data from one person to another across the globe.

PHYSICAL LAYER -

The physical layer contains information in the form of bits. It is responsible for transmitting individual bits from one node to the next. When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together.

FUNCTIONS OF PHYSICAL LAYER :

1. Physical layer is responsible for movement of individual bits from one hop to the next.

2. It defines the characteristics of the interface between the devices and the medium.

3. It also defines the types of encoding

4. The transmission rate is also defined by this layer.

5. Synchronization of bits is done by physical layer..

6. Physical topology is defined by this layer.

7. Physical layer also defines the transmission mode of the network.

DATA LINK LAYER -

The data link layer is responsible for moving frames the node to node delivery of the message. The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer.

FUNCTIONS OF DATA LINK LAYER :

1. This layer divides the stream of bits received from the network layer into Frames.

2. Physical Addressing is the most important function of data link layer.

3. Data flow control is done by this layer.

4. Error control and access control are also done by this layer.

NETWORK LAYER -

Network layer works for the transmission of data from one host to the other located in different networks. It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available.

FUNCTIONS OF NETWORK LAYER :

Routing: The network layer protocols determine which route is suitable from source to destination. This function of network layer is known as routing.

Logical Addressing: In order to identify each device on internetwork uniquely, network layer defines an addressing scheme. The sender & receiver’s IP address are placed in the header by network layer. Such an address distinguishes each device uniquely and universally.

TRANSPORT LAYER -

Transport layer provides services to application layer and takes services from network layer. The data in the transport layer is referred to as Segments. It is responsible for the End to End Delivery of the complete message. The transport layer also provides the acknowledgement of the successful data transmission and re-transmits the data if an error is found.

FUNCTIONS OF TRANSPORT LAYER :

1. Service point addressing

2. Segmentation and reassembly

3. Connection control

4. Flow control

5. Error Control

6. Delivery of a message from one process to another.

SESSION LAYER -

This layer is responsible for establishment of connection, maintenance of sessions, authentication and also ensures security. It is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.

FUNCTIONS OF SESSION LAYER :

1. Session establishment, maintenance and termination: The layer allows the two processes to establish, use and terminate a connection.

2. Synchronization : This layer allows a process to add checkpoints which are considered as synchronization points into the data. These synchronization point help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.

3. Dialog Controller : The session layer allows two systems to start communication with each other in half-duplex or full-duplex.

PRESENTATION LAYER -
Presentation layer is also called the Translation layer.The data from the application layer is extracted here and manipulated as per the required format to transmit over the network.

FUNCTIONS OF PRESENTATION LAYER :

1. Translation : For example, ASCII to EBCDIC.

2. Encryption/ Decryption : Data encryption translates the data into another form or code. The encrypted data is known as the cipher text and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.

3. Compression: Reduces the number of bits that need to be transmitted on the network.

APPLICATION LAYER -
The apllication layer enables the user to access the network. It provides user interfaces and support for services such as e-mail, remote file access and transfer, shared database management, and other types of distributed information services.