OSI Model

OSI Model - 3

By sajal bera - March 08, 2017

Open System Interconnection

(OSI) Model - 3

Summary on What we have studied in last tutorial :-

The data unit at Network layer is known as packet.

Network layer receives data from transport layer. It adds header to the data and passes data to the data link layer.

The data link layer provides physical addressing which is useful for a local network.

Routing is a process wherein a proper path is defined for the packets to reach destination.

Internetworking means connecting two or more computer networks together.

The data unit at Transport layer is called Segment.

Transport layer receives data from the session layer. It divides the data into segment with each segment having its own header and passes these segments to the network layer.

(1.5) Session Layer :-

This is fifth layer of OSI Model. This layer establishes, manages, synchronizes and terminates connection between the computers. It also sets up, co-ordinates and terminates interactions and dialogues between communicating devices. It provides either half duplex or full duplex service.

Functioning of session layer is shown in the figure :

Session Layer

Session layer receives data from the presentation layer. It adds header and synchronization bits to data and passes data to transport layer. At receiver side, session layer receives data from transport layer. It detaches header and synchronization bits from data and passes data to presentation layer.

Responsibilities of Session layer :

Dialog Control : The session layer is responsible for setting up sessions between communicating devices. It allows two devices to enter into dialog (communication process). These dialogs can take place either a half-duplex or full duplex mode. Dialog control is implemented using a data token. The user who had the token only allowed to send the data. When the user is done the operation the token is passed on to the next user.
Synchronization : At the session layer, checkpoints (synchronization bits) are added into a stream of data to synchronize the sessions. For example, if a device sending a file of 3000 pages, then you can insert checkpoints after every 100 pages to ensure that these 100 pages are received without an error and acknowledged independently. If an error occur while transmitting page 633, the only pages that should be retransmitted are from 601 - 633. Previous pages does need to be resend because the successful reception of the pages from 1 - 600 have been acknowledged before transmission of the pages from 601 - 700.

(1.6) Presentation Layer :-

This is Sixth layer of OSI Model. This layer deals with syntax and semantics of the data exchanged between two devices. It transforms the data in the format that will be accepted by the application layer.

Functioning of presentation layer is shown in the figure :

Presentation layer

Presentation layer receives data from application layer. It adds header to the data and passes data to the session layer. At the receiver side, the presentation layer receives data from session layer. It detaches header from the data and passes data to the application layer.

Responsibilities of presentation layer are :

Translation : The running on the computer exchange data in the form of character strings and the numbers. Before transmitting, the information should be converted to bit format. The presentation layer handles interoperability between device, presentation layer changes the information from its sender-dependent format into a common format that will accepted by the receiving device . At the receiving side, presentation layer changes the information from the common format into its receiver-dependent format. For example, consider one device that uses Extended Binary Coded Decimal Interchange Code ( EBCDIC ) to represent data and wishes to transmit data to a device using American Standard Code of Information Interchange ( ASCII ) to represent the same data. The information sent directly cannot be accepted by the device. To resolve this problem, the presentation performs the conversion of codes. At the sender's side the EBCDIC code is converted into common format. At the receiver,s end the presentation layers again converts the data ( common format ) into ASCII code.
Encryption : The presentation layer also adds security to data by encrypting it. Encrypting is a process in which the data is converted into form that prevents unauthorized users from reading. Encrypting allows a system to carry sensitive and private information. The encrypted data is decrypted by presentation layer at the receiving end. Decryption is a process opposite to Encryption. It converts the original message back to its original form.
Compression : Data compression refers to a process of encoding data using less number of bits. This process reduces the number of bits used to represent the information. It is important while transmitting multimedia information as text, audio and video.