The OSI MODEL

The OSI MODEL

The Open System Interconnection (OSI) reference model describes how a software application in one computer sends information to a software application on another computer. This model was developed in the year 1984 by the International Organisation for Standardisation (ISO), a multinational organisation responsible for the worldwide agreement on international standards. Further, the OSI model is an ISO standard, which covers all the aspects of network communication.

 The OSI model is a layered framework that allows communication between all types of computer systems. It is composed of seven separate but related layers. Each layer defines a part of the process of moving information across a network.

1.Application layer

This level provides services to the user such as user interfaces. The application layer provides the following services:

 • Network virtual terminal: It helps user to log-in from a remote host, and creates a software version of the terminal at that distant host.

 • File transfer, access and management: It allows user to access, retrieve or manage files from a remote host to a computer, locally.

 • Mail services: It provides e-mail forwarding, storing and receiving services.

• Directory services: It grants the user access to the distributed database of global information about various services.

2.Presentation layer

This level deals with the syntax and the semantics of information exchanged between two systems. It ensures that the data is readable between the applications of two systems. The presentation layer deals with the following services:

• Translation: Different computers use different kinds of encoding systems. The presentation layer is responsible for data translation. This is in order that two applications having two different encoding systems that can read data sent by each other. It translates data according to the encoding system used by the receiving system.

3.Session layer


This level is responsible for establishment, maintenance, and synchronization of interaction between communicating systems. it is known as the ‘network dialog controller’. The session layer deals with the following services:
Dialog Control: It allow two processes to enter a dialog. It enables these O to communicate either in half-duplex or full-duplex mode.


Synchronization: The session layer achieves synchronization by adding checkpoints or synchronization points, while transferring data. For example, if a node is transmitting 10 GB data, then the session layer inserts checkpoints. If the data is lost at some point, then it can be recovered and resent after the latest checkpoint. In such a case, there will be no need to resend the whole data.

 • Encryption: The session layer is also responsible for encryption, to ensure privacy. A system can carry sensitive information after applying encryption. Encryption is the conversion of original data into an unreadable format. Decryption is the conversion of unreadable data into the original format.

4 .Transport layer

This level is responsible for process-to-process delivery of packet. A process is an application running on the host. The transport layer ensures that the packet arrives in order, and checks both error and flow control. It also provides following services:

  • Service-point addressing: In some cases, source-to-destination delivery means delivery of packets from one specific process to another. At the same time, the transport layer header includes a new type of address called as service-point address (or port address).
  • Segmentation and assembly: The packets in the transport layer are divided into segments. Each segment contains a sequence number. This helps in sequential delivery of messages, and replaces lost packets.
  • Connection control: The communication provided at this layer can either be connectionless or connection-oriented. In a connectionless communication, the transport layer sends each segment independently to the destination. In a connection-oriented communication, the transport layer first establishes connection and then delivers the packet.
  • Flow control: Similar to the data link layer, this transport level also Provides flow control. However, the technique involves process-to-Process communication instead of a link across.
  • Error control: Similar to the data link layer, this transport level also provides error control. However, like flow control the communication technique is also performed process-to-process.

5. Network Layer

The network layer is responsible for receiving frames from the data link layer, and delivering them to their intended destinations among based on the addresses contained inside the frame. The network layer finds the destination by using logical addresses, such as IP (internet protocol). At this layer, routers are a crucial component used to quite literally route information where it needs to go between networks.

6. Data Link Layer

The data link layer is very similar to the network layer, except the data link layer facilitates data transfer between two devices on the SAME network. The data link layer takes packets from the network layer and breaks them into smaller pieces called frames. Like the network layer, the data link layer is also responsible for flow control and error control in intra-network communication (The transport layer only does flow control and error control for inter-network communications).

7. Physical Layer

The lowest layer of the OSI Model is concerned with electrically or optically transmitting raw unstructured data bits across the network from the physical layer of the sending device to the physical layer of the receiving device. It can include specifications such as voltages, pin layout, cabling, and radio frequencies. At the physical layer, one might find “physical” resources such as network hubs, cabling, repeaters, network adapters or modems.

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