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OSI Models Explaination


September 18, 2016
Published By : Pratik Kataria
Categorised in:

Physical Layer


  • Physical layer is the bottom(layer 1) of OSI model.
  • It is responsible for the actual physical connection between the devices.
  • The physical layer is responsible for movements of individual bits from one node to next.

Functions of Physical Layer

  • Convert bits to signals
  • Bit synchronization
  • Manage physical connection
  • Bit rate control
  • Line configuration
  • Physical topology
  • Transmission mode
  • Multiplexing
  • Switching

Data Link Layer


  • Framing:- divides the data from N/W layer into frames.
  • Physical Addressing:- Add a header to the frame to define the physical address of the source and the destination machines.
  • Flow Control:- It is the traffic regulatory mechanism implemented by Data Link layer that prevents the fast sender from drowning the slow receiver.
  • Error Control:- It provides the mechanism of error control in which it detects and retransmits damaged or lost frames.
  • Feedback:- after transmitting the frames, the system waits for the feedback.

Network Layer


  • It is responsible for the source to destination delivery of a packets across multiple networks.
  • Routing:- Provide mechanism to transmit data over independent networks that are linked together.
  • Logical addressing:- Adds Logical addresses of sender and Receiver.

Transport Layer


  • It is responsible for source process to destination process delivery of entire message.
  • Transport layer provides two types of services:
    • Connection Oriented Transmission: In this type of transmission the receiving device sends an acknowledgment back to the source after a packet or group of packet is received.
    • Connectionless Transmission:  In this type of transmission the receiver does not acknowledge receipt of a packet.

Functions of Transport Layer:

  • Segmentation and Reassembly: Divide the message received from Session layer into Segments and number them to make a sequence for reassembly at the receiving side.
  • Service point addressing: Transport layer makes sure that the message is delivered to the correct process on destination machine.
  • Error Control: Make sure that the entire message arrives without errors else retransmit.
  • Flow Control: Transport layer makes sure that the sender and the receiver communicate at a rate they both can handle.

Session Layer


  • It is responsible for beginning, maintaining & ending the communication between two devices, which is called session.
  • Establishment, maintaining and ending a session:

      Sends SYN packet – establish request

    • Receives ACK & SYN- established
    • To end – Sender sends ACK
    • Dialog Control: The session layer allows two systems to enter into a dialog.
    • Synchronization: Allows a process to add checkpoints to a stream of data.


Presentation Layer


  • This layer is concerned with the syntax and semantics of the information exchanged between two systems.
  • Data Translation: Encoding and Decoding
    • Sender to Common format on Sending side
    • Common to Receiving format on Receiver side
  •  Data Encryption: For security and privacy purpose.
  • Data Compression: Data compression reduces the number of bits contained in the information.

Application Layer


  • Provides User interfaces and support for Services, like e-mail, file transfer.
  • Network Virtual terminal: It allows a user to log on to a remote host.
  • File Transfer Access, and Management: This application allows a user to access files in a remote host.
  • Mail Services: This application provides various e-mail services.
  • Directory Services: This application provides the distributed database sources and access for global information about various objects and services.