Osi Layer






OSI -> Open Systems InterConnection

It is a layered abstract description for communication and for the design of Network Protocol which is a seven layered hierachial setup.


  • In Layman terms there is nothing called 7 layer setup present physically in the computer or networking device they are just logical divisions.
  • When a Network administrator enters a Server room where the server is down and he was asked to troubleshoot then the layering abstraction would give him an idea of where to start the troubleshooting like he may first check the pyhsical connections and network config and so on.
  • Because of layering developing of Network Software becomes so Easier.
  • To provide well defined Interfaces between Adjacent layers.
  • A change in 1 layer should not affect the other layers.
  • Allows structural development of Network Software
  • Interface must remain the same.









  • There are 7 layers in the OSI Layering model on which the first 4 layers are host to host layers and remaining three layers are point to point layers.
  • Host to host layers are concerned only about the host nodes at both the ends and does not worry about the intermediate layers where as the Point to point layers are the vice versa meaning it is aware only of the neighbouring nodes and not the host nodes.


  • Here the transmission is in the form raw bit stream over a physical medium.
  • For eg Consider a Copper wire where bits are sent as actual electrical Voltages.
  • The layer also defines what transmission technique is used to transmit bits over the physical medium.
  • In this layer i.e the wiring and cable part 95% of the network problems occur.
  • Protocol Data Unit – Bits


  • The purpose of Datalink layer is Reliable transfer of 'frames' over a p2p link.
  • It performs Flow Control and Error Control
  • Flow Control in the sense when a node transmit a bit and the next node is receiving it faster than its actual bandwidth then it asks for flow control
  • Error control checks for possible errors if any.
  • Datalink layer refer by MAC addresses,every individual networking devices has its own MAC address.
  • Protocol Data Unit – Frames.


  • The purpose of Network  layer is establishing , maintaining , terminating network Connections.
  • The Network layer is responsible for the routing of Packets.
  • The address here is IP Address
  • Protocol Data unit- Packet.


  • Transport layer is the first end to end layer (Intermediate links are not visible)
  • The layer is a Reliable layer that it performs Error control and Flow Control.
  • Also it performs Windowing [how long the message should be sent].
  • The Transport layer Protocols are TCP and UDP.
  • Protocol Data Unit-Segment



  • Everytime a Computer communicates with another computer we create a Session between them.
  • It estlabishes a connection with the web server and manages the established session.
  • Protocol Data unit- Data


  • As the Name suggest the Presentation layer provides ways for presentation of data.
  • It also Provides Data Independence.
  • The important work of Presentation layer is it provides Encryption.
  • It is Operating system Configurable.
  • Protocol Data unit- Data


  • Application layer makes provides interface point for user Application.
  • It Interacts with the user.
  • eg. Apps like FTP and HTTP
  • Protocol Data unit- Data


Data flow in Osi Layers












S-Source Node

A-Intermediate Node

B-Intermediate Node 2

D-Destination Node

  • When a node is transferred from Source Node to Destination node it travels from top to bottom from application node down on to the physical layer.
  • When travelling from top to bottom Headers and trailers are added as it moves downward in the Source node layers.
  • Then it moves to the intermediate node A then it moves up 3 layers
  • Then moves downward in A then moves the bottom layer in B then moves up the three layers in the intermediate node B.
  • Then it moves downward in B and moves to the Destination node D.
  • Finally it moves to the destination node D and moves up the Destination node
  • The headers and trailers are removed as it moves upward and the data is obtained in the application layer

Thank u for reading this Article hope it was useful !


Sathesh Bm

About Sathesh Bm

Sathesh Bm has written 26 post in this website.

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