Recent Post


  • Open system interconnection reference model,developed in 1984 by the international standards organization(ISO)
  • It is away of sub-dividing a communication system into smaller parts called layers.
  • A layer is a collection of conceptually similar function that provide services to the layer above it and receives services from the layer below it.
  • Provide a set of design standards for equipment manufactures so they can communication with each other.It is basic guideline for protocol development.

1) Application
2) Presentation
3) Session
4) Transport
5) Network
6) Data Link
7) Physical

Physical Layer:-
  • Convey the bit stream through the network at the electrical and mechanical level.
  • Defines physical means of moving data over network devices.
  • Interfaces between network medium and devices.
  • Defines optical , electric and mechanical characteristics:voltage levels, timing of voltage changes , physical data rates , transmission distance and physical connection.
Data Link Layer:-
  • Takes a string of bits and delivers it across a link.
  • Convey the bit stream through the network at the electrical and mechanical level (i.e Layer 1).
  • Turns packets into raw bits and bits into packets.  
  • Framing and Error detection:-
              i)  Break the bit stream up into frames.
              ii) Compute and error-detection code.
              iii) Transmit each frame seprately.

Network Layer:-
  • Translate logical network address and names to their physical address (e.g ,device name to MAC address).
  • Responsible for:-
      i) Addressing.
      ii) Determining routes for sending.
      iii)Managing network problems such as switching data  congestion and routing.
  • Breaks the data into smaller unit and assembles data.
  • Shields higher layers from details of how the data gets to its destination.

Transport Layer:-
  • Divides streams of data into chucks or packets.
  • Reassembles the message from packets.
  • Provide error-checking to guarantee error-free data delivery , with no losses or duplication.
  • Provides acknowledgment of successful transmissions.
  • Provides flow control and error-handing.

Session Layer:-
  • Establishes maintains and ends sessions across the network.
  • Responsible for name  recognition (identification) so only the designated parties can participate in the session.
  • Provide synchronization services by planning check points in the data steam.
  • If session fails , only data after the most recent checkpoint need be transmitted.
  • Manages who can transmit data at a certain time and for how long.

Presentation Layer:-
  • Translates from application to network format and vice-versa.
  • All different formats from all sources are made into a common uniform format that the rest of the OSI can understand.
  • Responsible for protocol conversion , character conversions , data encryption / decryption , expanding graphics commands and data compression.
  • Sets standards for different system to provide seamless communication from multiple protocol stacks.

Application Layer:-

  • Used for application specially written to run over the network.
  • Allows access to network services that support application.
  • Directly represents the services that directly support user applications(e.g., file transfer and email)
  • What the user sees or does. 

No comments