OSI Reference Model

     OSI stands for Open System Interconnection (OSI). It was established in 1947. OSI is a multinational body dedicated to the worldwide agreement on international standards that covers all aspects of  the network communication is the OSI model. OSI is the set of protocols that allows any two different systems to communicate regardless of their underlying architecture. OSI architecture is not a protocol it is a model for understanding and designing a network architecture. I.e., Flexible, Robustness and inter-operable.

This model was developed by the International Standards Organization (ISO) for connecting open systems. Hence, it is called Open System Interconnection (OSI) reference model. Open system is a system which are  open for communication with other systems. This model has seven layers which has been discussed below

Physical Layer

  • Physical Layer is deals with the movement of individual bits from one node to another node. It coordinates the function required to carry a bit streams over physical medium. It is  also deals with the mechanical and electrical specifications of the interface and transmission media. It is also define the procedures and functions that physical device and interfaces have to perform factors to occur.

Data Link Layer

  • Data Link Layer is a layer above the physical layer whose function is to break input data into frames and take care of acknowledgement. It regulates the flow of traffic in case of fast sender transmitting to a slow receiver.
  • It provides an error free transmission for the network layer and solve the problems that results from frames.
  • Media Access Control (MAC) sublayer is a sublayer of the Data Link Layer provides controlled access to the shared channel.

Network Layer

  • Network Layer routes packets from source to destination by applying routing algorithms. It controls the entire operation of the subnet besides controlling congestion.
  • It is also maintains the accounting information about the data sent by each user. Network Layer establishes the connections with other heterogeneous networks.

Transport Layer

  • The Transport Layer is responsible for accepting data from the session layer, dividing it into small pieces (TPDUS), if required and passing these pieces to the network layer and assure the correct reception of data at the other end. It sets up and terminates the connections across the network thereby regulating the flow of information.
  • It also multiplexes the data and establishes multiple connections, when a high throughput is desired.

Session Layer

  • Session layer is responsible for conducting session among various users on different machines. It manages the dialogue control in case of half duplex communication.
  • It prevents the occurrence of simultaneous operation at both sides by using a service called ‘token management’.
  • Data synchronisation is also provided by inserting checkpoint into the data stream, so that only the data after the last checkpoint is retransferred in case of a system crash.

Presentation Layer

  • Presentation Layer is concerned with the syntax and semantics of the information traveler.
  • It converts the information between the representations used in individual computers and globally accepted standards.

Application Layer

  • Application Layer contains the implementation of various protocols used for user interaction such as Telnet, FTP, SMTP, etc.

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