What is Open System Inter-connection (OSI)

OSI stands for Open System Interconnection, the OSI model defines the networking framework to put into action the protocols in seven different layers. The control is passed on from one layer to the next from the application layer down to the physical layer and works its way back up the hierarchy on the other end.

There is nothing really to the OSI. As a matter of fact, it is nothing tangible and as such has no function. It is purely a conceptual framework meant to help you understand the otherwise complicated interactions between two computers.

Physical Layer

This is the lowest layer in the model and is concerned with the reception and transmission of the data through a physical medium.

      It provides:-

  • Data encoding: – It modifies the binary language to better suit the characteristics of a physical medium.
  • Physical media attachment.
  • Transmission technique.
  • Medium transmission.

Data Link

This ensures that data moves from one node to another without any errors. In order to do this, this layer provides:-

  • Establishes and terminates a link.
  • Controls the traffic of the frames sent.
  • Frame sequencing.
  • Detects and also recovers from the errors that may develop in the physical layer.
  • Checks the frames received if they are free of errors.
  • Determines when a computer or a node can use the medium.

Network layer

This controls the subnet, determining which physical path the data being sent will follow depending on the network condition. It:-

  • Traffic control of the sub-nets
  • Determines whether the router on the downstream is utilizing its frame size to the fullest when buffering
  • Translates the names or logical addresses into physical addresses
  • Keeps track of the frames that have been forwarded by a particular subnet and provides billing details.

Transport layer

This ensures that the data set is delivered with no duplicates, losses and without errors. Its complexity and size will depend on the service quality of the network layer.

  • It splits the packets from the session layer into smaller units and passes them to the network layer
  • Provide end to end delivery of the messages for acknowledgement
  • It Controls the message traffic.

Session layer

It enables the establishment sessions of procedures that are running on separate stations.

  • Establishes, maintains and terminates a process
  • Supports the session

Presentation Layer

This layer formats the data and presents it to the application layer. You can think of it as the translator.

  • Translates character codes
  • Does data conversion
  • Compresses data
  • Encrypts data for security reasons

Application layer

This serves more as the window for the application processes and the users to access the network services.