Role of OSI reference model

- Basically OSI reference model is introduced by international standard organization (ISO).

- The OSI model is not protocol, generally it is model to show how to facilitate the communication between different system without changing any hardware and software.

- The OSI model is the layered framework that allows the communication between any kind of system.

- Generally OSI model is the 7 layer structure as follows

Physical layer :

Mainly physical layer is concerned to carry out a bit stream from one node to next node. Diagrammatically it is represented as follows

The physical layer defines the characteristics between the devices and the transmission medium. It also defines the type of the transmission medium. The physical layer data also concerned with the stream of data (i.e sequence of 0’s and 1’s). To be transmitted, these bits are encoded with the signals that must be optical or electrical. Physical layer also concerned with the data rate i.e the number of bit sent each second. In other words, it defines the duration of bit, which is how long it would last. 

Data link layer : 

The data link layer transforms the physical layer. The data link layer divides the stream of bit received from the network layer into manageable data units called as frames. If frames are distributed to the different system on the network, the data link layer add header to define the senders or receivers frame.

Network layer : 

The network layer is responsible from source to destination delivery of the packets. Whereas the data link layer oversees the delivery of the packets between different system on the same link (or network). If two systems are connected with the same link or network their is no need of the network layer. But if the two systems are connected with different network layer to accomplish source to destination delivery of the packets.

Network layer include the another responsibility such as logical addressing and routing. 

1) Logical addressing 

The physical addressing implementing by the data link layer handles the problem of addressing, but if packet passes the network boundary then we have to get another addressing mode to distinguish the source and the destination systems. 

2) Routing 

When independent network or links are connected to create the inter networks, then the connecting devices (Called as switch or routers) routes or switches the packets to their final destination. 

Transport layer :

The transport layer is responsible for process to process delivery of the entire message. The network layer is responsible for source to destination delivery of the individual packets, it does not recognize any relationship between those packets. Whereas the transport layer on the other hand ensures that the whole message is arrives in order and with the error-free at the source to destination level.

Session layer :

The session layer establish, maintain and synchronized the interaction among the communicating systems.

Presentation layer :
The presentation layer is responsible for translation, compression and encryption.

Application layer :
The application layer provides the user interface and support for services such as electronic mail, remote file access and transfer, shared database management and other types of distribution services.