Need of OSI layered Model
   Computer network helps users on the network to share resources like file, hardware, software application, and user communication. Sharing of resources is only possible when devices or computers connected to the network can communicate. Communication does not mean just the transmission of data or information.
Computer Communication
   In Computer Network, communication means meaningful exchange of data between two computers. In the early day computer of one brand could not understand the data created by the computer of another brand. It is because they differ in many ways the architecture is different, the hardware is different, the operating system is different, and the data format is different. It is very similar to a situation where two people want to communicate and do not know each other’s language. To understand this complex process of communication let us first understand person-to-person communication.
Example : Person to Person communication.
   In this example assume that Bhavika wants to exchange her ideas with Maria but Bhavika lives in India and knows English and Marathi and Maria lives in Spain and knows only Spanish. The problem here is language and distance. What do you think will they be able to exchange their ideas, of course, yes because the problem of language can be solved by appointing a translator or using a translator at both ends and the problem of distance can be solved by a Postal Service that will carry a message from one end to the other end.
   Communication between Bhavika and Maria happens in this way: Bhavika writes a letter in English the language she is comfortable with.  She then uses a translator machine that scans the letter and creates a secret code Bhavika then puts this letter in the post office box. The latter is carried out by Postal Service to the post office of the city where Maria lives in. The letter is delivered to Maria, then Maria uses her translator machine to change the secret code to a letter in the Spanish language.
3 layerd model
   With this example we can understand when the process of communication is not simple or when it is complex, we can make it simple by dividing the whole process of communication into several parts. In this example we have divided the process into 3 parts at one end we have Bhavika, a translator, and a Postal Service and at the other end, we have Maria, a translator, and a Postal Service. We can say, we have divided the whole process into 3 layers: layer 1 layer 2, and layer 3.
Computer Communication layers
   The point here is we want to understand how two computers or two digital devices with different operating systems, different data formats, and different hardware can communicate. But with the help of person-to-person communication we have understood when the communication process is complex, we can make it simple by dividing the whole process into several layers. Computer communication is much more complex than person-to-person communication, but we can make it possible by adding a few more layers to the communication process. It depends upon how many tasks we have to carry out for that communication process in the case of person-to-person communication we have seen there are 3 layers. Why 3 layers because there were 3 different tasks to be carried out.
   In the case of computer communication, if there are 10 different terms to be carried out we can divide the whole process of communication into 10 different parts or 10 different layers and assign one task to each layer in the communication process. We can also reduce the number of players or the number of parts in the communication process by assigning 2 functions to one layer.
ISO (International Organization for Standardization)
 International Organization for Standardization (or International Standards Organization) i.e., ISO which was established in 1947, developed a computer communication model which has 7 layers. That means ISO has divided the whole process of computer communication into 7 different parts maybe because there are several different 7 different tasks to be carried out. For computer communication, this model is called the OSI model i.e., the Open Systems Interconnection model. ISO released this OSI model in 1984 as a standard. The ISO OSI reference model is composed of 7 ordered layers:
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- Physical layer
- Data link layer
- Network layer
- Transport layer
- Session layer
- Presentation layer and
- Application layer
   Each layer is assigned a particular task the open systems interconnection model that is OSI model is very similar to a person-to-person communication model i.e. a layered model. The OSI model is also a layered framework for the design of network systems that allow communication between all types of computers. The term open systems mean, systems are designed such that they are open to all other systems. Which are also designed by using the same standard. This means every computer manufacturer will construct a system following this standard and therefore, every computer system will be compatible with each other. So, the purpose of the ISO OSI reference model is to guide vendors and developers so that the digital communication systems and software programs they create can inter-operate. They will be compatible with each other.
