OSI Model

Introduction 

The OSI model is the short form of the “Open Systems Interconnection” model. This model is based on concepts that standardise and characterise the communication functions involved in a typical computing system or telecommunication. According to the OSI model, the data flow from the sender to the receiver gets partitioned into 7 layers, in the communication system. A class of functionality is served by the lower layers to the layers above it. In other words, when it comes to the intermediate layers they not only serve the layer above it but at the same time get served by the OSI layer below it.

Features of OSI Model

• A layer of abstraction is created where such a specific level of abstraction is/are needed.
• Certain internationally standardised protocols must guide the function of an individual layer. 
• There should be a large number of layers to avoid the congestion of one layer with all the functions. However, the total number of layers also has to be kept short to avoid making the architecture complex. It is about finding the right balance in between.
• The function of a layer in the OSI model is, to large extent, dependent on the primitive functions performed by the layer below it. Therefore, in the OSI model, the lower layers must provide these services for the layers above. 
• When a change is made in one layer, it does not necessitate a change to be made in other layers. 

OSI Model Layers

The OSI Model has upper as well as lower layers. These are then partitioned into 7 distinct layers. 

Upper Layer (Software):-

                     Application Layer

                     Presentation Layer

                     Session Layer

Head of OSI:-

                       Transport layer

Lower Layer (Hardware):-

                        Network Layer

                        Data Link Layer

                        Physical Layer

Functions of the OSI Model Layers

• Physical Layer- This layer deals with physical and electrical details and specifications regarding the data connection. The relationship between a transmission medium and the device is established at this level. The instances of hardware in this layer involve network adapters, repeaters, Ethernet, so on.
• Data Link Layer- To establish or terminate a connection of two network devices that are connected, protocols need to be maintained. This layer allows in defining those protocols. Moreover, this layer allows understanding of the IP address. Therefore, the logical addressing or identification of the endpoint is possible because of this layer. Under this layer, Media Access Control (MAC) controls a device, in a certain network, in accessing a medium and making transmission of data possible. A layer of Logical link control also falls under this layer and identifies and encapsulates network-layer protocols and finds errors.
• Network layer- Procedural and functional means to transfer data sequences from a certain node to the other, connected via different networks, is provided in this layer. Layer-management protocols involve multicast group management, routing protocols, and address assignment in the network layer. 
• Transport Layer- This layer is built on the network layer. The transport layer allows the transportation of data from the process on the source device to a process on the destination device. The layer determines the quantity of data to be sent, where exactly to be sent and what should be the rate of sending. This layer provides acknowledgment of a successful data transmission. 
• Session Layer- This layer in the OSI model is responsible for controlling the dialogues between computers. The start and termination of the connection are established between the local and the remote application. Password validation, log-on are handled by this layer. Therefore, the utility of the layer is to permit two systems to form or enter into a dialogue. 
• Presentation Layer- This layer of the OSI model is also referred to as the syntax layer. It allows the transformation of data in a form that the application accepts. Handling data encryption and data compression adds to the utility of the layer.

Application Layer- This is the OSI model’s highest layer/level. Interaction of the users with different software applications is possible because of this layer. Examples include email, file transfer, so on.

Conclusion

The topic of the OSI reference model has been discussed in detail. The meaning and scope of the OSI model in a computer network have been established. Moreover, the features of the model have been enlisted to understand the model better. The OSI model layers have been mentioned and elaborated. The FAQs section provided deems to make the concept clearer. The additional information in the FAQs section helps to address the most probable queries of readers.