What is OSI Model (Open Systems Interconnection)?

In this post, I am going to discuss about what is OSI model (Open System Interconnection). By the end of the post you get to know what is OSI model and layers of the OSI model. Firstly, the OSI reference model is a multilevel communication and computer network protocol design.

History of OSI model:

Firstly, the development of OSI model is by ISO (International Standard Organisation) in 1984. However, this model allows two different systems to communicate of their underlying structure. Also the other name of this model is ISO OSI reference model. Each layer is different from one another and has a specific processing function.

What is OSI model (Open Systems Interconnection):

The construction of the World Wide Web is based on this OSI model. Thus, the OSI model is a framework used for all communications from the cloud. Hence, OSI model also describes the information flow from one computer to another using different layers.

Moreover, even OSI model has different names. The Open System Interconnection reference model or OSI reference model or OSI model. However, these are some conceptual description for multilevel communication and computer network protocol design. Therefore, OSI model divided into seven layers they are:

1. Application Layer – Data generation
2. Presentation Layer – Encryption and formatting
3. Session Layer – Establish connection
4. Transport Layer – Delivery and Sequencing
5. Network Layer – Routing and Destination
6. Data Link Layer – Local Network Host Delivery
7. Physical Layer – Access to Media

Therefore, the OSI model can be seen as a universal language for computer networks. In other words, it is based on the concept of division of the seven-layer communication system. Here, everyone is organised according to the list. Basically, each and every layer functionality is different from one another.

Layer 1 to layer 3 are called as lower layer or hardware layer. Where as layer 5 to layer 7 are called as upper layer or software layer. So, here Transport Layer is the heart of the OSI model.

Brief description on seven layers of OSI model :

Application Layer:

The Application Layer is the 7th layer of the OSI model that interacts directly with the end user. The application protocol that is widely used is HTTP (hypertext transfer protocol). This is the basis of the WWW (World Wide Web). So, this layer is a Graphic User Interface that allows users to view and move files and also do some other work related to network communication to operate through network.

Here, the application layer functions as an interface between application and the network. Thus, your message is sent in human readable format.

Therefore, some other functions that include in application layer are :

1. Mail Services
2. Network Virtual Terminal
3. Directory Services
4. Transferring of files
5. Distributing the results to the user
6. Simple Mail Transfer Protocol

Presentation Layer:

Presentation layer is the 6th layer of the OSI model serves as a translator and takes care of the data. And is sent in such a way that the recipient will understand the information and is able to use the data. So, the role of presentation layer is to work as a translator. Therefore, this layer is related to the syntax and semantics of information exchanged between two systems.

Responsibilities of Presentation layer:

1. Formatting and Representation
2. Encryption and Decryption
3. Compression and Decompression
4. Graphic handling

Session Layer:

Session layer is the 5th layer in the OSI reference model. This layer’s task is to maintain or set the dialogue between destination and origin application. For example, between two computers. Session layer keeps the communication active then coordinates and terminates the conversation.

Responsibilities of session layer:

1. Dialog control
2. Synchronization

Transport Layer:

This is the 4th layer in the OSI reference model. Thus, this layer do very important job in OSI model which plays as the heart of OSI model. So, the role of this layer is to perform the end to end connectivity and ensures the delivery of the data packets. So, this task is performed by TCP.

Responsibilities of transport layer:

1. Multiplexing and Demultiplexing
2. Identifying services
3. Error control or recovery
4. Flow control

Network Layer:

This is the 3rd layer in the OSI reference model. The main function of the network layer is to move data to the other networks. Hence, network layer provides the data routing paths for network communication. Therefore, here data is transferred in the form of packets and this layer is accountable for path determination.

Some responsibilities of the network layer:

1. Routing
2. Logical addressing
3. Error handling
4. Congestion

Data Link Layer:

This is the 2nd layer in the OSI reference model. So, this is the layer that communicate directly to the physical layer. Hence, It is intend to process packets from the network layer. Data link layer is divided into two sub-layers:

1. LLC (Logical Link Control): It gives details about WAN protocols
2. MAC (Media Access Control): It gives details about physical address

Responsibilities of OSI model:

1. Hardware addressing
2. Error detection and handling
3. Defining the physical layer

Physical Layer:

This is the 1st layer in the OSI reference model. This layer converts or encodes a frame from the data link layer to a binary signal then that reflects the pieces and transmitted to the physical destination. Hence, it defines the physical and electrical characteristics of transmission media such as, cables, connectors, coding and voltage.

Responsibilities of the physical layer:

1. Fast Ethernet
2. Communication mode
3. Bit synchronisation

Advantages of OSI model:

1. It is more secure and adaptable.
2. OSI model support connection free and connection oriented services.
3. It is a general model where it functions as a standard model in the computer network. Hence, OSI model separates complex function into simpler component.
4. Therefore, it ensures the inter-operable technology.
5. It solves the troubleshoot issue more quickly and effectively.
6. Thus, we compare the basic functional relationships of different networks with OSI model.

By the above explanation you have got a brief idea about OSI model and its layer. So, if you have any queries you can mention in the comment section below.