OSI Model of Computer Network - MetaTutorials


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OSI Model of Computer Network

Here, we will learn about the OSI model of Computer Network in detail with its characteristics, functions, advantages and disadvantages. 

What is the OSI Model?

OSI is an abbreviation of Open Systems Interconnection. It is a conceptual framework that describes how the data is transferred from the software application of one machine to the software application of another machine through the physical medium. 

This model uses the seven different layers for describing the network connections, so, sometimes this model is also known as the OSI seven-layer model. In 1984, the International Standard Organization introduced the OSI model for inter-computer communications.

In this model, the network architecture is divided into seven small different layers or sub-components. Each layer performs a specific task independently and interacts with the above and lower layer  automatically.

Characteristics of OSI Model

Following are the characteristics of the OSI Model:
1. This model helps to understand the relationship between the network devices easily over both small and wide area networks.  
2. This model helps the network administrators by dividing the large data chunks into smaller sub-components. 
3. As the data moves layer to layer, the function of each layer varies and the data complexity reduces. 
4. It also shows how the functions of software and hardware perform together.
5. The OSI model easily detects or finds errors at different layers.
6. The OSI model is categorized into the following two groups of OSI layers. 
a) Upper Layers:
Those layers which are closest to the end-user and deal with the issues related to the software applications.
b) Lower Layers: Those layers which are closest to the physical medium and which deal with the problems of data transmission. These OSI layers placed the data on the physical medium that needs to be transmitted. 

Function of OSI Model

OSI Model breaks down the network process into several layers. Each OSI layer performs a different function serially.  
Following are the seven OSI layers that play an important role in data communication:
1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer.

Let's discuss each layer in short one by one.

Physical Layer

The Physical Layer is the lowest and closest layer to the physical medium of the OSI model. This layer transmits the unstructured bits from one device to another over the network.

It establishes and maintains the physical connection between two or more network devices. It also specifies how the data is flowing between the two devices.

Data Link Layer

The Data Link Layer makes the data error-free which is transferred from one device to another. It divides the received data packets into frames and assigns the MAC address of sender and receiver to each data packet. 

Network Layer

The Network Layer finds the best path for sending the packet to the destination and places the IP addresses of sender and receiver in the frame header. 
This layer is responsible for breaking the segments into packets. It also resembles these packets received from the data link layer on the receiver device.  

Transport Layer

The Transport Layer receives the data from the higher layer and converts the data into segments. This layer makes the point-to-point connection between the sender and receiver device. 
It uses Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).  
This OSI layer controls the flow of data and performs the error control technique.

Session Layer

The Session Layer manages and synchronizes the interactions between the connected devices over the network. 
This layer also provides those services which include the functions of authentication and authorization. 

Presentation Layer

The Presentation Layer converts the data with the help of syntax and semantics for the application layer. Sometimes, it is also called the syntax layer. 
It uses encryption and decryption techniques for data security and privacy. 

This layer also performs the process of data compression. Data Compression minimizes the number of bits which are transmitted between the devices over the network. This mechanism transfers the bits at a fast speed over the network.

Application Layer

The Application Layer is the closest to the end-user and the uppermost layer of the OSI model. This layer directly interacts with the users and provides the network service to them with the help of software applications. 
HTTP and SMTP are the two protocols of the application layer.

Advantages of OSI Model

Following are the advantages of the OSI Model:
1. If the changes are done in one layer of the OSI model then those changes do not affect the other OSI layers.
2. It is a standard model and provides modular engineering in computer networking. 
3. This model supports both connectionless and connection-oriented services. 
4. Communication between the different types of software and hardware is easy.
5. As we know that, the model breaks the process into small subcomponents or segments. So, the design, development, and maintenance of the network are very easy in this model.
6. The security of this model is high.

Disadvantages of OSI Model

Following are the disadvantages of the OSI Model:
1. As we know, the layers do not work in parallel. So, each layer has to wait to take the data from its previous layer.
2. The initial implementation of this model is costly and time-consuming. 
3. Some services are common in more than one layer. Error Control and Flow control services are provided by multiple OSI layers.
4. The standards of this model are generic or theoretical. That's why practical implementation is not possible in this model.