What is OSI Model In Computer Network | 7 Layer OSI Model Explain

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9 Nov, 2023

How is data transferred from one computer to another in a computer network? For this we use two networking models, the first OSI Model and the second TCP/IP Model.

We talked about the TCP/IP Model in our previous article. In today's article, we are going to talk about the OSI Model.

So without wasting any time let us know what is OSI Model? How many layers are there in the OSI Model? And how does it work?

What is the OSI Model?

The OSI model is a general-purpose networking model developed by ISO in 1984.

This model describes how information is sent from one computer to another. It explains how two devices can send and receive data among themselves.

It is used as an architectural model or reference model. It is a logical and conceptual model using which the functioning of networking can be easily explained.

Because the OSI model can fit anywhere, it is called “open source”.

There are 7 layers in this model and each layer has its own different function. These 7 layers ensure that data can travel securely from one networking device to another. ,

These layers do not depend on each other, but data is transmitted from one layer to another.

The top three layers of the OSI Model are called upper layers which handle application-level tasks and the bottom four layers are called lower layers which control the end-to-end transport of data between devices.

Summary

OSI stands for “Open System Interconnection” and this model was named by “ISO”.
This is a reference model which consists of 7 layers.
It was developed by ISO in the year 1984.
It is a 7-layer architecture.
It runs without the use of protocols.
By the OSI Model, the functions performed during data transmission between computers and devices in the network have been defined in 7-layer. Therefore it is also called “7-layer architecture model”.
OSI is a standard model that is used as a reference model by various manufacturers of network devices while making their products so that networking devices of two different companies can communicate with each other.
This model is not used in real life, it is used only as a reference. In real life, TCP/IP Model is used instead of OSI Model.

7 Layers of OSI Model -

1) Physical Layer

The physical Layer is the first and lowest layer of the OSI model. In this layer, the digital signal is converted into an electrical signal.

The main function of this layer is to transmit data in the form of bits (0 and 1), hence this layer is also called Bit Unit.

This layer works to establish and maintain physical concessions in the network. This layer tells whether the communication between two devices will be wireless or wired.

Functions of the Physical Layer

The physical layer is responsible for various functions which are as follows -:

The physical layer represents and transmits data in the form of bits.
The physical layer determines the signals that transmit data.
It completes the work of network topology.
The physical layer provides synchronization of bits.
The Physical Layer tells how many bits of data will be transferred in 1 second.
It describes how two or more devices will connect to each other.
This states whether the concession will be wired or wireless.
This layer defines how data is transferred between two devices in the network.

2) Data link layer

This is the second layer in the OSI model which works to transmit data in the form of frames in the network.

Since it transmits data in the form of frames, this layer is also called a frame unit.

In this layer, packets of data sent by the network layer are decoded and encoded.

The data link layer ensures that there are no errors in data packets. Node-to-Node Delivery takes place in this.

The data link layer has two sub-layers, the first is MAC (Media Access Control) and the second is LLC (Logic Link Control).

In this layer, HDLC (High-Level Data Link Control) and PPP (Point-to-Point Protocol) are used for data transmission.

Functions of Data Link Layer

The data link layer encodes and decodes data packets received from the network layer and these data packets are called frames.
The data link layer connects the header and trailer to these frames.
The function of this layer is to control flow. In this, a fixed data rate is maintained from both the receiver and sender sides. So that the data does not get corrupted.
This layer also works to control errors. In this, CRC (cyclic redundancy check) is added to the trailer of the frame so that there is no error in the data.
This layer also performs the function of access control. It determines which device is given access when two or more devices are connected over a communication channel.

3) Network layer

This is the third layer of the OSI model whose function is to provide IP addresses to networking devices.

In this layer, switching and routing techniques are used to transmit data packets from one device to another.

The data in this layer is in the form of packets, hence this layer is also called packet unit.

The network layer works to transmit data packets from one device to another. Along with routing, and forwarding, this layer also performs the work of addressing, internetworking, error handling, and congestion control.

Network layer functions

Its function is to provide IP addresses to the devices.
The network layer adds the source and destination IP addresses to the header of data packets. Which is used to uniquely identify network devices on the Internet.
It works to send data packets from source to destination.
It determines the best and appropriate path by switching and routing.

4) Transport layer

This is the fourth layer of the OSI model. The function of this layer is to ensure that the data is received by the receiver in the order in which it was sent by the sender and there is no error in the data. If any error is found in the data then it retransmits the data.

The two main protocols of this layer are TCP (Transmission Control Protocol) and second UDP (User Datagram Protocol).

This layer is used to transfer data properly between networks.

The transport layer takes data from the session layer. It also manages end-to-end message delivery in the network.

Functions of Transport Layer

Transport Layer performs both flow control and error control functions.
Its function is to transmit data from a source device to a destination device.
It facilitates communication between two devices.
It provides the facility of point-to-point connection.
It also works to control the connection.

5) Session layer

It is the fifth layer of the OSI model which creates a session for communication between two devices.

The main function of the Session layer is to control the connection between devices.

The Session layer establishes, maintains, and terminates the connection between the sender and the receiver, and it also performs the functions of dialog control and synchronization.

It controls the communication between different computers.

Functions of Session Layer

The function of the Session Layer is to establish, maintain, and terminate the session between two devices.
It also acts as a dialog controller. It creates a dialog between two processes.
It also works for synchronization. That is, whenever there is an error in transmission, it repeats the transmission.

6) Presentation layer

This is the sixth layer of the OSI model whose function is to control the format of data.

In this layer, data is encoded and decoded and data is also encrypted and decrypted.

The presentation layer maintains the syntax of the data properly, hence it is also called the syntax layer.

The presentation layer is related to the operating system and is also used for compression of data.

This layer determines how the data is presented. IMAP, SSL, and SSH protocols work in this layer.

Functions of Presentation layer

Encryption and decryption are done to keep the data secure in the Presentation Layer.
In this layer, compression is also done to reduce the size of the data.
Along with compression, it also does the work of translation.

7) Application layer

This is the seventh layer of the OSI model whose function is to provide an interface between the real application and other layers.

It is very close to the end user, and it provides network services to the end users.

Protocols like HTTP, FTP, SMTP, and NFS come under the application layer.

The application layer controls how any application accesses the network. It acts as an interface between network services and applications.

Functions of Application Layer

Through the application layer, the user can remotely access files from the computer.
Through this, emails can be forwarded and stored.
Additionally, the directory can also be accessed from the database.

Features of OSI Model

The following are the characteristics of the OSI model:

The OSI model is a general-purpose networking model that serves as a reference model and has no practical use.
Problems occurring in one layer do not affect the other layers.
It is a flexible model that provides both connection-oriented and connectionless services.
It is used as a standard model.
Through this, the communication between two devices in the network can be easily understood.
It has 7 layers and each layer has its own function.

Advantages of OSI Model

OSI model has the following advantages:

The OSI model is a generic model that is used as a reference as a standard model.
This is a flexible model because we can use any protocol in it.
It can be easily administered and maintained.
This is a secure networking model.

Disadvantages of the OSI model

OSI model has the following shortcomings:

The OSI model does not define any specific protocol.
It is difficult to implement new protocols in this.
This is a complex model whose initial implementation is very slow and expensive.
In this, all the layers are internally dependent on each other.
OSI model is a reference model which is not actually used.

What is the difference between the OSI Model and the TCP/IP Model?

Following are the differences between the OSI Model and the TCP/IP Model -

OSI Model	TCP/IP Model
The full form of OSI is “Open Systems Interconnection”.	The full form of TCP/IP is Transmission Control Protocol/Internet Protocol.
The size of the OSI header is 5 bytes.	The size of the TCP/IP header is 20 bytes.
There are 7 layers in the OSI Model.	TCP/IP has 4 layers.
The OSI Model follows the vertical approach.	The TCP/IP Model follows the horizontal approach.
The OSI model is developed by ISO.	The TCP model is developed by ARPANET.
In the OSI model, the transport layer is only connection-oriented.	In the TCP/IP model, the transport layer is both connection-oriented and connectionless.
In the OSI model, the data link layer and the physical layer are two separate layers.	In the TCP/IP model, both the physical layer and the data link layer are combined as the network layer.
Both the session layer and the presentation layer are part of the OSI model.	There is no Session layer and presentation layer in the TCP/IP model.
In the OSI model, the transport layer provides guarantees for the delivery of packets.	The transport layer in the TCP/IP model does not provide guarantees for the delivery of packets.
The protocols in the OSI model can be easily changed as technology changes.	In the TCP/IP model, the protocol cannot be changed easily.
There is little use of the OSI model.	The TCP/IP model is widely used.

FAQs – Frequently Asked Questions

What is the full form of the OSI Model?

The full form of the OSI Model is the “Open System Interconnection Model”.

What are Computer Network Models?

A systematic approach was required so that two or more users could easily share data among themselves in a computer network. This approach is implemented using models in computer networks and is known as the computer network model. Computer network models are responsible for establishing the connection between the sender and the receiver and transmitting the data smoothly.

Learn More...

Conclusion

in today's article, we talked about the OSI Model and learned what is OSI Model? How many layers are there in the OSI Model? And how does it work?

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What is OSI Model In Computer Network | 7 Layer OSI Model Explain