What is the OSI Model? Layers of OSI Model

What is the OSI Model? Layers of OSI Model

OSI Model describes how computer applications can communicate over a network.

It was first adopted in the 1980s by all companies like telecommunication and computer companies.

In today’s Moden world OSI is not used, Modern technologies used simple TCP/IP models, but OSI 7 layer model is widely being used.

OSI Model shows how the network is operated, how is the network architecture from top to bottom, how it troubleshoots network problems. Different Layers have different functionalities.

What is the OSI Model? Layers of OSI Model
Layers of OSI Model

Let’s Start the OSI 7 Layer’s Model:-

7. Application Layer

Application Layer is used by you and I. Application Layer are end-user software like a web browser and email clients.

This Layer is used to interact with software applications that help users to access the Network Services. When any user you, me, and I want to share files, images, videos, audio, etc.. this layers allow to interact with the application and allow to communicate in a network.

A Network Service Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Post Office Protocol (POP), Simple Mail Transfer Protocol (SMTP), and Domain Name System (DNS) uses applications like Chrome, Firefox to deliver an HTML file and show the result as per user requirements.

6. Presentation Layer

The presentation layer is also known as the Translation layer as it Translates, Encrypts, Decrypts, Compression, Decompression the data.

  1. Translation: It Translates from ASCII to EBCDIC
  2. Encryption-Decryption: Encryption translates data into a secure code which is also known as ciphertext, whereas Decryption is a breakdown of encrypted code, which is plain text and can be read.

What is the OSI Model? Layers of OSI Model

  1. Compression: Compression reduces the size or number of bits which has to be transmitted on the network.
  2. Decompression: Decompression is a process in which data files come into their original form.

5. Session Layer

If anyone wants to send any information or any type of file to other devices there is a need for a session and session layer to create a communication channel between them.

First, a session is created between two devices that want to communicate which is an authentication step, and then the Second step authorization step establishes in which Session Layer gives access to the user for sharing information and communication.

Session Layer is responsible for the opening session(when data/information/communication) and for the closing of the session(When communication ends).

Session Layer checks during data session and checks the errors and interruptions causing and resumes it if any.

Here are some main functionality of the Session Layer:-

  1. Session Management: The session layer allows two devices to establish, use and terminate a connection between them.
  2. Synchronization: Session Layer creates checkpoints also called synchronization points into the data. These synchronization points in the session layer find the error and fix them and re-synchronize properly, so end-users are not disconnected and don’t lose data.
  3. Transmissions: half-duplex or full-duplex is used to communicate between devices.

Application Layer, Presentation Layer, Session Layer are known as the Upper Layers of OSI Model.

Let’s consider a scenario when there is a user who wants to send a message/file/audio/video/image via some application/software running in his browser(Chrome, Firefox). The “Application” here acts as an application layer that provides the user with an interface to create the data and this message/image/file… called Data and then compressed, and then encrypted (if any secure data), and then converted into bits (0’s and 1’s) so that it can be transmitted.

4. Transport Layer

Transport Layer takes the data from the session layer in an application and further breaks it into ‘segments(data in transport layer is known as segments)’ and transmits it to the network layer.

The transport layer is also known as the Heart of the OSI Model and is operated by Operating System.

Transport Layer is responsible for sending end-to-end delivery, it is also responsible for sending data at the desired rate of connection speed at the receiving end. In simple terms transport layer is responsible for checking errors, controlling errors, and checking is data received at the correct application port number.

As each application has a port number through which the transport layer can identify the correct data to be received by the correct application.

• Sender’s side: 

When the transport layer receives data from the session layer, the transport layer breaks it into segments and checks and implements the flow, and also controls error for proper transmission of data.

The transport layer assigns the Source and Destination Port numbers and forwards them to Network Layer.

Note: The default port number used by web applications is port number 80

What is the OSI Model? Layers of OSI Model
• Receiver’s side:

After, the message is sent by the sender’s side transport layer reads the port number from the receiver side and forwards it to the respective application.

Functions of Transport Layer :

  1. Segmenting & Reassembling: After receiving the message from the session layer, the transport layer breaks this message into smaller units called segments. At destination end/receiver’s end message is reassembled.
  2. Service Point Addressing:  For a correct message passing and passing a message to the correct destination transport layer assign a port address/number and the transport layer specify the correct address and pass it to the destination of the correct application.

Services of Transport Layer :

  1. Connection-Oriented Service:
    – Connection Establishment
    – Data Transfer
    – Termination / disconnection
    After receiving the data at the receiver’s end this layer sends an acknowledgment for confirming the secure and reliable transmission.
  2. Connectionless service: This service, does not acknowledge receipt of a packet from the receiver side. This is much faster and reliable than a connection-oriented service.

Multiplexing & Demultiplexing

The transport layer also used Mutilpexing, in which if there are many applications open in a computer and wants to send data in a single media channel, then there is a technique multiplexing used by the transport layer.

What is the OSI Model? Layers of OSI Model
Multiplexing & Demultiplexing

Also at the receiver’s end, there are many applications running on a computer and these applications want to receive data/message then demultiplexing is used by the transport layer which knows from where and which application data is coming from.

3. Network Layer

Network Layer is the third layer of the OSI Model which breaks segments(segments are known as packets in network layer) into network packets, and then collects/reassembles these network packets at receiving end.

The network layer also discovers the best and nearest path for sending and receiving the network packets and this is known as Routing Packets(use Internet Protocol address to route packets at destination end).

The network layer transfers data packets from one network to another network with the help of the sender’s & receiver’s IP addresses

Function Of Network Layer :

  1. Routing: The network layer routes data packets to the best suitable path which is known as routing.
  2. Logical Addressing: For reliable and distinguish uniquely Network layer defines IP addresses at the sender’s and receiver’s end.

2. Data Link Layer

Data Link Layer is responsible for breaking up data packets into frames(packets in the data link layer are known as Frames) coming from Network Layer and sending them from source to destination. It is also responsible for establishing and terminating connections between physical nodes connected on a network.

Switches and Routers are used to establish a Data Link Layer.

What is the OSI Model? Layers of OSI Model
Data Link Layer

The network layer has two parts:-

  1. Logical Link Control (LLC):- LLC is responsible for checking errors in connections, checking network protocols, and synchronizing frames.
  2. Media Access Control (MAC):- MAC in this part Data link layer check the MAC address of sender and receiver and deliver it to the desired destination source(receiver’s end).

It is the responsibility of the Data Link Layer to transmit the correct message to the correct destination on a physical layer.

When a message is sent to the receiver’s MAC address an ARP(Address Resolution Protocol) requests and asks Who has that IP address? then at the receiver’s end host’s MAC address of the device will reply.

Functions of Data Link layer :

  1. Framing: Framing of data, and transmits only a set of bits that are meaningful to the receiver.
  2. Addressing: After creating frames of data packets,  Physical addresses (MAC address) of the sender & receiver are assigned to the frames
  3. Error control: Checks the damages and retransmit frames.
  4. Flow Control: Data sent to the other side is checked and monitored by the data link layer and maintains a balanced flow of control to prevent data corruption.
  5. Access control: When there are multiple devices connected, MAC addresses helps to determine which device has control over a connection.

1. Physical Layer

Physical Layer as the meaning of this layer is in its name Physical which consist of cable and wireless connections between networks.

In the physical layer data (raw data – which is in form of 0 & 1) is transmitted through electrical cable (wired) or wireless. It’s only a physical connection between networks in the OSI Model, which has information in form of BITS.

Physical cables used in Physical layer- Fiber Optic cables, Twisted Pair Cables, Coaxial Cables.

Physical Devices used in Physical Layer- NIC cards, Repeater’s, HUB, etc.

What is the OSI Model? Layers of OSI Model

Functions of Physical Layer :

  1. Bit rate control: Physical layer also controls the rate of transmission which is managed by NIC cards, Repeater, HUB, etc.
  2. Physical topologies: In the Physical layer devices/nodes are arranged in a network Like a bus, star, or mesh topology.
  3. Transmission mode: In the Physical layer, there is a flow of data in which transmission data is flowing Simplex, half-duplex and full-duplex.

Advantages of OSI Model

  • Can Easily check the required hardware and software to establish their network.
  • Can Easily troubleshoot, by identifying the issue caused by layer.
  • Can Easily Communicate with users to check which network layers will operate their products.

You can also read these topics:

What is Hub? Types of hub


What is Switch? and types of Switch

What is a Router? and How it works?

What is Unicast, Multicast & Broadcast?

What is Network Topology? and its types

What is Computer Network and why we need it?

What are the components of a Computer Network?

What is Switching in Computer Networks? and types of Switching

What is Data Transmission Modes? Types of Transmission Modes?

What is Data Transmission Media? and Types of Transmission Media

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