What is OSI Model?

What is the OSI Model

Are you someone who is fascinated by the power of technology and the wonders it can do? There was a time when the only way people could interact was to meet or exchange written letters that took days to reach each other. Today, due to the boon of technological advancement, we can connect and transmit data anytime, anywhere in the world. Do you want to know how two devices with no visible connection between them interact? This blog elaborates on what is OSI Model which is a fundamental communication model that is the basis of inter-device communication. 

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What is OSI Model with Example?

The open systems interconnection (OSI) model is a reference model designed by the International Organization for Standardization (1984), allowing diverse communication systems to interact using standard protocols. In simple terms, the OSI provides a standard for different computer systems to communicate with each other.

Viewed as a universal language for computer networking, the OSI Model is based on the concept of breaking a communication system into seven abstract layers, with each one stacked upon the last, performing particular network functions. 

Let’s understand what OSI Model is with an example:

Think of the OSI model as a car composed of autonomous functions that combine to achieve the end goal of moving the vehicle forward. The battery powers the electronics, the alternator recharges the battery; the engine rotates a driveshaft, an axle transfers the driveshaft’s rotation to the wheels, and so on. The OSI model layers carry out the independent functions and communicate with the layer above and below it. 

Layers of the OSI Model 

The OSI model consists of two broad layers: upper layers and lower layers.

  • Upper Layer of the OSI Model:

The upper layer of the OSI model essentially deals with the application-related issues implemented only in the software. The application layer is closest to the end-user. Both the end-user and the application layer interact with the software applications. An upper layer means the layer simply present over another layer.

  • Lower Layer of the OSI Model:

The lower layer of the OSI model concerns the data transport issues. The physical layer is the lowest layer of the OSI model and is closest to the physical medium. It is mainly responsible for placing the data on the physical medium. The data link layer and the physical layer are implemented in hardware and software. 

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List of Layers in the OSI Model 

A top to bottom list of the functional layers of the OSI Model is given below: 

  1. Application Layer
  2. Presentation Layer
  3. Session Layer
  4. Transport Layer
  5. Network Layer
  6. Data-Link Layer
  7. Physical Layer

7. Application Layer

This layer is the only layer that directly interacts with data from the user. Software applications like email clients and internet browsers depend on the application layer to initiate communications. However, you must note that client software applications are not part of the application layer. Instead, the application layer is responsible for the protocols and data manipulation that the software relies on to offer meaningful data to the user. Application layer protocols consist of HTTP and SMTP. 

6. Presentation Layer

This layer of the OSI Model is principally responsible for preparing data for the application layer to use it. In other words, it makes sure the data is presentable for applications to use. The presentation layer is responsible for the translation, encryption, and compression of data. Two devices communicating may be using different encoding methods, so the presentation layer is liable for translating incoming information into a syntax that the application layer of the device on the receiving end can understand.

Suppose the devices are interacting over an encrypted connection. In that case, it is responsible for adding the encryption on the sender’s end and decoding the encryption on the receiver’s end to provide the application layer with unencrypted, readable data. Finally, the OSI presentation layer also compressed data from the application layer before delivering it to the fifth layer: the session layer. This function assists in improving the speed and efficiency of communication by reducing the volume of data transferred.

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5. Session Layer

This layer of the OSI Model opens and closes interaction between the devices. A session refers to The time between when the communication is opened and closed. The session layer ensures that the session remains open long enough to transfer all the data to be exchanged and then swiftly closes the session to avoid wasting resources.

This layer also synchronises data transfer with checkpoints. For example, suppose a 150-megabyte file is being transferred, the session layer could place a checkpoint every 5 megabytes. If a disconnect or a crash occurs after 53 megabytes transferred, the session resumes from the last checkpoint; that is, only the part left after the first 50 megabytes of data need to be moved. Without the checkpoints, the complete transfer would need to restart from scratch.

4. Transport Layer

This layer of the OSI model is responsible for end-to-end communication among the two devices. It includes receiving data from the session layer, separating it into segments and then re-assembling the segments into data that the session layer is able to consume.,; then

The transport layer also determines the flow control and error control. Flow control determines an optimal transmission speed to guarantee that a sender’s fast connection doesn’t overwhelm a receiver having a slow connection. This layer performs error control on the receiving end. It ensures that the data received is complete and requests a retransmission if it isn’t.

3. Network Layer

The network layer facilitates data transfer among two diverse networks. If the two devices interacting are on the same network, then the network layer is futile. The network layer segregates the segments from the transport layer of the OSI model into smaller units referred to as packets on the sender’s device. It reassembles the packets on the receiving device. This layer also determines the best physical path for the data to reach its destination, a process known as routing.

The data link layer and the network layer are similar. The difference is the data link layer facilitates data transfer between devices on the SAME network. Packets from the network layer and split into smaller parts called frames. This layer is responsible for flow control and error control in intra-network communication like the network layer. The transport layer only performs flow control and error control for inter-network communications.

1. Physical Layer

The physical layer comprises the physical equipment required in the data transfer, like the cables and switches. This layer is where the data gets converted into a bitstream, a string of 1s and 0s. The physical layer of both the devices must agree on a signal convention required to distinguish the 1s and 0s on both devices.

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Data Flow Through OSI Model

For readable information to be carried over a network from one device to another, the data must move down the seven layers of the OSI Model on the sender’s device and then travel up the same seven layers on the receiving end.

For example, Mr Bing wants to send an email to Ms Green. Mr Cooper writes his message in an email application on his personal computer and then hits ‘send’. His email application will transfer his email message over to the application layer, which will then pick a protocol (SMTP) and give the data to the presentation layer. The presentation layer will then Perform its functions, and then the data will reach the session layer, which will initialise the session.

The data will then reach the sender’s transportation layer, where it will be segmented. Segments are divided into packets in the network layer, which will be broken down further into frames at the data link layer. The data link layer will then pass those frames to the physical layer, transforming the data into a bitstream of 1s and 0s and sending it through a physical medium, such as a cable.

Once Ms Green computer receives the bitstream through a physical medium (such as her wifi network), the data will move through the same set of layers on her device, but in the opposite order. 

The physical layer will transform the bitstream into frames which get passed to the data link layer. The data link layer will reassemble the frames into packets for the network. The network layer will then make segments out of the packets—the transport layer, which will integrate the segments into one piece of data.

After this, the data flows into the receiver’s session layer, which will pass the data to the presentation layer and mark the end of the communication session. The presentation layer removes the compression and passes the data in raw form to the application layer. The application layer will then feed the readable data to Ms Green’s email software, which will allow her to read Mr Bing’s email on her personal computer.

Why does the OSI Model Matter?

The modern Internet doesn’t strictly follow the OSI Model. The Internet protocol suite is considered more straightforward, while the OSI Model is still widely used for troubleshooting network problems. The OSI Model as a reference model can assist in breaking down the issue and isolating the origin of the trouble. If the problem is narrowed down to one particular layer of the model, it reduces unnecessary work and time.

Differences Between OSI and TCP / IP Reference Models

  • OSI model is a conceptual,  reference model based on each layer’s functionalities, whereas the TCP/IP model is a protocol-oriented standard. 
  • OSI model has a clear distinction between the three concepts: services, interfaces, and protocols. TCP/IP does not distinguish between these concepts.
  • OSI model gives general guidelines on how communication between two devices needs to be done, while TCP/IP protocols layout standards on which the Internet was developed. TCP/IP has a more practical use.
  • In the OSI Model, the structure of the model was developed first, and then the protocols of each of the layers were formed. In the TCP/IP suite, the standard protocols were developed first.
  • The OSI Model is seven-layered, while the TCP/IP has four layers.
  • The OSI follows a vertical approach. TCP/IP follows a horizontal path. 
  • The ISO model was developed by ISO (International Standard Organization). The TCP/IP model was developed by ARPANET (Advanced Research Project Agency Network).

Know what is OSI Model in Hindi here!

We hope this article familiarized you with what is OSI model, its layers and examples. Are you interested in Computer Science and are planning to pursue an esteemed degree from one of the world’s renowned universities? Let our Leverage Edu experts help you find the best course and university which can equip you with the requisite skills, knowledge and exposure to build a fulfilling career in the tech industry! Register today for a free e-meeting and take a step forward towards learning about the intriguing world of computers!

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