The OSI (Open System Interconnection) Model - Careerneeti

The OSI (Open System Interconnection) Model

OSI Model and its layers with example, Logo

What is the Open System Interconnection (OSI) model?

The Open System Interconnection model (OSI) is a seven-layer model used to visualize computer networks. In 1970, the International Organization for Standardization (ISO) conducted a program to develop general standards and methods of networking. A similar program started at the International Telegraph and Telephone Consultative Committee. Both bodies developed documents that defined similar networking models. In 1983, these two documents were merged to form a standard called The Basic Reference Model for Open Systems Interconnection. The standard is usually referred to as the Open Systems Interconnection Reference Model, or simply OSI model. The OSI model is often viewed as complicated and many fear to have to learn the model. However, the OSI model is an extremely useful tool for development and problem-solving. 

There are seven layers of the OSI model. These layers can be broadly divided into two parts:

  • Host Part or Software Parts: This part is closer to human e.g. Web-browser, Operating system, etc.
  • Media Layer or Hardware Part: This part is where all the machines reside.

Let’s discuss each of these layers in details. While discussing we will move from top to bottom layer.

  • Application Layer:  The application layer is where user inputs data. The application layer is usually software that is run off the local machine. The application layer provides services for email, Telnet and File Transfer, etc. For example, when you have to access you will have to open a web browser and enter the web address to the address bar.
  • Presentation Layer:  The presentation layer is where the data get formatted or encrypted. The operating system resides in this layer. This operating system could be Windows 10, Mac OS, Linux, etc. This is the layer where human interacts with the application layer and the application layer interacts with the Presentation layer.
  • Session Layer:  This layer is responsible for creating and maintaining the session between the operating system or presentation layer and third-party application or web servers. For example, when you entered the address in the browser, the application layer interacts with the presentation layer and the session layer, which allows the operating system to create a session with the web server.
  • Transport Layer:  The Transport layer is responsible for the transmission of data. When we request for any web page, the request gets split into multiple packets and transport layer make sure how much and what information gets transferred between the operating system and web server.
  • Network layer:  The Network layer is where all the hardware components start to work. The router operates at this layer. A router is a hardware device which is responsible for forwarding the packets of information to a different computer via the internet. This is the layer where the IP addresses come into play and based on this address router determine where to send or receive information. In your computer command prompt or terminal, if you type ipconfig, you can find your system IP address.
IP Address Command Explained

  • Data Link Layer:  Another hardware component Switch works at the Data link layer. These switches provide a reliable link between two connected nodes (A node is any computer connected within a network/Internet). This layer also fixes any packet error might occur at Physical Layer (we will discuss next). This layer can be divided between two layers, Media Access Control (MAC) and Logical Link Control (LLC). When you type ipconfig /all in command prompt as shown below, with your computer IP address you can see Physical Adress as known as MAC address, which also gets associated with the packet network layer send over to the web server.
MAC address command explained

  • Physical Layer:  The Physical layer is what makes a network. The Ethernet cable or LAN cable, Bluetooth, etc. are few of the example which makes a network. This layer where we need to determine the Network transmission mode (Simplex, half duplex, full duplex, etc. we will discuss on these in our next post), Network topology, Protocols, and any other physical specifications.

Let’s understand all these with an explanation provided below:

OSI Model Explained, Logo

When you type in in your web browser (Google Chrome), you basically raise a request for the web page (home page) of  "Careerneeti" website to the web server. This Request then gets passed from one layer to another from Top to bottom as shown in figure step 1. Once the request reached the physical layer it gets connected to the server-side physical layer and then at step 2 your request gets forwarded to each layer from bottom to top. Once the Server has the response ready for you it sends the response in the similar fashion as shown in step 3 and steps 4. Once the response reached to your nearest layer i.e. Application layer, you are able to see the home page of Careerneeti.

Easy way to learn the layer's name.

All = Application Layer
People= Presentation Layer
Seem = Session Layer
To = Transport Layer
Need = Network Layer
Data = Data Link Layer
Processing = Physical Layer

We hope you find this article helpful. If you have any question leave it in the comment section.

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