OSI Model Layers — “Explained”

The OSI Model sure seems weird and hard to understand to many people, therefore I am writing this short and to the point article to help out whoever is having trouble understanding the OSI Model.

Definition

The OSI (Open Systems Interconnection) Model is a conceptual model that characterises and standardises the communication functions of a telecommunication or computing system without regard to its underlying internal structure and technology. Its goal is the interoperability of diverse communication systems with standard protocols.

Let’s Understand

Okay, that was the proper definition of OSI model. The definition looks quite simple and to the point, but when it comes to the elucidation of OSI model, many of us get confused and are not able to realize what it really is.
First thing that comes to people’s mind when they hear the OSI model is that they will have to do a lot of memorization and cramming. This was exactly what happened when I read about OSI Layers the first time.

So, in this article, we will see about the OSI model and it’s seven layers. Therefore, read this article if you need to understand what the OSI model is all about.

The OSI Model

OSI stands for “Open Systems Interconnection”. The exact definition of the OSI Model has been already given above. In simpler words, to be precise, the OSI model is a tool used by IT professionals to actually model or trace the actual flow of how data transfers in networks. So, basically, the OSI model is a logical model/representation of how the network systems are supposed to send data (or, communicate) to each other.

What is it composed of?
The OSI Model breaks down this data transfer/communication procedure into different components (called layers). Why layers, because those components follow a proper order of execution. For example, the Physical Layer in which the “physical” wiring and connections take place, the Data Link Layer in which switching takes place, etc. In a total, there are seven layers which together make up the OSI Model.

Why OSI Model?

The purpose of the OSI reference model is to guide vendors and developers so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools. Most vendors involved in telecommunications make an attempt to describe their products and services in relation to the OSI model.
Moreover, it is critically very important for an IT professional to have a clear idea about the OSI Model. This is due to the fact that, in case of some network problems, using the OSI Layers, they can narrow down and figure out in which part the problem is. So having an OSI Layered approach for troubleshooting network problems is a very useful.

Advantages Of OSI Model

  • It creates a common platform for software developers and hardware manufactures that encourage the creation of networking products that can communicate with each other over the network.
  • It helps network administrators by dividing large data exchange process in smaller segments.
  • Due to the independence of layers, it prevents changes in one layer from affecting other layers.
  • Standardization of network components allows multiple-vendor development.
  • It structures very well the functions particular to each layer.
  • It reduces complexity and accelerates evolution
  • It simplifies teaching and learning

Seven Layers Of OSI Model

As discussed in the previous section, the OSI Model is composed of seven layers with the application layer, which is closest to the end user, at the top, going all the way down to physical layer, in which the actual data transfer happens with the use of a transmission medium.

7. Application Layer

This is the topmost layer in the seven OSI Layers. This is the layer that the end-user (can be a computer programmer, or a regular PC user) is actually interacting with. This layer allows access to network resources.

6. Presentation Layer

This is the layer in which the operating system operates with the data. Main functions of this layers includes translation, encryption and compression of data. Basically User interacts with Application layer, which sends the data down to Presentation layer.

5. Session Layer

This layer has the job of maintaining proper communication by establishing, managing and terminating sessions between two computers. For example, whenever we visit any website, our computer has to create a session with the web server of that website.

4. Transport Layer

This layer has a very important job. It decides how much information should be sent at a time. So, when you are communicating with a website, this layer will decide how much data you can transfer and receive at a given point of time. Also, this layer provides reliable process to process message delivery and error recovery.

3. Network Layer

The main job of this layer is to move packets from source to destination and provide inter-networking. This is the layer that the routers operate on. Since routers operate at the network level, hence we can say that the IP address is at the network level.

2. Data Link Layer

This layer is responsible for organising bits into frames and ensuring hop to hop delivery. This is the layer on which the Switches operate on. Since routers operate at the network level, hence we can say that the MAC address resides at the data link layer. All the computers in a specific network get plugged inito a switch so that they can communicate with each other.

1. Physical Layer

This is the layer on which the real transmission of data bits takes place through a medium. This layer is, as the name suggests, all the physical stuff that connects the computers together.

A simple mnemonic for memorising the names of OSI layers

There is a simple way to learn the OSI Layers, just remember this sentence

All People Seem To Need Data Processing

A — Application — All

P — Presentation — People

S — Session — Seem

T — Transport — To

N — Network — Need

D — Data Link Layer — Data

P — Physical — Processing

Concluding Words

This was a short and to the point article explaining the OSI Model, to help out all those who are having hard time understanding the model. Hope it will help you understand the OSI model, in case you are stuck and trying to by-heart and cram the OSI Layers.

Also, you can refer this awesome youtube video to get a clear picture of OSI Model, which is explained by taking analogy with train and railway system.

Hope it helps :)

Thanks a lot for reading till end. 
All the best and Happy Learning!
You can contact me in case of any doubts or if you need any assistance:
Email: madhavbahl10@gmail.com
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References

  1. https://searchnetworking.techtarget.com/definition/OSI
  2. https://www.youtube.com/watch?v=-6Uoku-M6oY