Every guy studying or working with computers has heard about OSI model. People working in networking have to deal with this on a daily basis, where as students need to understand this for their networking subject. There’s a lot of confusion around this OSI model which makes it look more sophisticated than it actually is, making students hate it even more. However, if understood properly OSI model will be one of the easiest topic in networking.
In this post, we’ll try to breakdown each layer systematically and try to explain it briefly in simple terms. You may come across books and long 4-5 pages articles or 40-50 minutes video explaining it. Those are great, but they dive right into the depth explaining each and everything in great details. In this post, we’ll try to paint a basic idea in your mind. Keep it simple to you can easily understand what happens where. Once you get an overview, you can then dive deep into it and understand it if you want to. We’ll give you enough knowledge to even write and explain it in your own words if you want to.
What is OSI model?
The Open Systems Interconnection model (OSI) is a conceptual model which was introduced to standardized the way data is transferred and received on the network all over the world. In simple words, it is a standard model for systems to communicate with each other. Following the OSI model has helped IT professionals deal with problems easily, because every country, every city is using the same set of rules and protocols for transferring and receiving data.
When a data packet is being transferred and received over any network various layers take part. Collection of all these layers is OSI model. To understand OSI model, we need to understand each layer. There are 7 layers in OSI model:
Physical, Data-link, Network, Transport, Session, Presentation and Application layer.
We can use a mnemonic to remember these layers. A great mnemonic will be
Please Do Not Throw Sushi and Pizza Away.
OSI model!
Let’s simplify each layer.
1. Physical Layer:
This layer deals with the physical connection between the two devices which are trying to communicate. Which means the patch cable connecting two computers is the Physical layer of the OSI model. The first thing while troubleshooting a network problem is to check for physical layer connection. 95% of time this will solve your problem.
2. Data-link Layer:
Computers usually store the logical address of the recipient. Now the data it is about to send will be addressed to the logical address but cannot be sent directly to it, because the logical address is just for the understanding of the computer. We work with physical address. So, we need a device which translates the logical address into physical address and this process takes place in data-link layer with the help of Switches. Encapsulation of data into frames takes place in this layer.
3. Network Layer:
The network layer is the most critical layer of OSI model. In this layer, basic questions like which protocols to be used? The best route to destination? Or the shortest path to the recipient? Routers work in the Network Layer. Network layer is also responsible for encapsulating segments into packets.
4. Transport Layer:
This is where http comes into the picture. A connection is established with the recipient using the TCP protocol. A number is assigned to each packet. Now, Transport layer determines the number of packet to be sent at a time and packets to be sent to avoid congestion. It basically deals with how much information should be transported in one go.
5. Session Layer:
When you connect to a web server or login into one, the server starts a session between your machine and the web server. A token is used to recognize each session, usually a number which is unique. Creating a new session is like creating a new identity. The web server will then find you by your session id. Apache and Php config files usually define how to create a session.
Sessions play an important role during online transactions and in e-commerce websites. Http is a stateless protocol, hence when you move from one page to another, you will lose your data as http doesn’t care about which data was with you before. Sessions makes sure your data is intact irrespective of which protocol or page you’re on.
6. Presentation Layer:
Every webpage has texts, images, videos, html files. Presentation layer recognizes these common formats and displays them. This is where your Operating System comes into picture. If you don’t have audio driver and you try to play a song, the presentation layer will tell you it can’t do so because of absence of audio drivers.
7. Application Layer:
Students usually consider Firefox or Chrome or Internet Explorer as Application Layer. These browsers are not the application layers. The common protocols used by these browsers are application layer. This is where the data is actually shown to the users via the browsers. But the common protocols used by the browsers and applications like Skype or Yahoo Messenger are application layer.
To troubleshoot any problem, we can simply put our understanding of OSI model into use.
If you have understood each layer and know its working, we can find our any problem within few minutes. Once we know the problem, we can then start working on it.