OSI Model (Open Systems Interconnection Model)

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Catalysts Reachout

4 min readOct 29, 2022

What Is the OSI Model?

The Open Systems Interconnection (OSI) model describes seven layers that computer systems use to communicate over a network. It was the first standard model for network communications, adopted by all major computer and telecommunication companies in the early 1980s.

When I was in college, I remember the first time I saw the OSI model. My first thought was W…. T…. F…. Of course, the diagram my professor showed was overly confusing and I instantly felt overwhelmed. Early on, my goal was just passing the class but as I went through, I quickly learned that the OSI model is one of the most important things you can learn.

Read until the end for some ways to use the OSI model to help solve your biggest issues with concepts explained.

Let’s get into the OSI model that help explain the different layers!

Application Layer

The application layer is where the humans interact the most. When you think of the application layer think about the different tools that you use like the web browser or your local email client. This layer is where things like HTTP, FTP, and DNS live. These are the applications that run on the network. This is usually the last layer that you will use when you are troubleshooting issues on the network.

Presentation

The presentation is probably the least talked about layer. It has one job and that is determining what data should look like. When you hear words like encoding, encryption or compression you are in the presentation layer. It’s job is to take any data that is transmitted from the application layer and prepare it for the session layer.

Session

The session layer is in charge of making sure that the communication channels stay operational so that the application on one device can continue to talk to the remote device. Throughout the communication, the session layer will send different checkpoints to make sure that the link is still up and operational. If for some reason there is an interruption, the session layer will attempt to re-open the connection so that devices can resume sending data.

Transport

My favorite layer when it comes to cyber security and that is the transport layer. The transport layer takes the data from the session layer and breaks it into segments that can be sent over the network. This layer is in charge of controlling how much data is sent at one time over the network and it also has the *ability to check to make sure that data makes it to the remote device.

Network

Now we are onto the network layer, and this is where packets come into play. The purpose of the network layer is to break segments up into packets, so they are ready to be sent. The network layer on the remote end will reassemble those packets and pass them off to the upper layers. The network layer uses IP Addresses in order to route packets through the network.

Data Link

Have you ever heard of a MAC address? If so, you are talking about the layer 2 protocol or the data link layer. Connecting two physical interfaces like a network card is the purpose of the data link layer. There are two main components of the data link layer, and they are the Logical Link Control or LLC and it is responsible for identifying what network protocols are being used and it helps to synchronize ethernet Frames. The second portion of the data link layer is the MAC address which is used to identify the different network devices that are transmitting and receiving data.

Physical

The physical layer is where cables and wireless cables come into play. At this layer the 1’s and 0’s or bits are transmitted as electrical pulses or pulses of lights when it comes to technology like fiber.

Now before we go, lets talk about why we need to know the OSI model, how can we apply this to our job as an IT specialist or cyber security professional. When it comes to the OSI model, it is important to know the different layers so you can properly troubleshoot issues that may popup.