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What are the OSI and TCP/IP models?

Christophe September 6, 2021

The OSI Model

Also known as the OSI reference model, it separates the technologies used in networking into seven parts, also called layers. You might have heard people saying things like “that’s a layer 3 issue.” They are referring to the third layer in this model.

The seven layers of the OSI reference model, starting from the lowest layer, are: 

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

A popular mnemonic to help you remember is “Please Do Not Throw Sausage Pizza Away.” 

The layers are usually represented in a stack, that looks like the following:

Now, you might be wondering. Why seven layers? What do they do? Why is the image starting from the 7th layer?

Don’t worry, we will discuss these layers a little more in-depth in this section of the course. Also, we will discuss them even more in-depth in future sections. But for now, we need to cover other introductory topics.

What is the TCP/IP model?

The TCP/IP model, just like the OSI reference model, separates technologies into layers. But the TCP/IP model only uses 4 layers. Depending on the source, you can also find this model referenced as TCP/IP stack or Internet Protocol Suite. These are the layers for the TCP/IP model:

  1. Link
  2. Internet
  3. Transport
  4. Application

The TCP/IP model was developed before the OSI Model, so by the time the OSI model was finished, the TCP/IP model had already gained traction and acceptance from different organizations like the Department of Defense and IBM.

The TCP/IP model became the standard for modern networks and is the model that we all use now on the modern Internet. But the OSI model was used to influence the improvement of the TCP/IP model and is still used for academic purposes and as a reference model. 

The OSI model is so prevalent, that in the IT field when we refer to a layer with a number, like Layer 7, we are referring to the OSI Model and not the TCP/IP model.

But both models, in general terms, are not that different. This is how they compare to each other

As you can see, the first two layers on the OSI model (Physical and Data Link) are equivalent to the Link Layer on the TCP/IP Stack. The 3rd layer (Network Layer) is equivalent to the Internet Layer, the Transport layer stays the same, and the last three layers (Session, Presentation and Application) are merged into one single layer called Application.

Since this is a Networking Fundamentals course, we will be covering from layer 1 to 4, on the OSI model. As the TCP/IP stack shows, the last 3 layers are considered the Application layer, so we will only briefly mention those last 3 layers in this course, and will instead focus most of our time on the first 4.

Why do they exist?

Now, what’s the point of these models and their layers? Let’s use the same example from our introduction, where are currently watching a video or reading this text. To simplify a little bit, let’s imagine that you have a video library (or text) stored on another computer. So you connect your personal computer to the computer storing the videos using an ethernet cable. Think of this as the smallest network that can exist. A network with two devices. Or, think of it as your computer being a Client, and the other device being a Server. This is called a Client-Server architecture.

Communication is easy. Sure, you need to do some quick configuration to share folders and files, but once it’s done you can share files and information quickly.

After this, your family also wants to have access to your video library. Some of them have computers from other manufacturers, and some connect via wireless instead of an ethernet cable. Maybe they need to access the video library from a different room or house. So your home network starts to grow.

Now, even your neighbors are asking to have access to the video library! So you end up with something that looks like this.

That’s a big infrastructure, and you made a big investment

But there’s an issue. You have network devices from different manufacturers, and each vendor uses a proprietary stack or model, and therefore it can’t “speak” to other devices.

Maybe your customers or partners are using a different brand of computers, and it’s impossible to communicate with them, so you can’t properly use the advantages of networks. There is zero interoperability: meaning there is no ability for computer systems or software to exchange and use information.

That’s why organizations like the IETF (Internet Engineering Task Force) and ISO (International Standardization Organization) developed standards – TCP/IP model and OSI model respectively – to guarantee the communication between different devices, regardless of manufacturer.

Now regardless of what vendor you should choose, there are no problems with interoperability. That’s why you can have a brand of computer connected to a wifi router from a completely different vendor to access the Internet.

Think of it as languages. In the past, every company created its own “language” for network communication. But as new technologies emerged, the devices using these new technologies were not able to “speak” with other devices.

So the models give us a neutral language that all devices can speak. With this new language, you can grow your network as much as you want, regardless of what kind of device your family, friends or neighbors have, as long as the devices comply with the standard. Or should I say, talk the same language?

Ok, now that we have covered why these models are needed, let’s explain each layer in more detail.

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