Open Systems Interconnection (OSI) Model Overview

Jason Dion • 500,000+ Enrollments Worldwide
A free video tutorial from Jason Dion • 500,000+ Enrollments Worldwide
CISSP, CEH, Pentest+, CySA+, Sec+, Net+, A+, PRINCE2, ITIL
4.6 instructor rating • 25 courses • 320,066 students

Lecture description

This video discusses the OSI model as covered by the Network+ exam. This video is a broad overview, with subsequent videos diving into the details of each layer.

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English -: What is the OSI Model? Well, it stands for the Open Systems Interconnection Model and it was developed all the way back in 1977 by the International Organization for Standardization. This organization is responsible for creating different standards which we refer to as ISO and then some number behind it. For example, if you see ISO 7498, that's the standard we refer to as the OSI Model. Now, for the exam, you don't have to go and worry about memorizing the ISO number. I just wanted to introduce you to this concept because everything we cover in computing has some kind of standard associated with it. Now, in the case of the Open Systems Interconnection Model, most people simply call this the OSI Model but sometimes, you're going to hear them refer to it as the OSI Stack. Either way, we're talking about the exact same thing. This OSI Model is extremely important to the Network+ exam because it's a fundamental thing that we use to discuss all the pieces and parts of the network. It's so important that you're going to see a lot of questions on the Network+ exam about this concept. The OSI Model is made up of seven different layers and we're going to talk about each of those seven layers in the next seven lessons covering one layer per lesson because this model is just that important. These seven layers of the OSI Model are useful when we're trying to troubleshoot a network as well because if I have a problem and I can think about all the different seven layers, I could start to identify where that problem is and then I can troubleshoot it appropriately. This allows the OSI Model to serve as what's known as a reference model. Now, you may be wondering, what's the exact purpose of a reference model? Well, a reference model is simply something that we use to categorize functions of a network into a particular layer and that's what we use the OSI Model for. In fact, when you start looking at the OSI Model and you start learning it in depth, you're going to find out that the OSI Model doesn't actually cleanly or easily or accurately refer to the way that our networks actually operate today. For example, some things are going to operate at multiple layers of this OSI Model, especially when we're discussing the upper layers. Now, the reason the OSI Model isn't perfect for the way our networks operate today is because our networks today operate under what's known as a TCP/IP model but the OSI Model was designed to refer to how all networks operate, not just TCP/IP networks. Now, I know this is a little bit confusing. Don't worry too much about it yet. After we talk about all seven layers of the OSI Model, we'll go back and we're going to compare the OSI Model versus the TCP/IP model and we'll see why we use the OSI Model as our reference model. Now, this is because, again, it's generic in nature and so, it works for all networks, not just TCP/IP networks. Now, the idea of using a reference model like the OSI Model is going to allow us to compare different technologies across different manufacturers, as well. So, if I'm going to look at a particular wireless network card for my computer, I can compare how it operates through the seven layers of the OSI Model and compare it equivalently to a different manufacturer's card, as well. But understanding the functions at each and every layer that's being performed, it's going to help us better understand the flow of data in the network, how it communicates, and how we can troubleshoot it. Now, I've mentioned that there are seven layers of the OSI Model several times in this lesson already, but I still haven't even shown you what they are. So, let's take a look at the seven layers of the OSI Model. First, we'll start at the bottom working from Layer 1 all the way to the top which is Layer 7. These are the physical layer, the data link layer, the network layer, the transport layer, the session layer, the presentation layer, and the application layer. For the exam, you need to be able to remember these seven layers in the proper order. To help you with that, I like to use a memory aid called a mnemonic. In the case of the OSI Model, I like to think about it with my favorite kind of pizza, sausage pizza. So, if you want to remember the seven layers of the OSI Model for the exam, then just remember what Jason always says about sausage pizza. Please do not throw sausage pizza away. I mean, sausage pizza is pretty good, right? And so, you shouldn't throw it away. But this sentence will also help you remember the seven layers from bottom to top. Just take the letters PDNTSPA and replace it with the words physical, data link, network, transport, session, presentation, and application. And there you go. We have our seven layers of the OSI Model. Well, at least that's how I like to remember the seven layers, but you can create your own mnemonic, if you like. I've heard a few interesting ones over the years from my students. I was once teaching this class to a bunch of navy sailors who told me please do not tell shore patrol anything. I'm guessing they didn't want to get in trouble with their navy captains, right? Another one of my students told me please do not teach students pointless abbreviations. Whatever works for you is fine. Just remember that you need to memorize these seven layers before you take the exam because on the exam, you're going to get questions about these seven layers. They're going to ask you things like, what is the seventh layer of the OSI Model? And you have to say application. Or they might say, what layer is the session layer at? And you're going to have to say Layer 5. You're going to get more complex things also like asking you about a particular device and figuring out what device goes on to which layer, such as a router being a Layer 3 or network layer device. Because of this, we're going to cover each and every layer of the OSI Model in this section of the course. All right, before we finish up this lesson, we need to talk about one more thing and that's data. Well, specifically, how data is going to flow through the network and has different names based on where it is as it goes through the network. As data goes through the different layers of the OSI Model, we're actually going to call it different things. When it interacts with that data at Layers 5, 6, and 7, that's session, presentation, and application layers, we actually call it data. That's pretty simple, right? Now, as it moves down the layers going from 4 to 3 to 2 to 1, though, it's actually going to change the name. And when it's at Layer 4, it's segments. Then, it goes to Layer 3 as packets and then Layer 2 as frames and finally, Layer 1 as bits. This is something that you need to memorize for the exam, as well. But don't worry, I have a mnemonic for this one, too. Do some people fear birthdays? Well, it's a valid question, right? I mean, as someone gets older, they tend to start fearing their birthday because they don't want to age. I know this is a bit silly, but hopefully, it helps you remember the four types of information as they flow through the layers. So, if we have Don't some people forget birthdays, this is going to align with the top three layers with data, the next session layer with segments, the network layer with packets, the data link layer with frames, and the physical layer with bits. And that's it. So, for the exam, I want you to remember the seven layers and the five different types of data. Now, with that said, let's jump in to the first layer of the OSI Model.