When it comes to networking, a deeper understanding can help you design more effectively and operate more efficiently. But, that understanding doesn’t always come easily. Professionals spend a lifetime acquiring knowledge and growing and studying. Still, you can break down some essential knowledge pretty quickly, and to help you do that, this is a crash course in the OSI networking model. It’s a staple in the industry, and if you can grasp the basics, it can help you take steps into a deeper and wider world.
What Is OSI?
The Open Systems Interconnection model (OSI) is a standardized way to look at the various functions within a network. The model breaks everything into sets of rules and concepts that allow devices and software to communicate across a network.
The network consists of seven layers, starting with the physical layer at the bottom and ending with the application layer at the top. Each layer will be discussed in detail in its own section below.
The OSI model was originally published by the International Organization for Standardization (ISO) in 1984, so it’s been around for a while. Despite its age, it still proves completely relevant in modern networking.
The Seven Layers
The OSI model really boils down to the seven layers, so to understand the model, we have to break down each of these layers. In general, you can compare networking to building roads (although it’s not a perfect analogy in all respects). Each layer of the network is comparable to an aspect of roads, traffic, and driving, so we’ll try to stick with this analogy as we go through everything in more detail.
The first layer is the physical layer. As the name suggests, this layer describes the physical aspects of the network. More specifically, it describes how things in the network physically communicate.
That communication could be through copper lines, in which case the signal is electrical. Then, the layer would need to define voltages and maybe even pin settings for devices in the network.
The physical layer can also include fiber optic lines and even wireless frequencies.
In the road analogy, the physical layer is the material that makes the road. It’s the asphalt, concrete, or dirt that you drive on to get from one destination to another. It’s not the whole network, cars, and traffic lights, and all kinds of other things are important to the system, but it’s the physical pathways that your data can take.
Next up is the data link layer. This is where data is packaged into pieces that can be sent and received. Those packets (also called frames) are the little nuggets that actually travel the physical layer.
In other words, the data link layer is where information is digitized so that it can travel through the network.
The data link layer is uniquely complex in that it has two sublayers of its own. The media access control (MAC) layer controls flow and multiplexing (or which direction data flows through the infrastructure). The logical link control (LLC) manages error control and identifies protocols. As a result, the data link layer is also where a network can correct information errors that occur in the physical layer.
It’s also worth noting that the data link layer is the typical switching layer (although switching can also operate in Layer 3).
Using the road analogy, the data link layer would be your parking lots, driveways, and bus stops. It’s where the passengers enter a vehicle before the vehicle travels the road.
We have seven networking layers, and one of them is called the “network layer.” While that might seem confusing, this layer is aptly named because this is where the route is chosen for your data. Most of routing takes place in this layer. IP addresses are defined in Layer 3, and that’s why it’s the routing layer.
Sticking with our road analogy, this is the map. This is where you decide which roads you’re going to take to get to your destination. It’s worth noting that routing is smart just like you as a driver. If a route is troubled along the way, you can change your road choices on the fly, and routers do similar actions on a regular basis.
The transport layer manages the delivery of information packets. It also handles error checking.
This is where the system can figure out the size of a packet, how to sequence information, and make many other decisions necessary for successful communication. The transport layer chooses the protocol (like TCP) and follows it accordingly.
The transport layer manages the rules of the road. It’s the list of traffic laws.
Meanwhile, the session law is what establishes, maintains, and kills communications between devices. If one computer is going to talk directly to another computer, they have to establish a session. That happens, surprisingly enough, in the session layer.
Authentication is one of the most important aspects of the session layer and it’s how the systems know that each user is authorized to communicate with the other. As an example, when you check your email, you have to establish a session with a mail server. The session layer verifies that you own the email account for the server, and it identifies that you’re talking to the right server for your sake.
The session layer is like the unwritten rules of the road. It governs how you interact with other drivers, and it can customize those interactions for each unique situation.
Nearing the top of the model, the presentation layer handles data formatting. It follows syntax and semantics rules (which is why it’s also called the syntax layer). This is where encryption and decryption take place.
To put it in simpler terms, the presentation layer translates information from the network to an application (like your web browser) or back in the other direction.
The presentation layer doesn’t fit as cleanly into the road analogy, but in a sense, you can consider the driver to be the presentation layer. The driver views the road and translates that information into steering and pedal actions that control the car.
The application layer is at the top of the model, and it’s where the end-user interacts with applications that use the network. Web browsers are the easiest example. You type things into your web browser, click on it, and view it with your eyes. The browser is the application.
Meanwhile, the browser has to interact with your network in order to upload and download information according to what you do. That’s the application layer. It’s where applications directly work with the network.
For the road analogy, the car itself is the application layer. It’s the steering wheel that responds to your guidance, the gas and brake pedals, and even the windshield that allows you to see the road as you travel it.
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