There’s a lot that goes on behind the scenes anytime you browse the internet. Communication channels open and close, things get encrypted, and there are a bunch of 1s and 0s floating around.
It can get pretty complicated, but luckily, whenever there’s a complex process going on between machines, there’s a model to help people understand it. For example, there’s the open systems communication (OSI) model.
Ahead, we’ll walk you through the OSI model and each of its layers, from top to bottom.
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What is the OSI model?
The open systems interconnection model (OSI) is a conceptual model that describes how different communication systems talk to each other on a computer network.
For example, when you clicked on the link to get to this blog post, your request involved software applications (your browser) encrypting and transmitting information over physical cables. The OSI model divides the whole process into seven steps or layers (but more on that in a moment).
By “conceptual model,” we mean that OSI describes a process that’s hard or impossible to see in real life, like a diagram of the water cycle. OSI isn’t a type of software or technology standard. Instead, different communication technologies define themselves as handling one of the seven OSI layers.
So, what are these OSI layers anyway?
What are the 7 layers of the OSI model?
The OSI model is made up of seven independent layers. Let’s start from the highest layer and work our way down.
7. Application layer
The application layer is considered to be the highest level of the OSI model. It’s the only layer that directly interacts with you, the user.
When you use your browser, email app, or other application, the software initiates communications with the application layer. How? With application layer protocols.
Application layer protocols are standards for transmitting data between different computers and servers. They usually include standards for how messages are put together, how other parts should be interpreted, and how to respond to a message.
Examples of application layer protocols include:
- Hypertext Transfer Protocol (HTTP): For viewing web pages and accessing web resources
- Domain Name System (DNS): For quickly accessing web pages using names instead of IP addresses
- File Transfer Protocol (FTP): For transferring computer files
- Internet Message Access Protocol (IMAP): For accessing and storing email
- Simple Mail Transfer Protocol (SMTP): For sending outgoing email
Think of application layer protocols like language dictionaries. Imagine that you’re getting a message from someone in German. To make sense of the message, you’d want to use a German translation dictionary for help.
Part of the application layer’s job is to help convert or translate the message before sending it to the software.
6. Presentation layer
When you send or receive data, it isn’t sent in the same form that you see on your computer. Instead, it’s often encoded, encrypted, or compressed for security and efficiency in the presentation layer.
If two computers communicate over an encrypted connection, it’s the presentation layer that adds the encryption and decodes the encrypted message on the receiving end. And if two computers are using different encoding methods, the presentation layer also translates the data into a form that the receiving computer can understand.
5. Session layer
When two computers start communicating with each other, they start a “session.” Ideally, the session is active for just long enough to transmit all of the required data. Closing the session too soon means that some data wasn’t transmitted, and keeping the session open for too long wastes valuable resources. The session layer ensures that a session is open for just the right amount of time.
The session layer also sets up checkpoints for data transfer in case of a sudden disconnect. Have you ever started downloading a file only to lose your connection halfway through? It’s happened to everyone — but it’s thanks to the session layer that you’re able to pick up where you left off in your download rather than having to start again from zero.
4. Transport layer
So far, we’ve translated our data into a standard protocol, packaged it up for secure delivery, and opened up the connection between the two computers. It’s time to start moving that data with the help of the transport layer.
Before sending anything, the transport layer first breaks down data into smaller chunks called segments. On the receiving end, the transport layer reassembles the segments into the original data structure. The transport layer also manages the flow of data so that a sender with a fast connection doesn’t overwhelm a receiver with a slower connection.
Finally, the transport layer confirms that the data received is identical to the data sent. If not, then the transport layer will attempt to retransmit the data.
3. Network layer
In most cases, computers that talk to each other are on completely different networks. The network layer enables cross-network communication by breaking down data segments from the transport layer into even smaller pieces called packets. The network layer on the receiving end then reassembles packets back into segments.
The network layer is also responsible for routing, which is finding the best physical connection path between the two computers.
If you’re communicating with another computer on the same network — like using your company intranet or playing Mario Kart at a LAN party — then the network layer isn’t needed.
2. Data link layer
The data link layer is similar to the network layer but is used for transferring data between two devices on the same network — for example, two computers or a modem and a computer. The data link layer breaks down network layer packets into even smaller chunks called frames.
1. Physical layer
In the end, all data travels through physical cables or radio waves. The physical layer refers to the cables, switches, wireless transmitters, and other physical routes through which the data moves. It’s also in the physical layer where data is converted into a bitstream — a string of 1s and 0s.
OSI vs TCP/IP
You may see OSI compared a lot to Transmission Control Protocol/Internet Protocol (TCP/IP). Both TCP/IP and OSI involve conceptual models that describe how computers communicate with each other. The TCP/IP model contains four layers rather than seven — some of the OSI layers are combined into single layers in the TCP/IP model.
But while OSI is only a model, TCP/IP is also a standard protocol that’s used in every network, including the internet. In fact, every major application layer protocol (HTTP, SMTP, and so on) is a part of the TCP/IP family.
OSI helps Cybersecurity, and IT specialists work better
Even though the internet itself is based on the TCP/IP model, the OSI model is the more widely used model for visualizing and communicating how networks operate. It helps network equipment manufacturers easily define what their products do and which parts of the network the products work with.
And if you’re interested in becoming a Cybersecurity specialist, Network Engineer, or IT specialist, then you’ll need to memorize every part of the OSI model. That’s because OSI is how these professionals isolate, troubleshoot, and fix networking issues.
Now that you know more about the OSI model, where do you go from here? If you’re interested in becoming a Cybersecurity specialist, try one of our online programming courses to build the skills you need to succeed in your new career.