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7 Layers Of The OSI Model
Every piece of software, every computer technology we have ever worked with, is built on the OSI model. When a developer says they are “full-stack”, what they mean is that they can manage an application from the top layer – user interface – all the way down to the physical firing of electrons through cables and computer chips. The OSI model is how we make computers do what they do, they are the evolution of technology that, today, seems like auto-magic. It is the seven-layer dip, without which, we would not be partying here in the wireless information age.
So what are the seven layers, and how can understand them to help us to make more robust, efficient, and satisfying technology today?
The 7 Layers of the OSI Model
The OSI model starts at the lowest, physical layer with electrons pulsing through copper wire and ends with handy apps we can tap or click to achieve our goals without a second thought. In the middle is what makes it all work. The layers are:
- Physical
- Data-Link
- Network
- Transport
- Session
- Presentation
- Application
Technicians who work with some or all of the stack often use fun and memorable mnemonic devices ranging from not tossing out sausage pizza to not touching pet alligators belonging to “Steve”. Some people, mainly those who work the top-of-stack, even reverse the list with “All Pros Search Top Notch Donut Places” and other reversed mnemonics. The best stack, however, starts with the building blocks at the bottom, so let’s dive into the Physical layer and build our way through the OSI model from there.
1. The Physical Layer: Electrons and Binary
The physical layer is where all computers start, as electronic signals move through copper wires. This is where the 1s and 0s are happy. In a network cable, for example, the signals sent are either above a certain voltage (1) or below that voltage threshold (0). From this binary, all data is formed.
In fact, the physical layer encompasses all cable and signal baselines including the type and quality of cable you’re using, the number of pins in your connector, and the microscopic channels of wire in every microchip. Interestingly, the physical layer also includes your wifi and radio signals, as pulses through the air are actually physical and send those essential binary messages to the next layer, creating the data we work with.
2. The Data-Link Layer: Data Frames and MAC Addresses
The Data-link layer is how computers know, understand, and interpret the signals that are sent. Those 1s and 0s mean nothing unless your computer knows how to read them. Here is where data is arranged into meaningful frames. Data frames form information we can work with, which is arranged into packets in the network layer (3) and transmitted through the transportation layer (4).
The Data-link layer, for example, defines MAC (Media Access Control) addresses, the unique address for every device, the LLC (Logical Link Control, not limited liability corp), and the ARP (Address Resolution Protocol).
Most network switches operate on the Data-link layer, which integrates with our third layer, Networking.
3. The Network Layer: Routing the Information Super Highway
If you’ve ever worked with routers and modems, you’re familiar with layer 4, the Network layer. This layer is how computers find, reach, and ping each other. The network layer helps to define now just the connection between computers, but the correct route to take for two computers to communicate.
Networking began with hard-line internal LAN networks, but with the introduction of the internet, networking through larger and more universal routing systems became necessary. Today, many technicians do most of their work in the network layer managing both routing with cybersecurity measures to prevent the wrong kind of routing connections from opening a line to the protected system.
4. The Transport Layer: Ports, Data Chains, and Loss Prevention
The transport layer is among the most interesting if you like data packets. In the transport layer, you will find all your TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) that gets your data from place to place in an orderly fashion. UDP relates to the ports systems. You might be familiar what the fact that IP (Internet Protocol) connections almost universally use port 80, FTP takes Port 21, and HTTPS connects through port 443. UDP alone throws “packets” of data through the networked route and hopes the other computer catches it. What keeps it orderly is TCP.
TCP creates packet chains, and each chain is numbered with an index at the front and a closing packet at the back. When a stream of data is sent, TCP tells the receiving computer how many packets to listen for and to watch for missing or misordered packet numbers. Then the receiving computer sends an ACK (acknowledge protocol) to say it got the stream. This provides loss control and error correction.
5. The Session Layer: Continuous Data Connections
The session layer is how two or more computers can communicate continuously without opening a new data stream every time. The original use of X-Window existed on the session layer, though today you may be more familiar with the common uses of file-sharing or opening an application on a remote computer. Your control over that remote computer, or access to a remote server, is encapsulated in a session.
Every time computers start a conversation, they open a session. The session has definitions, like how long to wait for a response and when to terminate the session based on live signals or an inactive line.
6. The Presentation Layer: Data Translation and Encoding
The presentation layer, today, is a bit of a misnomer. It’s not about UI, but rather, how data is presented from one layer to another. This is a transition layer that helps to translate, usually between the application layer and the network layer. A great example of the presentation layer in action is data encryption and decryption. This takes active data from an application and creates an “encrypted presentation” of the data that can then be transported through the network safely. At the receiving end, decrypting and displaying the data in a receiving application is also the presentation layer.
Outside of cybersecurity, the presentation layer also helps to work between different types of data, as different types of programming language encoding. For this reason, the presentation layer is essential for application developers who must choose their encoding methods and how to handle interfacing with data encoded in a different way.
7. The Application Layer: Connecting the Stack to the User
Finally, the layer everyone is most familiar with today is the application layer. This is where the end-user human interface happens and where low-code development takes place. When the rest of the stack is built and humming in the background, we can build applications that function on this foundation and interact with applications that make use of some or all of the OSI stack.
The importance of the application layer is that it communicates with the layers below, translating the user’s actions into signals that trigger network activity, data packaging, and closing and opening ports, and so on. Both web browsers and TelNet interfaces exist on layer seven, with roots far lower in the OSI model, making this layer far more than the garnish top of a seven-layer dip.
Conclusion
Where does your development lie in the OSI model? Does your team handle full-stack, top-of-stack, bottom-of-stack, or even mid-stack development? The complexity and importance of each layer is exactly why IT professionals, teams, and products aren’t interchangeable. Even two apps that seem exactly the same on the surface may have extremely different stack protocols underneath. By understanding the OSI model from bottom to top, you can add depth do your development vision and empower your team to achieve results that are more robust, dynamic, and with performance customized to exactly your goals from the electrons all the way to human interface.
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