OSI Layers: Understanding the OSI Model & Its 7 Layers

The open system interconnection model (or OSI model) is purely conceptual. These aren't standards or firm rules that everyone must follow when developing or maintaining a product. 

Instead, OSI layers can help you visualize how the parts and pieces of network systems should communicate with one another. 

If you've ever encountered a system-wide failure and you're not sure where to find the problem, the OSI layers could help. And if you're working with a vendor that sells something very specific that works on just one part of your system, you could talk about that application in shorthand with the OSI model. 

If you're confused, don't worry. We'll dig deeper in this article, so you can understand just what this model is and how it works. 

What is the OSI model?

In the late 1970s, plenty of companies entered the computer age. Telecom companies, hardware manufacturers, and programmers in France, the United States, and elsewhere were all hoping to build a network of connected devices. Unfortunately, they couldn't decide on a common language or set of tasks. 

Enter the OSI model. Everyone in these various groups and locations pitched in to decide how devices should connect and communicate. Then, the International Organization for Standardization (IOS) stepped in to formalize the model. 

OSI officials say it's their job to develop standards that bridge creativity and functionality. On the surface, the OSI model does just that. 

But again, the OSI model isn't a firm, accepted set of rules within the industry. It's not a standard. Instead, it's a conceptual framework students, IT professionals, and software/hardware developers use.

The OSI layer model explained 

If you've ever heard an IT student muttering, "A priest saw two nuns doing pushups," you've heard about the OSI layer model. The terms are a bit technical, and students often struggle to remember them. But dig in, and you'll discover that the layers are both economical and elegant.

Seven layers make up the model, and people often describe them from high to low.

• Layer 7: Application
  Human/computer interactions happen here. A user works directly with some type of software, such as a web browser.
   
• Layer 6: Presentation
  Two devices prepare to connect. This is a bit like a preparation area before the real work begins. Encryption and decryption tasks happen here.
   
• Layer 5: Session
  The establishment, management, and termination of connections between devices happen here. The communication channels are known as sessions, which is how this layer gets its name.
   
• Layer 4: Transport
  The means to transport data (of almost any length) from one source to a recipient device are part of this layer. If the data must break into pieces, the source waits to send the next pieces until the first ones arrive.
   
• Layer 3: Network
  Mapping between devices happens here. Messages might move through several nodes or routers, and the specific coordinates are chosen here.
   
• Layer 2: Data link
  Switching between two connected nodes happens here.
   
• Layer 1: Physical
  This layer includes everything from cables to pins.

Notice that we don’t name specific programs or protocols within this model. It's meant to be completely agnostic and open, so you could use this theory on almost any kind of network. 

Each layer is served by the layer below it, and it provides some kind of functionality to the layer above it. But there are some cross-layer functions, such as security, that could affect more than one layer simultaneously.

What makes the OSI model unique?

At one point, developers thought the OSI framework would define everything we did with computing. But now, most people use it as a theory rather than a set of rules. 

People building the brand-new internet felt the OSI framework was too long, too cumbersome, and too hard to apply to an online, connected world. Instead, these experts favored the Internet Protocol Suite, sponsored by the Engineering Task Force. 

The internet control message protocol (ICMP) model recognizes just four types of functionality (not seven). And that model doesn’t list those functions in a hierarchical form. Instead, the functions connect and intertwine based on the goal of the specific type of network communication.

Even though you might use ICMP, you might lean on the OSI model to help you with critical tasks, such as:

• Creating a new network. What hardware do you need? What software would be helpful? What pieces should a vendor tackle? 
• Finding a problem. Where does your issue likely originate based on the symptoms you're seeing?
• Talking with outsiders. How can you explain how your system works to people who don't use your software or servers?
• Selling a product. How can you explain what specific part of a connection process your product can tackle? 

Most IT professionals are familiar with the OSI model, and its specificity allows for quick and comprehensive conversations.

Get help from Okta 

At Okta, our solutions work across almost every layer in the OSI model. We can walk through each part and piece with you and explain how our solutions can strengthen your defenses.

And if you'd rather use different terminology to talk about the help you need, we can handle that too! Contact us to get started.

References

How Standards Nurture Innovation in the Cold Light of Dawn. (February 2006). ISO Focus. 

OSI: The Internet That Wasn't. (July 2013). IEEE Spectrum.