One of the most frustrating yet relatable experiences shared by almost everyone in this day and age is suddenly finding your internet connection is down… for no apparent reason.

When it’s just a matter of not being able to watch your favorite Netflix show, the “no-internet” problem is annoying.
A network failure that takes your business offline? That’s a huge headache with major consequences to your bottom line.

Sometimes the reason for network failure is apparent—there’s a power failure or a natural disaster—but other times, it takes an IT expert to make a proper diagnosis and get it back up and running again.

What if your IT person isn’t there? What if you don’t have an IT person?

For businesses that don’t have the budget for a full-time IT staff, it may be in your best interest to become familiar with OSI layers and how they can be used to treat several IT-related business problems, including troubleshooting your network issues.

When you can successfully troubleshoot these kinds of issues independently, you may no longer have to tolerate any significant amount of network (and thus business) downtime.

What is the OSI seven-layer network model?

What are osi layers

The OSI model stands for Open Systems Interconnection. Think of it as a conceptual framework that breaks down the functions of a networking system in a way that sets a standard and is easy to understand.

So instead of having to comprehend all sorts of highly technical computing functions that you may not be able to wrap your head around, you can use the OSI model to translate those complex functions into a universal set of rules.

OSI layers also support interoperability between different types of technologies, products, tools, and software solutions.
To better illustrate the advantage of this, consider that, in the past, if IBM manufactured a computer, it would be able to communicate with another computer or networking device manufactured by IBM.

In this way, the OSI model created a common platform that both software developers and hardware manufactures could use to generate networking products that could work with each other, rather than siloing everything between different companies.

This was best both for consumers and manufacturers. Something to note is that this model was developed at a time when network computing was still a new thing. In 1984, the OSI layers model was published by the International Organization for Standardization (ISO).

The model is by no means perfect, but it’s still an excellent way to describe Network architecture even today. It does this by dividing the concept of data communication into seven abstract layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

Understanding the seven OSI layers

understanding osi layers

The OSI model is made up of two major parts: 

  1. The reference model
  2. The protocols

The reference model is simply another way to characterize the 7 OSI layers. In this model, each layer within your network is compatible with the layers immediately below and above it, so software and tools in layer 3 can work directly with tools and software in layers 2 and 4. 

The protocols enable the OSI layers on a host to properly communicate with the same layer on a different host, i.e., you can use Gmail (a Layer 7 application) to email someone, hundreds of miles away from you that uses Microsoft Outlook’s email service (another Layer 7 application). 

Now let’s dive into each of the OSI layers in greater detail.

Layer 1: Physical 

When troubleshooting any issue related to electronic devices, Layer 1 is where you ask and answer the question of whether or not the device is plugged in.  

This is the lowest layer of the OSI model and deals primarily with the physical transmission of data between network devices and infrastructure. This is accomplished through electrically (or optically) transmitting raw unstructured data bits over the network.  

Important specifications to consider at the physical layer include: 

  • Voltages
  • Pin layout
  • Cable size
  • Signal frequencies

Layer 2: Data Link

This layer focuses on media and is where you’d be dealing with the switches used to start up or turn off communication between connected devices. It’s made up of two sublayers: Media Access Control (MAC) and Logical Link Control (LLC).

Getting more technical, the layer specifically acts as a medium for node-to-node transfers of data between frames, which are simple containers used for a single network packet, between two devices that are physically connected. 

MAC is what determines how the devices in a given network attain access to a medium and get permission to transmit data. LLC deals with the identification and encapsulation of network layer protocols, checks and corrects for errors made at the physical layer, and handles frame synchronization. 

Important specifications at the data link layer include: 

  • Network topology
  • Flow control
  • Physical address
  • Error notification
  • Frame sequence

Layer 3: Network

Layer 3 is another media layer and is home to routers and IP addresses in search of the most efficient comms pathways, specifically for packets that contain control information and or user data (AKA, a payload). 

If the packet layer is too big for transmission, it can be split apart into smaller fragments which are shipped out before being reassembled upon arrival. This layer is also home to firewalls and 3-layer switches. 

The frames received at the Network Layer come from the Data Link Layer below it. It identifies the destination of the data it received through the use of logical addresses such as Internet Protocol (IP). 

However, for the Network Layer to do its job, it needs to route information to its proper destinations.  

Important specifications at the Network Layer include: 

  • Best available path
  • Traffic controls
  • Priority of Service
  • Network characteristics 

Layer 4: Transport 

Think of the Transport Layer as a kind of digital post office that coordinates data transfers between different systems and hosts. That’s its function in a nutshell. 

This layer acts as a host that figures out how much data to send, at what rate to transmit the data, the data’s destinations, and more. Appropriately, additional firewalls live in this layer and so do gateways. 

Additional functions of this layer include checking for errors, identifying duplication of data, handling failed deliveries that need to be resent, and keeping track of all the data packets.  

Layer 5: Session

The Session Layer of the OSI model functions like a moderator that controls the communication between devices and or servers—think of it as a meeting host or mediator between different computers (though it’s not limited to just computers).

It establishes pathways, creates limits for response wait time, authenticates sessions—a session is an interactive exchange of data between two entities in a given network—and terminates them. 

A common example of this layer in action includes HTTPS and sessions, which enable users to visit and browse sites on the internet.

Layer 6: Presentation 

Here is where data is translated and formatted so that networks, devices, and applications can actually understand and work with the data they’re receiving. This layer relates to data compression, conversion, code translation, encryption, and decryption. 

This layer is also known as the syntax later because it maps the semantics and syntax of data so that any received information is able to be understood by any distinct network entity. 

When you’re using an encryption-based communication app and transfer data, that data is formatted and encrypted at this layer before being launched across the network. 

Once the data arrives where it’s supposed to it’s decrypted and formatted into plain text or media information, this layer is also responsible for the serialization of complex data into transportable formats—and then deserializes and reassembles it into its original format destination.

Layer 7: Application 

The Application layer is likely what most end-users are familiar with as it is where Application Programming Interfaces (API) lives, which allow remote file access, resource sharing, and other things. When you think of this layer, think of web browsers and apps like Gmail and Slack. 

If your business creates applications like that, then this is the layer where your customers will likely be engaging with your products. The layer determines resource availability, ensures the quality of communication, and identifies communication components.

It also allows for network resources and uses common protocols like Telnet, Hypertext Transfer Protocol (HTTP), Simple Mail Transfer Protocol 9SMTP), and File Transfer Protocol (FTP). However, it should be noted that most traffic monitoring solutions don’t operate in Layer 7.

It would actually benefit your business to use traffic monitoring solutions that utilize Layer 7 because you’d be able to gain visibility into a significant amount of end-user behavior that can help mitigate risks of cyberattacks and even help with load balancing, among other things. 

For instance, where you or an IT professional analyze Application Layer analytics, you may be able to detect suspicious behavior (like DDoS traffic) and deal with the threat without hurting the experience of legitimate users.  

The OSI Model in Action

osi model in action

Let’s explore email, which lives in the Application Layer of the OSI model, to get a better idea of how all the layers come together. 

  • Specifically, the Application Layer is the email client, such as Microsoft Outlook, which users use to send data.
  • At the Presentation Layer, the data gets converted into ASCII format and goes through the process of compression and encryption. 
  • Then, in the Session Layer, a session is started, and here, a header is added to label it. Then a connection is established between nodes. 
  • The Transport Layer is where the message is split into segments and transmission via the Transmission Control Protocol (TCP), and then the source and destination ports are added. 
  • At the Network Layer, the best physical routing path for the data segments is determined, and the right headers are also added. The segments that were created in the previous later are converted into data packets. 
  • Those data packets are passed on to the Data Link Layer, which converts the packets into frames and contributes the physical address of the node. 
  • It is at the Physical Layer that the data packets are received at the physical medium for transmission. 

As you can see, there’s a lot of technical work that goes into something as seemingly simple as sending an email through the OSI model.

Why are OSI layers a useful model?

osi layers useful model

Unless you’re an administrator or IT professional, you likely won’t be thinking of the OSI model all too often, but it’s good information to know of regardless. 

Because of the way the OSI model distinctly separates the technical work across different layers, it makes it easier to isolate and troubleshoot the issues plaguing your network connection. 

It also allows administrators and IT professionals to determine what type of software and hardware to use and informs manufacturing so that they’re able to create devices that can communicate with other devices built with the OSI model in mind.

It’s also just a secure model for transmitting data. 

How to troubleshoot a network issue using the OSI model

troubleshooting with osi model

At this point, we’ve not only covered the history of the OSI model, but we’ve also taken a deeper look into each of its layers. 

Now we’re ready to explore how to investigate a network issue using this model and systematically troubleshoot a network problem so that you reliably avoid downtime and get back to business.

Unsurprisingly, most issues that plague network connections happen at the Physical Layer, Data-Link Layer, and Network Layer. For this reason, a good rule of thumb is to start your investigation here before proceeding to the other layers. 

  1. Check to see that all physical devices related to your network are plugged in and working; it’s not uncommon that damaged cabling is the culprit of network problems. If you have a wireless connection to your network, the cause of the problem is likely to be excessive signal attenuation or wireless interference. 
  2. If all your physical components are working properly and you’re still experiencing issues, it’s time to check the Data Link Layer. You can do so by verifying that all the VLAN and switch configurations are working properly; if those are good, move on to see if there are IP address conflicts—try to remove any duplicate IP addresses. Next, check the STP to make sure it’s functioning properly.
  3. Moving onto the Network Layer, this is where most routing and network addressing issues live. Make sure that your networking devices are not damaged, have the wrong configurations, are authenticated, and that you have enough network bandwidth. 
  4. At the Transport Layer, network issues typically come from ports that are damaged or blocked. Disabling quality of service (QoS) may resolve the issue, but otherwise, you’ll want to make sure that none of your firewalls are blocking TCP/UDP ports. 
  5. It’s rare that the network issue originates from the Session and Presentation layers because they play a less crucial role in keeping the network up and running. 
  6. Finally, at the Application Layer, you’ll find that network issues are often related to the DNS, most of which can be fixed by using the “nslookip” command. You can also try using the “tcpdump” command, which filters TCP/IP packets and analyzes network packages.

The broader impact of the OSI model

impact of osi model

Over the years, some people have come to question the purpose of the OSI model, and it’s perfectly fine to do so as it can lead to a greater understanding of network systems and even the internet as a whole. 

Some argue that the OSI model is obsolete because of its more theoretical layer, but because the model helps frame discussions of protocol and helps contrast various technologies, it’s likely to remain relevant for a long time to come. 

In summary: 

  • What is the OSI model? — OSI stands for Open Systems Interconnection. It’s a conceptual framework that splits the functions of a networking system into 7 layers that are easy to understand. 
  • The Seven OSI Layers — The layers include the Physical Layer, Data Link Layer, Network Layer, Transport Layer, Session Layer, Presentation Layer, and lastly, the application layer. 
  • OSI Model Use-Cases — While IT professionals and administrators are the ones who will engage with the OSI model most, it’s still useful to end-users and business people alike not just because of how it can help with network troubleshooting but also for how it breaks down network communication in an easy to understand way.
  • Troubleshooting with the OSI Model — The OSI model helps with troubleshooting network issues by making it easy to investigate the problem step by step, analyzing the critical components of each layer, starting at the physical and then moving all the way up to the Application Layer. Network issues rarely stem from the Session or Presentation layers.

Managing your business’s Network Services

managing network services

Understanding the OSI model can certainly help with DIY network troubleshooting, but that’s probably not a responsibility you want to deal with on top of managing the day-to-day of your business. 

While it’s great to be able to handle your own network troubles, why not off-load that task to us with our Network & Wireless Connections service? 

After all, a functioning network is a crucial component of your business’s IT security. Your network is the primary piece of infrastructure that facilitates access and delivers connectivity to all your servers, files, devices, and more. 

We’ll take a close look at your unique situation to come up with a working solution that fits the way you do business.