OSI layers: Everything you need to know

November 9th, 2018 - Get new posts sent straight to your inbox, click here. Aaron O'Keeffe

If you’re not heavily into networks and telecommunication systems, then you probably haven’t heard of the seven OSI layers. In fact, if you have heard of it then you’re probably thinking about baking, because cakes today are all about the layers. However, the seven OSI layers have nothing to do with cakes, but nonetheless this analogy can be useful to understand when talking about technology – we’ll get into this in more detail later.

OSI stands for “Open Systems Interconnection” and refers to how applications communicate with one another over a network. Within that, there are two layers of particular importance when you’re looking at the efficiency, speed and security of your network – these are generally referred to as “Level 2 Networks” and “Level 3 Networks”.

But before we begin that, it’s important to know what each of the levels are. As the name suggests, there are seven different layers to consider in this model, which you can think of very much as a seven layer cake.

Layer 7: Application

This is the part that users see – web browsers and software such as Skype, Outlook and games are all part of this application layer. It’s the point in the seven layer stack where data starts its transmission (from the user’s interaction), and it’s the finishing point for receiving data once it has been processed through the stack.

Put simply, this layer allows users to access network resources.

Layer 6: Presentation

The presentation layer is where the data is taken from the application and then put into a format that can be transmitted, or ‘presented’, through the rest of the network. The best and easiest way to understand what this layer does is to think of encryption – the machine encrypts data before sending it through the network to make sure there aren’t any compatibility or security issues.

Layer 5: Session

For any two devices – for example computers, tablets, phones, or servers – to interact with one another, a link or “session” needs to be created between them. This happens at the session layer, as after the data has gone through the presentation moment it’s ready to be sent to the receiver.

The session layer also coordinates conversations and exchanges between the two devices at either end, and once the data has been transmitted, the session layer terminates the communication between the devices.

Layer 4: Transport

This is a very important layer in the model, as it’s where the data transfer between end systems actually happens. You’ve probably seen terms like Transmission Control Protocol (TCP) and Internet Protocol (IP), which are elements of the transport layer in motion. Layer 4 coordinates the data, as well as how much data needs to be transferred and where it needs to be delivered.

Layer 3: Network

The network layer is where the routing happens. One device can connect to another through any number of different paths, and it’s up to the router to determine which path the data will actually take. As such, the network layer is one of the more visible parts of the stack, and often the one where a lot of troubleshooting and efficiency solutions are focused.

Layer 2: Data Link

At the data link, any errors from the physical layer (below) are managed, and node-to-node data transfer processes are undertaken. This layer is divided into two sublayers, being the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer.

If you’ve ever managed a switch before, then you’ve managed the data link layer.

Layer 1: Physical

Finally, the physical layer. As the name suggests, this is quite a straightforward layer as it involves hardware. It’s where all the cables are connected and refers to all the pins and chips that go into the typical networking solution. Often when something goes wrong in a networking solution, the physical layer is the most likely to be at fault, and the easiest to diagnose and repair.

Why is all of this important?

Once you understand how the seven OSI layers function, and the role of each of them, you can start to really work with them to deliver a better networking experience. For example, you now know that the three bottom layers are the physical layer, the data link layer, and the network layer. Collectively, these three layers are known as the media layers. Once you understand that, you can start playing with them.

So, for example, a “Layer 2 network” forwards all traffic, so any data transmitted by a device on the layer will be forwarded to all the devices on the network. This is a very, very fast way to transmit data, but has the disadvantage that once the network has hit a certain size there will be congestion and inefficiency with all the data flying around.

Meanwhile, the Layer 3 networks will restrict broadcasts. An administrator gets greater control over what is broadcast between subnetworks, which helps to limit the congestion on the larger networks.

This is not to say that one is inherently “better” than the other, since they both have a roll in network performance – they don’t exist independently of one another.But focusing on layer 2 networks can be more useful when computers are in a close geographic location to one another, while layer 3 networking is proven to be most effective when managing network traffic over the internet – for example, between head office and satellite offices.

For more information on how Layer 2 and Layer 3 networks breakdown, and why you might want to focus on one or the other,read this handy blog post. And to bring it back to the question about why any of this is important: how your organisation manages the various layers in the networking stack will determine how efficiently the overall network operates.

What else is there to know?

Of course, unless you’re a network administrator, you’re not going to have to concern yourself with the management of the network itself, but it helps to be able to ask pointed and accurate questions to make sure your provider and staff are getting as much out of your investment into the Internet as possible.

To request a consultation to discuss your enterprise’s internet requirements, contact the Australian experts at Aussie Broadband today.

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About the author

Aaron O'Keeffe

Aaron works from an office in the tropical Northern Territory, inciting intense jealousy from his Victorian workmates during winter. He’s an expert in IT solutions from the ground up. Aaron is National Sales Manager of Aussie Broadband, which specialises in bespoke telco solutions for corporate and government customers.