Monday, 16 May 2022 | 5 min read
OSI layers: Everything you need to know
Written by Aaron O'Keeffe, Chief Growth Officer
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 are all about layers. Although cakes have nothing to do with the seven OSI layers, the analogy is useful in better understanding how the model works.
What is the OSI model?
OSI stands for “Open Systems Interconnection”. It refers to how applications communicate with one another over a network. There are seven different layers to consider in the OSI model, which you can think of very much like a seven layer cake — except less delicious!
Within these seven layers, two layers are particularly important when 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 get into that, it’s important to know what each of the levels are and what they do.
What are the 7 layers of the OSI model?
The seven layers of the OSI model are:
Application: Layer 7
Presentation: Layer 6
Session: Layer 5
Transport: Layer 4
Network: Layer 3
Data link: Layer 2
Physical: Layer 1
Layer 7: Application
Layer 7 is the topmost layer. The application layer is the part that users see. Things like 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 put into a format that can be transmitted, or ‘presented’, through the rest of the network.
The best and easiest way to understand what the presentation 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. After 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. Once the data is transmitted, the session layer terminates the communication between the devices.
Layer 4: Transport
The transport layer is important in the OSI 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 to transfer and where to deliver it.
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:
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 at fault, and is the easiest to diagnose and repair.
Why is the OSI layers model 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 the media layers. Once you understand that, you can start playing with them.
So, say a “Layer 2 network” forwards all traffic, so any data transmitted from 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 play a role in network performance and 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 most effective when managing network traffic over the internet, for example, between head office and satellite offices.
Given this, the OSI layers model is important because the way that 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 probably not going to have to concern yourself with the management of the network itself. Yet knowing how the layers work helps you to better understand how your network functions and enables you to ask pointed and accurate questions to ensure your provider and staff are getting the most 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.
Editor’s note: This article was originally published on Nov 9, 2018 and has been refreshed for accuracy and comprehensiveness.
Chief Growth Officer
Aaron worked as an IT professional for 10 years before shifting into telecommunications sales. He joined Aussie Broadband as a Business Development Manager in 2008, was promoted to National Sales Manager of the Company’s business division i...See all articles
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