The OSI Model

Are you ready for some networking theory? Let’s talk about the OSI Model!

I know, I know! Try to control your excitement on this! I
know it’s difficult (major sarcasm inserted here).

But really, seriously, if you want a job or career in the field
of computer networking you’re going to have to know all about the OSI model.

The OSI Model

OSI stands for Open Systems Interconnection.

The Open Systems Interconnection Model is a structured model developed for computer networking in the 1970’s.

It was developed by the International Organization for Standardization (ISO). (And I know this can get confusing because the OSI was developed by the ISO, but I digress).

It was basically developed as a way, it was developed and adopted really, as a way of standardizing network technologies.

The first and most important thing this did – this model did – was it allowed computers from different manufacturers to communicate with each other.

Because it modeled networking hardware software and protocols and modeled them in such a way that they could communicate with each other.

Manufacturers had a model to work under which allowed them all to kind of leverage off of each other by being compatible with each other’s devices. They had kind of a standardized model to work with.

The OSI model is divided into 7 Layers or groupings of standardization that makes up kind of a hierarchy of sorts.

I’m going to cover these 7 Layers in more detail in my “New to Networking” Course.

There are also other study materials out there that kind of go into deeper detail about the OSI Model.

Also keep in mind just as a side note that most of what is done in each of these Layers is actually done on the actual networking card (the NIC) of a computer. Not all of it.

But for all intents and purposes we can technically say that at this level of understanding networking.

So what are the 7 Layers?

Application Layer

The top layer is the Application Layer.

This is the layer where users communicate and interact with the computer and applications, as well as where the applications interact with all the other layers of the OSI Model over a network.

Presentation Layer

The Presentation Layer is the next layer down. This layer presents data coming from the other layers to the Application Layer.

Now it also translates data as needed from the Application Layer down to the Session Layer below it.

Session Layer

Obviously, the next layer is the Session Layer.

This is the layer that is responsible for each networking “session” between this computer and the other devices on a network.

This is also the layer – this Session Layer – where Presentation Layer data is set up managed and dismantled.

Transport Layer

Then you have the Transport Layer…

This layer – the Transport Layer – takes data from the upper layers – the Session the Presentation the Application Layers – it takes data from those and it combines it into a data stream for end-to-end transport between devices.

So the Transport Layer kind of makes sure that it is transported from this device to the other, and from the other device to this device correctly.

It also takes data received from other devices and delivers it up to the Session Layer to go back up the hierarchy if it needs to. That’s data that it received FROM the network.

Network Layer (Layer 3)

Then you get into the Network Layer…

Now this is also known as “Layer 3”

So when we’re talking about “Layer 3” in networking we’re talking about the Network Layer.

This Network Layer is the layer where device addressing (see IP addresses as a side note) where network addressing and data packet tracking actually takes place.

This is where Routers tend to do their “magic”…at the Network Layer.

Anything dealing with IP addresses and network addresses, this is the Network Layer that’s involved.

Data Link Layer (Layer 2)

Next you have Layer 2 which is the Data Link Layer.

Now this Data Link Layer is responsible for transmitting the data from the upper layers out ONTO the Physical Layer and the network.

It also receives data from other devices on the network from that Physical Layer below it and takes it up into the upper layers of the OSI Model.

You also see – at the Data Link Layer – you also see flow control and error notification. Those things are done at this Data Link Layer – this Layer 2.

Physical Layer (Layer 1)

Last but not least you have the Physical Layer…

The Physical Layer is the actual physical media or medium used to transmit and receive data in bits…either 0’s or 1’s.

Now this is essentially the physical hardware that the NIC interface, the cables, the antennae, any physical hardware that makes up the network is basically in the “realm” of the physical layer.

As you further your experience and knowledge of networking, you will reference troubleshooting issues with physical network media as “Physical Layer” issues, as these tend to be the first thing to rule out when you have network connectivity issues.

To Sum It Up…

So that’s the 7 Layers of the OSI Model.

And again it’s just a structured standardized model that manufactures of all these different devices and contraptions on a network can kind of stick to to make sure that they’re on the same page, so to speak, with all the other manufacturers of all of the other devices out there for networking so that they can all be used together on the same network or networks.