Original subtitles

It takes an amazing amount of software and hardware working together to make a network function.

So the challenge to all of this is that if you have different pieces of software and hardware they don't

have to be able to talk to each other and interact and able to make your network go.

You don't have a network card from 3Com and you might be running a Windows 8 operating system and all

of these devices have to work together. To help make interconnectivity work

they came up with something a long time ago called the OSI seven-layer model. The OSI seven-layer model is seven

distinct functions that a network must do.

Now the OSI is being supplanted with something called the TCP model.

So, let's do a quick overview and take a look at OSI and TCP. Now for OSI we're going to start,

basically there's no real bottom or top but I tend to look at it at the bottom which is your most basic

part which is what we call the physical layer. The physical layer is, that's like the easiest part of

the OSI seven-layer model.

It's just what type of cables do I use and things like that.

Now coming up from that is the data link layer.

Now the data link layer a little bit more important.

Basically anything that works with a MAC address works at the data link layer.

So when we're talking about data link we start talking about things like the network cards themselves,

start talking about switches things like that.

The third layer is the network layer.

Now the network layer is important because the network layer has to do with logical addresses.

So when you see the network layer you're talking about IP addresses.

So things like routers in particular work at Layer 3. The next layer up is the transport layer the transport

layer is kind of interesting.

Data is big,

and the actual chunks of data that go through a network are relatively small.

So there has to be a assembly disassembly area for data as it gets broken down into individual packets

and that's what the transport layer is all about. The transport layer

not only disassembles packets but make sure the packets get to the other system in good order.

Next one is the session.

Now the session is kind of interesting. When we work at a session layer here we're talking about the

actual connection between two systems.

Are we making a TCP connection between a web server and a web client?

Are we just sending e-mail?

Are we sharing a folder between two computers on a local network?

And it's the session layer that defines what's taking place in terms of how that connectivity really

works.

Next is the presentation layer. The presentation layer is old and weird, but since it is on the OSI we

need to at least make a quick mention of it. Just because a big chunk o data comes into your computer

does that necessarily mean that this data is in a form that your computer can read it?

Now, let's use Microsoft Office as an example.

If I open up my Microsoft Office I can open up almost any kind of document you can dream of.

Office has the smarts built into it to automatically read it.

But that wasn't always the case.

So the presentation layer used to be used to convert data into a format that your applications can read.

hippy you're at the top is applications.

Applications is important because it's the applications themselves and it's not really the applications

you have to be careful here.

What we're talking about is the smarts in the applications that make them network aware. Little simple

things that you and I never think of. For example, if we were to open up Microsoft Word and we could literally

go to the network to go find a particular file.

And that's because Microsoft Office itself is network aware.

We often use the term API, or application programmers interface, as the definition of the smarts that

are built into an application that allows us to make the application network aware.

So this is the OSI. Alright, the TCP/IP model is more simplistic than the OSI simply because if you're

using only TCP/IP there's a lot of parts don't really come into play as much.

Let me show you what I'm talking about.

First of all we have what they call the network interface layer. So the network interface layer covers

all the physical cabling, MAC addresses, network cards, pretty much everything in terms of hardware with

a couple of small exceptions, routers, are all covered here under network interface. So that ties similarly

to the data link and physical for the OSI.

Now your next one is your internet link.

Your internet for IP addresses. So things like routers or anything it has to do with an IP address

works at the Internet layer.

The next one is transport.

Now transport does all the assembly and disassembly but it also has to do with whatever it takes to

connect to the other system to make sure the data gets there.

So in this case we're talking about things like, for example TCP or UDP or whatever it takes to get those

chunks of data from one application to the next.

Now the last one is called application.

So application actually takes into consideration the old OSI application, presentation and session layer.

So everything that has to do with the application itself works at this layer.

Now keep in mind that the TCP/IP model looks at this a little bit differently than OSI.

TCP model looks at applications as applications for example email or FTP or telnet.

All of these very distinct things that each have their unique port numbers all play into this one particular

area.

So if it were up to me, and it's not but to me, I would change this a little bit.

First of all I would go ahead and take the transport layer and put the old OS I session layer into that as

well as the transport layer.

But that's really not the way we look at it for the exam so we put that back.

And that's pretty much your TCP/IP model.

Now what I want to do is let's take a look at both of these guys.

First we have the OSI model, older more detailed.

And then we have the TCP/IP model, more modern,

not as much detail but not as complicated.

Make sure for the network plus exam that you know both of these models. Now don't worry about it.

We've got a lot more to talk about.

We've got lots of episodes that are going to break down all the different layers in these models and

a lot more detail. But for right now the important thing you need to appreciate is that each one of these

layers has a distinct function.

Each one of these layers has a name but not only that it's also got a number.

So let's use OSI as an example.

If I'm talking about OSI and I'm looking at the network layer. Now understand its network layer but

there's also numbers associated with it.

They go from one up to seven.

So if I'm talking about OSI and I say Layer 3, you need to know that I'm talking about the network level.

That's also very true with TCP/IP.

So whenever we're talking about the TCP/IP model there are numbers associated with it.

Make sure for the network plus you know each name of each layer as well as the number associated with

it.