>> Our next topic is going to be the OSI reference model.
The OSI reference model
provides us a framework so that we can
understand the different elements of
networking and how they work together.
There are a couple of purposes with
the OSI reference model to cover first.
Then we'll get into exactly
>> what it is and how it works.
>> It's the idea of interoperability between vendors.
What we want is an environment where
different components from
different vendors all work together.
We don't want to have to buy everything
on our network from a single vendor.
We don't want vendors to have
any monopoly of services because they can
then charge whatever they want and we're
locked into that specific vendor.
Whereas if instead we have a variety of
components and I'm connected to a Netgear switch,
and that fails, I can go out and buy
a Linksys switch that makes my life easier.
We prefer standardization.
Usually when we add vendors
that have proprietary equipment,
it may take additional training in
order to work with those devices.
There's usually additional costs
and sometimes additional complexity.
The OSI model promotes standardization.
Ultimately, what we have is seven layers of
the OSI model and standards at each layer.
If I have a Layer 2 device and I built it to
the standard of an OSI Layer 2 device,
then it will be interoperable
and a standard space environment.
That's great for me as a vendor
because standardization sells,
it doesn't mean proprietary stuff doesn't sell,
but standardization really is what
most network administrators are looking for.
There might be times when we purchase
a proprietary device or component.
But for the most part, we like standardization.
Each layer of this OSI model is going to give us
a description of certain services based on the standard.
What we're also going to see with
the OSI model is an idea of encapsulation.
When we talk about encapsulation, it is packaging.
It's placing headers around data,
or it's putting a protocol in a protocol.
The best example I can offer is this.
Let's say my uncle Steve in Seattle has an anniversary.
In order to help him celebrate,
I'm going to send him some champagne glasses.
Technically, I could write his name on
the champagne glasses and
say Happy Anniversary Uncle Steve.
Put a stamp on them and put them in the mailbox.
But of course, as soon as I do that,
I'll hear clink, clink, clink.
What I'm going to do instead is
wrap the champagne glasses in bubble wrap,
put them in a small box,
and put bubble wrap around that box.
I'm going to gift wrap the box and write
happy anniversary or best wishes on the box.
I do want to point out that when I
write best wishes uncle Steve,
that the uncle Steve piece is an addressing element.
It's not worldwide addressing but
when the package find Steve's house,
he's going to see that it's for him.
Some of what we're doing is for protection and padding,
and some of it is for addressing.
Ultimately, we start out with
a very small payload, the champagne glasses.
As we package and add
addressing and protection and all those things,
by the time we're ready to pass this off to FedEx,
we wind up with a much more sizable package.
We continue to add.
We put in a FedEx box,
we get a FedEx label we handed off to
the FedEx carrier, that's encapsulation.
It's not the same as encryption though,
sometimes people make that mistake.
I never encrypted the champagne glasses.
I didn't magically turn them into coffee mugs.
There still champagne glasses.
They're just all wrapped up.
That's our goal with encapsulation.
Of course, the only piece that I missed in
the instructions is that if you have pets,
at some point in time,
I can guarantee you there will be a cat in
the packing peanuts or
perhaps a pug wrapped up in bubble wrap.
If you need to [inaudible] the animals,
you can add those steps in here.
I do it want to mention no animals were
harmed during the making of this video.
This encapsulation idea's illustrated here on the right.
At the top, we have seven layers.
Application, down a presentation, session,
transport, network, data link, and finally physical.
The idea here is data conceptually
travels down through the OSI model.
When I say that, what I mean is that
the sending computer starts with
an application layer protocol.
Application layer protocols are
those protocols that drive user application.
If you think about a web browser, you got HTTP.
If you think about mail, you've got POP3
or IMAP or some of those other mail protocols.
Data starts at the very top of the OSI model,
and as it conceptually travels down, headers are added.
You can see we start with the data
alone at the application layer.
But as we move down to presentation,
we get a presentation header,
a section header next, a transport header,
then network cutter, data link cutter,
all the way down to the point where
we have the conversion in
the bits that relay electronic signal on the network.
At each layer, something is added that will
help the data get to where it needs to go.
All the way to the destination host.
There's formatting,
there's perhaps receipt of acknowledgments.
There's IP addressing, there is MAC addressing,
and all these things we'll talk about as we move on.
But I just want you to have this idea that at each layer,
something gets added until the computer is
ready to put that data out on the network.
Now, I would definitely
recommend that you know the OSI model
top down and know it by number and by name.
All people seem to need Domino's Pizza is
a good little trick that a lot of
folks use from the top down.
All people seem to need Domino's Pizza.
That will help you remember.
My personal favorite from the bottom up,
people don't need to see Paula Abdul.
Change my mind, I'm standing by that.
But at any rate, find some a little trick.
I do not recommend that you go out and Google
mnemonic tricks for the OSI reference model.
There are some things you cannot unsee and
there are people of questionable intent on the Internet.
That's all I'm going to say about that.
What I've probably just done is guaranteed that 2,200
people go on the Internet and Google
in mnemonic tricks for the OSI model.
But at any rate, find some trick
that helps you remember it and go with that.
The other thing I asked you to think about is
that at each layer of the OSI model,
we refer to data a little bit
differently based on the headers.
When data is at
the application presentation session layer,
we still refer to it as data but
once it comes down to the transport layer,
we have a segment down to the network layer.
We have a packet down to the data link layer,
we have a frame, and at the physical layer,
it's converted into bits.
Segment, packet, frame, bits.
Sometimes you'll hear these terms used
interchangeably and that's okay.
But I just want you to know
that really these terms should
vary based on the headers
that have been added to the data.
A packet has more headers than a segment.
A frame has more headers than a packet,
and so specifically at the transport layer,
there is specific transport information
that's added to the header.
Next, at the network layer,
specific information is added.
Then at the data link layer,
specific information is added.
This gives you an idea of
the broad purpose of the OSI model.
Just keep in mind, it's a seven-layer model.
It's designed to promote interoperability
>> among vendors.
>> There are standards at each of the layers,
and really what it does is it helps us describe
the process of encapsulation and
the adding of headers so
that data can get from destination
to destination with the correct information.