In this section of the course,
we're going to learn all about the OSI model.
Now, you may be wondering what is the OSI model?
Well, OSI stands for the open systems interconnection.
And when we talk about the OSI model,
this was developed all the way back in 1977
by the International Organization for Standardization.
Now, this organization is responsible
for creating different standards, which we refer to as ISO
and then some number behind it.
For example, if you see ISO 7498, that is the standard
that refers to the OSI model.
Now for the exam, you do not need
to worry about memorizing any kinds of ISO numbers,
but I do want to introduce you to this concept
because everything we cover in computing is going to be
associated with some kind of standard with it.
Now, in the case of the open systems interconnection model,
most people simply call this the OSI Model,
but you'll also hear it referred to
as the OSI Stack sometimes.
Either way, we're really talking about the exact same thing.
Now, the OSI model is an extremely important thing in
Network Plus because it's a fundamental thing
that we're going to use to discuss all the pieces
and parts of our networks.
In fact, it's so important
that you're going to see some questions on the exam about this
concept and how it relates to other things in our networks.
The OSI model is going to be made up of seven different layers,
and we're going to talk about each of these seven layers in the
next seven videos covering one layer per video
because this model is just that important
to our understanding of our computer networks.
Now, as we work through this section of the course,
we're going to be focused mainly on domain one,
networking concepts,
and exclusively we'll be covering all of objective 1.1,
which states that we must be able
to explain concepts related
to the open systems interconnection or OSI reference model.
Now, the seven layers that make up the OSI model are useful
when we're trying to troubleshoot a network,
because if we have a problem
and we think about it from all the different layers in the
seven layers, we're going to be able
to start identifying exactly where the problem is
and then we can troubleshoot it more accurately.
Now, the OSI model is also called the OSI Reference model,
and you may be wondering what do you mean
by a reference model?
Well, a reference model is simply something that we use
to categorize the functions
of a network in a particular layer,
and that's what we're going to do when we use the OSI model
during our troubleshooting efforts.
Now, when you start to look at the OSI model in depth,
you're going to start to notice
that the OSI model doesn't actually line up cleanly
or easily or accurately with the way
that our modern networks are going to operate.
For example, some things are going to operate at multiple
layers of the OSI model in our modern networks,
especially when we're discussing the upper three layers.
Now, there's a good reason for why the OSI model isn't
perfect the way it is the way our networks are operating
today, and that's
because our networks today operate under a model known
as the TCP/IP model.
Now, to keep things simple for right now, just remember
that the OSI model is a reference model,
and we're going to use it in our network operations
and troubleshooting because again,
it's very generic in nature
and it's going to allow us to work with
any and all network types
we come across, not just the most commonly used network
that we use today, which is actually TCP/IP.
Now, one of the benefits of using a reference model like the
OSI model is that it can be used equivalently across lots
of different technologies and devices and manufacturers.
So if I'm going to look at a particular wireless network card
for my computer, I can compare how it operates
through each of the seven layers of the
OSI model and then compare it equivalently
to a different manufacturer's wireless networking card.
When you can understand the functions at each
and every layer that's being performed, this helps you
to better understand the flow of data in your network
through the card, how it communicates,
and how you can troubleshoot it too.
All right, now that I've mentioned
there are seven layers of the OSI model
several times in this video,
I still haven't really showed you
what those seven layers are.
So let's take a look at the seven layers of the OSI model.
First, we start at the bottom layer,
and we work our way from layer one all the way to the top,
which is known as layer seven.
These layers are the physical layer, the data link layer,
the network layer, the transport layer, the session layer,
the presentation layer, and the application layer.
Now for the exam, you need to remember these seven layers
and know them in the proper order.
You need to know them going from bottom to top
and from top to bottom.
Now, to help you with that, I like
to use a memory aid known as a pneumonic.
Now in the case of the OSI model, I like
to think about my favorite kind
of pizza when I'm listing the things from the
bottom to the top layer.
Now, my favorite kind of pizza is sausage pizza.
So if you want to remember the seven layers of the OSI model,
just remember what I always say about sausage pizza.
Please do not throw sausage pizza away.
After all, I think sausage pizza is pretty darn tasty, and
therefore we shouldn't be wasting it
by throwing it in the garbage.
Now, by using this sentence, you'll be able
to remember the seven layers from bottom to top.
Just take the letters from the sentence,
please do not throw sausage pizza away or P-D-N-T-S-P-A
and replace it with the words physical, data link, network,
transport, session, presentation, and application.
And there you have it, the seven layers
of the OSI model from bottom to top, at least that's
how I like to remember it, but you can use
whatever mnemonic you want.
I've heard many interesting ones over the
years from my different students.
Once I was teaching a class to a bunch of navy sailors
and they told me their saying was,
please do not tell shore patrol anything.
For those of you who don't know, shore patrols basically
like the military police officers in the Navy.
So I'm guessing they didn't want to get in
trouble with their ship's captain.
So they believed in the mantra of,
please do not tell shore patrol anything.
And this kind of thing is like saying
what happens in Vegas stays in Vegas, right?
Exactly.
Now, another one of my students told me
that their favorite mnemonic was,
please do not teach students pointless abbreviations.
Now, I thought this was kind of funny
because the abbreviation is exactly what we're trying
to do here to remember the seven layers of the OSI model.
Now, whatever works for you is fine with me.
Just remember that you need to memorize these seven layers
before you take your exam
because on the exam, you are going to get questions
about these seven layers.
Now you might see something easy like
what is the seventh layer of the OSI model?
And in that case, you would just pick
application and be done with it.
Or they might give you something a little bit harder
and say, what layer is the session layer at?
And you would say, layer five.
Now again, they might go a little bit more complex
and ask you things about a device
and ask you what layer is that device operating in?
For example, a router is going to operate at layer three
or the network layer of the OSI model,
but a hub only operates at layer one
because it's considered a physical device and
therefore it works at the physical layer
of the OSI model.
Now because of this, we're going to cover each
and every layer of the OSI model in this section
of the course, so you can learn what devices are used at
what layer, as well as which protocols
and other information about them too.
Alright, before we finish up this lesson, we need
to talk about one more thing, and that's data
because our networks are designed for the purpose
of making data flow across those networks.
Now, data moves through the layers of the OSI model,
and as it does, the name of it actually gets changed,
and it's not called data anymore.
So if you hear the word data,
what we're really talking about is things at layer five,
six, and seven of the OSI model
because all three of those layers use the term data.
As you move down the layers
and you go from four to three to two to one,
that name is going to change each and every time.
Now, when we're at layer four
or the transport layer, we are going to call it a segment.
When we get to layer three
or the network layer, we call it a packet.
When we get to layer two or the data link layer,
we call it a frame.
And finally, if we're operating at layer one
or the physical layer, we convert the data into ones
and zeros and we send it across the media
and we refer to this as bits.
Now, this is something you're going to need to memorize
for the exam too, but don't worry,
I've got another pneumonic to help you remember this one.
And this one is really just a simple
question you can ask yourself.
Here it is. Do some people fear birthdays?
That's it.
I think this is a valid question
because as we get older,
sometimes people start fearing their birthdays
because they don't want to show their age
and they don't want to appear older.
But again, I know this is really silly pneumonic,
but hopefully it helps you remember the four types
of information as they flow from data
through those upper layers and down to the lower levels,
going from data to segments to packets, to frames to bits.
Alright, if you can remember some people fear of birthdays,
that's going to give you D-S-P-F-B as your pneumonic.
And then again, we have those data types going from layers
five, six, and seven with data moving down to segments,
packets, frames, and bits going from layers
four, three, two, and one.
So one more time for the exam.
It is really important to understand the seven layers
of the OSI model and how the data moves up
and down through those layers
as you're transmitting the data
and encapsulating and decapsulating.
And that's what we're going to be talking about.
Alright, as we begin this section,
we're first going to spend one lesson on each
of the different layers of the OSI model, as I said.
Then we're going to move into our coverage of encapsulation
and decapsulation.
Encapsulation is the process of wrapping data
with protocol information at each layer of the OSI model
as it passes down the networking stack.
Decapsulation, on the other hand, is the process of removing
that protocol information layer by layer
as the data unpacks itself on the receiving end
as it moves up the OSI model.
After that, I'm going to demonstrate
how you can use a software tool known as Wireshark.
Now, Wireshark is a network protocol analyzer that captures
and displays data as it travels back
and forth over a network.
And this can be done in real time
or captured and analyzed later.
Now, we do this for the purposes of inspection,
troubleshooting, and analysis.
And in our case, we're going to use Wireshark
to analyze a packet capture so you can better see
how the data is broken down into these different
layers of the OSI model.
And then finally, at the end of the section,
we'll take a short quiz to see if you remembered
what you learned in this section.
As we do that, we'll also review your answers
to make sure you know why the right answers were right
and the wrong answers were wrong.
So with all that being said,
let's jump into our lessons on the seven layers
of the OSI model in this section of the course.