Welcome to my complete CCNA, Cisco Certified
Network Associate course.
This is Jeremy’s IT Lab.
This course aims to be a complete course for
the CCNA, including everything you need to
pass the exam, all 100% free.
Stay tuned till the end of the video for the
quiz to test your knowledge of the material
in this video.
Also, remember to download and use the Anki
flashcards with the link in the description.
Let’s get started.
This third lesson is about two networking
models.
The first is the OSI Model, and the second
is the TCP/IP suite.
Note that the TCP/IP suite is a networking
model, too, but it’s usually referred to
as the TCP/IP suite, or just TCP/IP.
Unless you’re familiar with networking,
you probably haven’t heard of the OSI model.
However, you probably have heard of TCP/IP,
or at least the second part of that name,
IP, the Internet Protocol, for example you’ve
probably heard of an IP address before.
So, what is a networking model?
Well, networking models categorize and provide
a structure for networking protocols and standards.
Do you remember what a networking protocol
is?
I talked a bit about Ethernet, and various
standards for copper and fiber-optic cables
in the Day 2 video.
That’s an example of a protocol.
A networking protocol is a set of rules defining
how network devices and software should work,
including how they should work together.
By the way, protocols refer to logical rules
about how devices should communicate, not
physical standards.
So, you could visualize it like this.
This is a networking model, and it categorizes
and provides a structure for various networking
protocols
like these.
The different colors represent different categories
of protocols and standards.
These protocols and standards could define
something like the structure and usage of
IP addresses, or maybe physical details like
electrical voltage used on copper cables when
transmitting data.
Let’s talk about the OSI model first, and
then we’ll look at the TCP/IP suite after.
So, what if there were no standard networking
models and protocols?
Here’s a few Dell PCs and a few iMacs.
If Dell and Apple each made their own networking
model, each with their own set of networking
protocols, these Dells would be able to communicate
with each other, and these iMacs would be able
to communicate with each other, but because
the Dells and iMacs would speak a different language
than each other, with a different set of rules
about how to communicate, the Dells wouldn’t
be able to communicate with the imacs.
This is obviously a problem in modern networks,
such as the Internet, where we expect devices
from various makers in various countries to
be able to communicate with each other.
The OSI model is one attempt at standardizing
network communications.
Although it isn’t actually in use today,
it has had a big impact on how network engineers
think about networking, and we still refer
to it today.
OSI stands for ‘open systems interconnection’
model.
Open, means that it is an open standard, not
a proprietary model developed and used by
an individual company.
It is a conceptual model that categorizes
and standardizes the different functions in
a network.
It was created by the international organization
for standardization, the ISO, in the late
1970s and early 1980s.
Network functions are divided into 7 layers.
For example, look at the bottom Layer, layer
1.
It is the physical layer.
The standards for cables and interfaces we
learned about in the Day 2 video all fit into
the physical layer of the OSI model, but there
are 6 more layers on top of that to make the
network operate properly.
Let’s examine each layer one by one, starting
at Layer 7, the application layer.
The application layer is the layer that is
closest to the end user.
The application layer interacts with software
applications that have some communication
component, such as your web browser, whether
that’s Brave, Firefox, Chrome, or whatever.
For example HTTP and HTTPS are Layer 7 protocols.
Notice https before cisco.com, indicating
that https is being used to get this website
and view it in the browser.
Keep in mind that Layer 7 doesn’t include
the application itself, like Chrome or Firefox,
but rather the protocols that interact with
the application, like HTTP or HTTPs.
A couple functions of Layer 7 are: identifying
communication partners and synchronizing communication.
Let me explain how it does this.
Here I have two OSI model stacks, representing
two computers that will communicate with eachother.
The software application, maybe a web browser,
interacts with Layer 7, the application layer,
and wants to send some data to the system
on the right.
This data is processed through the OSI stack,
each layer adding something to the original
data.
This is called ‘ENCAPSULATION’, because
the original data is encapsulated inside this
additional information which is added on.
By the time it reaches the physical layer,
it is electrical signals on a wire, and is
sent to the neighboring system.
Then, the neighboring system performs the
opposite process, the additions of each layer
are stripped off until the data reaches the
application layer of the neighboring system.
This process is called ‘de-encapsulation’,
as the additional information is removed as
the data is processed up the stack.
Both the encapsulation and de-encapsulation
processes are examples of ‘Adjacent-layer
interaction’, interaction between the different
layers of the OSI model.
However, the communication between the application
layers of the two different systems, is called
same-layer interaction.
This same-layer interaction between application
Layers is what allows the application layer
to perform its functions of identifying communication
partners, synchronizing communications, etc.
Layer 6 of the OSI is the presentation layer.
Data in the application is in an ‘application
format’, a format that applications understand.
It needs to be translated to a different format
to be sent over the network.
the presentation layer’s job is to translate
between application and network formats.
One example of a function of the presentation
layer is encryption of data as it is sent,
so that only the intended recipient can read
it, and of course decryption as it is received.
The presentation layer also translates between
different application-layer formats, to ensure
that the data is in a format the receiving
host can understand.
To summarize, the presentation layer translates
data to the appropriate format.
That’s all you really need to know about
the presentation layer.
Layer 5 of the OSI model is the session layer.
The session layer controls dialogues, also
known as sessions, between communicating hosts.
It establishes, manages, and terminates connections
between the local application (for example
your web browser) and the remote application,
for example, YouTube.
YouTube’s servers are being used by countless
people at every moment, and there has to be
a way to manage all of these sessions.
That’s the purpose of the session layer
of the OSI model.
So, we’ve looked at the top 3 layers of
the OSI model.
From Top to bottom, application, presentation,
and session.
It’s important to know what the functions
of these layers are, but actually network
engineers don’t really work with these layers
of the OSI model.
That is the job of application developers.
Application developers work with the top layers
of the OSI model to connect their applications
over networks.
Let’s return to this diagram.
Remember what I said about encapsulation?
Well, data prepared at the top 3 layers is
then sent over to the bottom 4 layers, which
actually do the work of sending it over the
network.
After the top 3 layers hand data over to the
bottom 4 layers, The next step before sending
data is that Layer 4, the transport layer,
adds a header in front of the data, like this.
So, let’s look into the Transport layer
a little bit.
Layer 4 of the OSI model is the transport
layer.
It segments and reassembles data for communications
between end hosts.
To reword that, it breaks large pieces of
data into smaller segments which can be easily
sent over the network and are less likely
to cause transmission problems if errors occur.
For example, if data wasn’t segmented and
you were trying to watch a video, if an error
occurred that prevented the video from reaching
your computer, you wouldn’t be able to watch
the video at all.
However, if the data is segmented into many
small units, and only one fails to reach the
destination, that’s not a big problem.
The video might skip for a second, but then
will continue on just fine.
Also, the Transport layer provides host-to-host
communication, also known as end-to-end communication.
This also provides process-to-process communications
for applications.
Now let’s review.
Data is prepared by the top 3 layers.
a Layer 4 header is added on.
Note that at this point in the process, this
unit of data plus Layer 4 header is called
a segment.
Remember, if the data being sent is large
enough, it will actually be segmented into
smaller parts, and a Layer 4 header will be
added on to each segment.
Next, that segment is passed on to Layer 3.
And another header is added on to the end,
like this.
Layer 3 of the OSI model is the Network Layer.
The network layer provides connectivity between
end hosts on different networks, for example
outside of the local area network, or LAN,
which we mentioned briefly in the Day 1 video.
Layer 3 provides logical addressing, in the
form of IP addresses, something you’ve probably
heard of before.
Layer 3 also provides path selection between
source and destination.
Often there are many possible paths which
data can take to reach its destination, especially
over a huge network like the Internet.
Layer 3 provides the means of selecting the
best path.
Routers operate at Layer 3.
Remember in Day 1’s video I said that routers
are used when end hosts need to reach a destination
outside of their LAN?
That’s because routers are Layer 3 devices.
Let’s review the encapsulation process again.
Data is prepared by the upper layers, the
transport layer adds a layer 4 header, and
this combination of data plus layer 4 header
is called a segment.
next the network layer adds a layer 3 header,
including information like the source and
destination IP address, to the segment.
This combination of data, layer 4 header,
and layer 3 header, is called a packet.
Next, the packet is further encapsulated at
Layer 2, , this time with both a Layer 2 header
AND a Layer 2 trailer.
Layer 2 of the OSI model is the Data Link
Layer.
The data link layer provides node-to-node
connectivity and data transfer.
For example, direct connections between a
PC and a switch, or a switch and a router,
or a router and a router. because Layer 2
is adjacent to Layer 1, the physical layer,
it also defines how data is formatted for
transmission over a physical medium, like
copper UTP cables.
Layer 2 also detects, and possibly corrects,
errors that occur on the physical layer.
Like layer 3, layer 2 also uses an addressing
system, however it is separate and different
from layer 3 addressing.
Finally, switches operate at Layer 2.
Switches look at the destination Layer 2 address
to determine where to send the data,
Let’s look at the encapsulation process
once more.
Data is prepared by the application layer.
A layer 4 header is added to the data to make
a segment.
A Layer 3 header is added to the segment to
make a packet.
Remember, the IP address is included in this
Layer 3 header.
Then, a layer 2 header and layer 2 trailer
are added to the packet.
At this point, the combination of data, layer
4 header, layer 3 header, layer 2 header and
layer 2 trailer is called a frame.
Now, the data is not further encapsulated
at Layer 1.
This frame is then sent over the connection,
whether it’s electrical signals over a wire
or wireless signals in the case of wifi, to
the neighboring system.
Let’s look at layer 1 of the OSI model a
bit before moving on.
Layer 1 of the OSI model is the physical layer.
The physical layer defines physical characteristics
of the medium used to transfer data between
devices.
For example, voltage levels, maximum transmission
distances, like the maximum cable lengths
mentioned in Day 2’s video, physical connectors,
cable specifications, etc.
Digital bits are converted into electrical
signals, for wired connections, or radio signals,
for wireless connections, like Wi-Fi. as I
mentioned before, all of the information in
Day 2’s video, such as cables and pin layouts,
is related to the physical layer of the OSI
model.
Okay, now we’ve got a complete frame, and
that frame will be sent from the local device
over this cable, let’s say its an ethernet
cable.
Once it reaches the remote device, the reverse
process of encapsulation, de-encapsualtion,
takes place.
the data link layer translates the raw physical
data into a complete frame once again.
Then the layer 2 header and trailer are removed,
leaving the Layer 3 packet.
The layer 3 header is removed, leaving the
Layer 4 segment.
Finally, the layer 4 header is removed and
we are left with the original data prepared
by the upper layers of the original device.
That’s the process of de-encapsulation.
Before moving on, I want to review some terms.
So, when an application wants to send data
to another system, it interacts with the application
layer of the OSI stack and the data is prepared
to be sent.
When the layer 4 header is added at the transport
layer, what is this combination of data plus
Layer 4 header called?
It’s called a Segment.
Then, the Layer 3 header is added on at the
Network layer, remember that includes the
IP address.
What’s this now called?
It’s called a packet.
Finally, a Layer 2 header and trailer are
added at the Data Link layer.
What’s this complete unit called? It’s
called a frame.
Now, there is one new term that is used to
refer to all of these.
These are all called Protocol Data Units,
or PDUs.
For example, segment is the term for a Layer
4 PDU, Packet is the term for a Layer 3 PDU,
etc.
By the way, at Layer 1, the physical layer,
the name for the PDU is bit, referring to
the bits being transferred on the wire.
So, that’s a lot of information.
If you need help remembering the order of
the OSI model layers, the flashcards included
in the description should be a big help, but
here are a couple acronyms too.
I don’t use acronyms to remember things
myself, but many people like them, so here
are a few examples to help you remember.
The first one goes from layer 7 to layer
1, ALL PEOPLE SEEM TO NEED DATA PROCESSING.
Here’s another one, from Layer 1 to
Layer 7 this time.
PLEASE DO NOT TEACH STUDENTS POINTLESS ACRONYMS.
If you know any other good ones post them
in the comment section!
Now let’s move on from the OSI model, to
the TCP/IP suite.
Like the OSI model, it is a conceptual model
and set of communications protocols, and in
TCP/IPs case it is used in the Internet and
other networks.
It is known as TCP/IP because those are two
of the foundational protocols in the suite.
It was developed by the United States Department
of Defense through DARPA, which is the Defense
Advanced Research Projects Agency.
It has a similar structure to the OSI model,
but with fewer layers.
And, it is the model actually in use in modern
networks, NOT OSI.
Note, however, that the OSI model still influences
how network engineers think and talk about
networks today, which is why it's important
to learn.
Now let’s compare the two.
On the left is the OSI model you’re familiar
with by now, and on the right is the TCP/IP
Suite’s networking model.
The Application, Presentation, and Session
layers of the OSI model are essentially equivalent
to the Application Layer of the TCP/IP model.
Combining them into one actually represents
how network engineers tend to think about
networks, since we don’t work much with
anything above the Transport Layer, however,
when TALKING about networks, we use the OSI
numbering.
For example, i f you say ‘there is a Layer 4 problem in
the network’, network engineers will think
of OSI’s Layer 4, the transport layer, not
TCP/IP’s Layer 4, the Application Layer.
Moving on, the OSI model and the TCP/IP model
both share the transport layer.
The network layer of the OSI model maps to
the Internet layer of the TCP/IP model.
Finally, the data link and Physical layers
of the OSI model are equivalent to the Link
layer of the TPC/IP model.
Once again, though, if people say, for example,
there is a Layer 2 problem in the network,
they are referring to the OSI Model’s Layer
2, the Data Link Layer, not TCP/IP’s layer
2, the Internet layer.
Keep that in mind.
Although TCP/IP is the model actually in use
in modern day networks, we still talk about
networks using the OSI model language.
Now, you might hear different names used for
these layers.
This is a chart off of Wikipedia showing different
naming systems used.
Don’t worry about memorizing all of these,
but just keep in mind you might here different
names than the ones I taught you.
For example, the Link layer might be called
the network interface, or network access layer.
I actually like this 5 layer model, because
it combines the top 3 layers that we, network
engineers, don’t really think about into
one, but it keeps the data link and physical
layers separate, and I think its good to think
of them as two separate layers.
Before finishing up this lesson and moving
on to the quiz, I want to show you this excellent
diagram which I found on Wikipedia.
It demonstrates the process of a host, Host
A, sending data to Host B, with two routers
in between.
Here you see the four devices, Host A, connected
to a router, connected to another router,
connected to Host B. Here is the TCP/IP stack
on each device.
Note that, for forwarding data from host to
host, these routers don’t need to be aware
of the higher layers, so only the Internet
and Link layers are present here.
So, let’s walk through the process.
An application on Host A wants to communicate
with an application on Host B. Let’s say
it’s a Skype conversation, so Host A is
sending a little bit of video and audio data
to Host B. Skype interacts with the application
Layer, and the data is encapsulated via the
Transport, Internet, and Link Layers.
Then, it is forwarded over to the Router, probably
via Ethernet UTP copper cabling.
Remember, Routers are Layer 3 devices, so
they want to know the Layer 3 IP address to
know where to forward the data next.
So, de-encapsulation occurs at the Link layer,
and then at the Internet layer the router
looks at the destination IP address to know
where to send the packet.
Then, the packet is once again encapsulated
to make a Frame.
From here it is sent to another router, perhaps
over long-distance fiber cabling.
At the Link Layer of this second router, de-encapsulation
occurs again, and this router also checks
the destination IP address.
Once it knows where to send the packet, it
is once again encapsulated, and sent over
some medium, Ethernet cabling in this case,
to Host B. Now de-encapsulation takes place
once more.
Here at the link layer it is a frame.
The header and trailer are removed, and it
becomes a packet.
The header is removed, and it becomes a segment.
Finally, the transport layer header is removed,
and the application layer on Host B receives
the data and interacts with the application
on Host B. So, this has achieved process-to-process
communication, between Skype on Host A and
Skype on Host B. Of course, this process will
happen many many times in both directions
during the duration of a skype call.
As I mentioned before, this is known as same-layer
interaction.
Also, I mentioned the transport layer
providers host-to-host communications before.
This diagram indicates that as well.
This Transport layer segment was never changed
during this whole process, it is as if it's
direct communication between the two hosts.
Finally, remember that, because the TCP/IP
protocols are all industry standard protocols
used by all makers, it doesn’t matter what
kind of PCs or router you’re using.
An Apple iMac can communicate with a Cisco router,
which can communicate with a Juniper router,
which can communicate with a Dell PC.
That’s the importance of having industry
standards.
Okay, that was another lesson with a lot of
information.
Once again, there will be supplementary materials
to help you remember everything.
There will, of course, be an end-of-video
quiz, coming up from the next slide.
There will also be pre-made flash cards to
help you memorize what you need to remember,
check the link in the description.
Finally, for this video I will again include
a Packet Tracer practice lab, which will use
an interesting feature of packet tracer called
‘simulation mode’ to analyze network traffic.
Now let’s go to the quiz.
HTTP data sent from a YouTube web server is
displayed via your web browser.
This is an example of what?
A, adjacent-layer interaction.
B, same-layer interaction.
C, encapsulation.
Or D, de-encapsulation.
Pause the video to think about your answer...
the answer is B, same-layer interaction.
Let’s check the options.
Adjacent-layer interaction refers to interaction
between different layers of the OSI model.
In this case, both YouTube’s web server
and your web browser are operating at Layer
7 using HTTP.
So A, adjacent-layer interaction, is incorrect.
Although encapsulation and de-encapsulation
of data surely happened many times as the
data was sent from YouTube’s web server
to your web browser, they are not descriptions
of the interaction between YouTube and your
browser.
So c, encapsulation, and d, de-encapsulation,
are incorrect.
Same-layer interaction refers to interaction
between the same layer on different hosts,
in this case the application layer of YouTube’s
web server and the browser on your PC.
The concept of same-layer interaction allows
you to ‘ignore’ the other layers involved
and focus on interactions between a single
layer on different devices.
So b, same-layer interaction, is the correct
answer.
let’s go to question 2.
HTTP data has been encapsulated with three
separate headers and one trailer.
What is the appropriate name for this PDU?
A, packet.
B, segment.
C, frame.
Or D, data.
Pause the video to think about your answer...
The answer is C, Frame.
Let’s check.
Packet refers to the OSI Layer 3 PDU.
It would have two headers (the Layer 4 header,
and the Layer 3 header) and no trailer.
So a, packet, is incorrect.
Segment refers to the OSI Layer 4 PDU.
It would have one header (the Layer 4 header),
and no trailer.
So b, segment, is incorrect.
Data refers to the upper-layer data before
being encapsulated.
It would have no headers or trailer.
So d, data, is incorrect.
Frame refers to the OSI Layer 2 PDU.
It has three headers (Layer 4, Layer 3, and
Layer 2 headers) and one trailer (the Layer 2
trailer).
So c, frame, is the correct answer.
Let’s go to question 3.
Which layers of the OSI model are most relevant
to the role of a network engineer?
A, transport, network, data link, and physical.
B, transport, network, and data link.
C, network only, or D, applicaton, presentation,
and session.
Pause the video to think about your answer...
the answer is A, transport, network, data-link, and physical.
Let’s check.
Although these layers are very relevant to
the duties of a network engineer, the physical
layer is missing.
So b, transport, network, and data link, is
incorrect.
Although the Network layer is very relevant
to network engineers of course , it is not the only one.
So c, network only, is incorrect.
These layers of the OSI model are not generally
relevant to the role of a network engineer.
They are relevant to application developers.
So D, application, presentation, and session,
is incorrect.
These four lower layers of the OSI models
are all very relevant to the role of a network
engineer.
So a, transport, network, data link, and physical,
is the correct answer.
Lets go to the next question.
The Link layer of the TCP/IP Model is equivalent
to what layer, or layers, of the OSI Model?
A, transport and network.
B, network and data link.
C, data link.
Or D, data link and physical.
Pause the video to think about your answer...
the correct answer is D, data link and physical.
Let’s check.
The OSI Transport layer is equivalent to the
TCP/IP Transport layer, and the OSI Network
layer is equivalent to the TCP/IP Internet
layer.
So a, transport and network layers, is incorrect.
The OSI Network layer is equivalent to the
TCP/IP Internet layer.
The OSI Data Link layer is equivalent to part
of the TCP/IP Link layer, but it is not totally
equivalent.
So b, network and data link layers, and c,
data link layer, are incorrect.
The combined functions of the OSI Data Link
and Physical layers are equivalent to the
TCP/IP Link layer.
So d, data link and physical layers, is the
correct answer.
Let’s go to the last quiz question for this
video.
Which layer of the OSI model provides host-to-host
communications?
A, application, b, network, c, transport,
or d, data link.
Pause the video to think about your answer...
the answer is c, transport.
Let’s check.
The Application layer provides process-to-process
communications, not host-to-host.
So a, application layer, is incorrect.
The Network and Data Link layers do not provide
end-to-end, host-to-host communications.
So b, network, and d, data link, are incorrect.
The Transport layer provides host-to-host
communications, as shown in this diagram.
So c, Transport, is the correct answer.
Thank you for watching.
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