In this video, we're going to continue the discussion of the OSI Model
by looking at traffic flows between hosts. I want to show you a practical demonstration
of the OSI Model by using Wireshark to capture traffic between devices
Now when communication takes place between 2 hosts so in this example we have
HostA talking to HostB, the information that's going to be transmitted across the
network must undergo a process of conversion at both the transmitting device
as well as the receiving device, this is commonly known as encapsulation
and de-encapsulation and I'm going to cover this in more detail in the next
few minutes. At the end of this video you should be comfortable with the
fact that each layer of the OSI Model only talks to the corresponding layer
of the communication partner, in other words, Layer7 communicates with Layer7
Layer1 communicates with Layer1 and so forth and so on
So let's look at that in more detail
So when the sender wants to transmit data, let's say in this example
your PC is opening up a Web connection to Cisco.com and the PC is sending
some information, that information has to go through the OSI Model from Layer7
all the way down to Layer1, to then be transmitted across the physical media
So in this example user data is transmitted, so at Layer7 of the OSI Model
the data is encapsulated with the Layer7 header, that in turn is encapsulated
with the Layer6 header at the Presentation Layer and that in turn is encapsulated
with the Layer5 header at the Session Layer, which in turn is encapsulated
with the Layer4 header at the Transport Layer and this continues all the way
down, at Layer3 the data is encapsulated with the Network Layer header, at Layer2
the data is encapsulated with the Data Link Layer header and in this case a
frame check sequence is added to the data.The frame check sequence ensures
that the data is not corrupted when sent from the sender to the receiver
Lastly, the data is transmitted across the wire or physical media as bits
in other words, 0s and 1s and that data is then sent from the sender to the receiver
What I'd like you to notice here is that each layer of the OSI Model prepens a new
header on the data, so data is encapsulated with the Layer7 header, Layer6 header
and so forth and so on through all the layers of the OSI Model
and is then transmitted across the wire. In a moment I'll demonstrate this
by using Wireshark which is a great sniffing application and Wireshark allows us
to view data in real time but please note that the sender is transmitting the data
the data is encapsulated at each layer of the OSI Model and is then transmitted
to the receiver. So A is sending the data to B, this is a very simple network
where I've got 2 PCs connected back to back with say a cross-over cable
but in the real world, the network between this devices could be really
complicated, device A may be in England and device B may be in the USA and
there's going to be multiple devices and multiple media types between the 2 PCs
when data is transmitted from A to B. But for now let's keep it simple
the data is transmitted across a single cable from A to B