In this video we're going to continue the discussion of the oversized model by looking at traffic flows
between hosts.
I want to show you a practical demonstration of the OSA model by using wire shock to capture traffic
between devices.
Now when communication takes place between two hosts.
So in this example we have a host a talking to host b 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 d 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 OS I model only
talks to the corresponding layer of the communication partner.
In other words a Layer 7 communicates with Layer 7 layer one communicates with Layer 1 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 P.C. is opening up a web connection
to Cisco dot com and the P.C. is sending some information that information has to go through the OS
model from Layer 7 all the way down to layer one to then be transmitted across the physical media.
So in this example user data is transmitted so at a Layer 7 of the OS I model the data is encapsulated
with the Layer 7 header that in turn is encapsulated with the layer 6 header at the presentation layer
and that in turn is encapsulated with a layer five header at the session layer which in turn is encapsulated
with a layer for header at the transport layer and this continues all the way down at Layer 3 the data
is encapsulated with a network layer header at layer to the data is encapsulated with a 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 zeros and ones 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 OS model pre pens a new header on the data
so data is encapsulated with a layer 7 header layer 6 header and so forth and so on through all the
layers of the OS I model and is then transmitted across the wire.
In a moment I'll demonstrate this by using wire shock which is a great sniffing application and why
shock allows us to view data in real time.
But please note that the senders transmitting the data the data is encapsulated at each layer of the
OS our model and is then transmitted to the receiver.
So you're sending the data to be.
This is a very simple network way I've got two pieces connected back to back with say a crossover cable
but in the real world the network between these devices could be really complicated device a maybe in
England and device B maybe in the USA and there's going to be multiple devices and multiple media types
between the two pieces 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.