WEBVTT

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In this video, we're going to continue the discussion of the OSI Model

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by looking at traffic flows between hosts. I want to show you a practical demonstration

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of the OSI Model by using Wireshark to capture traffic between devices

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Now when communication takes place between 2 hosts so in this example we have

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HostA talking to HostB, the information that's going to be transmitted across the

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network must undergo a process of conversion at both the transmitting device

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as well as the receiving device, this is commonly known as encapsulation

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and de-encapsulation and I'm going to cover this in more detail in the next

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few minutes. At the end of this video you should be comfortable with the

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fact that each layer of the OSI Model only talks to the corresponding layer

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of the communication partner, in other words, Layer7 communicates with Layer7

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Layer1 communicates with Layer1 and so forth and so on

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So let's look at that in more detail

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So when the sender wants to transmit data, let's say in this example

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your PC is opening up a Web connection to Cisco.com and the PC is sending

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some information, that information has to go through the OSI Model from Layer7

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all the way down to Layer1, to then be transmitted across the physical media

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So in this example user data is transmitted, so at Layer7 of the OSI Model

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the data is encapsulated with the Layer7 header, that in turn is encapsulated

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with the Layer6 header at the Presentation Layer and that in turn is encapsulated

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with the Layer5 header at the Session Layer, which in turn is encapsulated

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with the Layer4 header at the Transport Layer and this continues all the way

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down, at Layer3 the data is encapsulated with the Network Layer header, at Layer2

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the data is encapsulated with the Data Link Layer header and in this case a

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frame check sequence is added to the data.The frame check sequence ensures

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that the data is not corrupted when sent from the sender to the receiver

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Lastly, the data is transmitted across the wire or physical media as bits

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in other words, 0s and 1s and that data is then sent from the sender to the receiver

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What I'd like you to notice here is that each layer of the OSI Model prepens a new

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header on the data, so data is encapsulated with the Layer7 header, Layer6 header

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and so forth and so on through all the layers of the OSI Model

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and is then transmitted across the wire. In a moment I'll demonstrate this

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by using Wireshark which is a great sniffing application and Wireshark allows us

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to view data in real time but please note that the sender is transmitting the data

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the data is encapsulated at each layer of the OSI Model and is then transmitted

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to the receiver. So A is sending the data to B, this is a very simple network

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where I've got 2 PCs connected back to back with say a cross-over cable

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but in the real world, the network between this devices could be really

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complicated, device A may be in England and device B may be in the USA and

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there's going to be multiple devices and multiple media types between the 2 PCs

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when data is transmitted from A to B. But for now let's keep it simple

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the data is transmitted across a single cable from A to B