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q Now that we've talked about the OSI Model and the TCP/IP Stack, let's but it all together and talk about how network communication actually works. Now, as we've talked about, data travels from the top of the OSI Model down if you're the source or the sending computer. The computer sends it through all the different layers of the computer and then when it comes to the receiving computer, it travels up the OSI Model from the bottom to the top on the receiving computer. Now, throughout this sending process, the data is encapsulated. That means that headers and footers are added to the data is it goes down. Now, as it travels up the protocol stack on the receiving computer, it's re-encapsulated, for lack of a better word. As we mentioned before, data is called different units as it travels from layer to layer. Whatever you call the data at the moment depends on which layer you're talking about. If you're talking about data at the transport layer, it's called a segment. If you're talking about data at the IP layer, it's called a packet. To try to explain it a little bit better, let's look at this diagram. We've got two computers on the left and right. The sending computer is the left-hand one. Now, as it sends data out, let's say there's a person sending an email going to a web page. Data goes to each layer on its way down. It's called data through the first three layers. Application talks to the presentation and sends data to the presentation layer. The presentation sends data to the session layer. Once it hits the transport layer, it's called segments and then it's transformed by adding headers on it and then it gets sent down to the network layer which adds more headers on it and the data as it comes from above the layer it's currently on is packaged and that header is tacked on to that data. So as the packets send it to the data link layer, it's called frames and at the data link layer, a frame header and footer is put on and then it's sent to the wire where it's converted to actual electrical signals, bits, ones and zeros. Now, of course throughout all this process what actually happens is it's being sent from the computer to various network devices, from the application on the computer to the network card, then from the network card across the cable to a switch, from the switch to a router. The router may be a long-haul communication line. So all of these things are happening as well throughout this process. Now when it goes and hits its final destination, the computer it's going to, it reverses this entire process. It's converted from bits on the wire from electrical signals to frames and frames it's converted up to the network layer to packets and as it goes up through these layers, these frames and these frame headers and footers and other headers are stripped off of the packet so that you get less and less data as you go up until you finally get to the application layer where you have the original data. Think of it like wrapping up an item and putting that item inside of a box and then putting that box inside of another box every time you give it to another person. And then when they turn around and give it to the receiver, they start taking each item out of the box so when it finally gets to the last person, it's the original data, the original item. So that's kind of how communication actually works in the context of the OSI Model and the TCP/IP Stack.