Time
1 hour 53 minutes
Difficulty
Beginner
CEU/CPE
3

Video Description

In this section we'll begin to dissect the OSI (Open Systems Interconnect) reference model. As its name implies, it's an open standard maintained by the ISO (International Standards Organization) used to ensure interoperability of network devices. Such interoperability means that a wireless router from Linksys can be swapped out for similar products from the likes of Netgear or Edimax without missing a beat. As previously discussed, the various layers of the OSI model define specific functions handled exclusively at each layer. This specialization is referred to as "encapsulation." So much has to happen in order to transfer data, that no one device or protocol is expected to handle it all. We'll see how various network devices map to the individual OSI layers. The data payload size remains the same as it's passed down and up through the 7 layers of the reference model. Headers are added to the data payload as it gets passed up the layers. This packaging up of data is the encapsulation process. At the upper layers (5-7) the data payload plus headers is referred to as a PDU (protocol data unit). As we move down towards the physical layer, headers are stripped off during the sending process. We have a segment at the transport layer, packet at the network layer, frame at the datalink layer, and finally, bits at the physical layer. The unpackaging process is referred to as "decapsulation." It should be noted, that in terms of the CISSP exam, it's important to understand the security threats and specific attacks that exist at each layer. There are several nifty mnemonic devices for remembering each of the OSI layers. One of the more popular is "All People Seem To Need Data Processing" which translates to: Application, Presentation, Session, Transport, Network, Datalink, and Physical layers. Feel free to come up with one that works for you!

Video Transcription

00:04
Now we begin our discussions of Tellem communications and networking with looking at the S hi reference model. It really is a good place to start if we're gonna talk about the various elements of networking. So when you take a look at the O. S I reference model before we get too deep into it, let's talk about what it is and what it's about. Okay,
00:23
so it stands for open systems interconnect,
00:27
and it comes to us from the I s o the Internet National Organization of standards. Okay, so the S I comes from the I S O. But if international standards organization
00:40
open systems interconnect now, if you listen to those words, we know that this isn't a proprietary model, right? This is ah, model designed to promote interoperability among vendors. So when we talk about that, think about it. If we've got 15 computers connected together via Netgear switch
00:59
and that Netgear switch fails, can we run out by links his switch and connect everybody in resume communications?
01:06
Sure began right. That's because we have interoperability. We don't have a proprietary switch. In that scenario, we have a switch that does
01:15
what a switch does because it meets the standards and their standards air specified through the S I reference model. So it allows me is a vendor to build a product to a certain standard. And if I build to that standard than we know that that device will be interoperable with others, Okay. Now,
01:33
the other thing that I think is most useful about the S I model is the fact that
01:38
there is so much that has to happen in order to get data from my computer to yours. Whether you're across the room or across the world, no one device or protocol has to do it all. So, for instance, if you've ever heard that phrase Jack of all trades,
01:53
the rest of that is master of none. That's not good. I am really lousy at a whole bunch of stuff, right? So if you look at different devices, protocols are gonna look at a home, and I realized we're not using hopes today. But thank back to 10 years ago are, uh are about 10 years ago to when we were using cops.
02:14
The best thing about a hub
02:15
was it was treated.
02:16
That was the biggest reason I said Oh, I'm gonna put a hub here, right? She 40 bucks get a hub with many, many ports. All right, Now, um, what is a hub do? What type of addressing does a hub do?
02:30
None. Since all data out all ports all the time. What type of error? Detection and error correction does a Humpty none doesn't help. Reassemble or disassemble packets Know, does Ah, hub. Ah, do flow control? No, no, no. And if I made a hub that had to do all that stuff,
02:50
it would be a very, very expensive home.
02:53
And again, it would be that Jack of all trades, Master of none. So what we do is we have components that each layer of the O S I model and they do their job.
03:02
And if I do my job but layer one, I'm gonna count on those other devices that later 234567 to do their jobs. And completely throughout the s I model, we will do the things that it takes tohave the communication. All right, So what the S I model really does, is it?
03:21
Each layer kind of describes a set of functionality that happens.
03:24
It also describes a process called encapsulation. Now, if you set through the section on I p sec, we talked about what encapsulation is If you didn't sit through that section, encapsulation is essentially packaging and what you'll notice on it's a little bit easier to see on this slide
03:42
encapsulation at each layer of the O S. I model. So when somebody is sending this is the sending computer over here to the left. And as someone sends, whether they're making nature keep he requests, they're sending a message, whatever it is.
03:55
Um, so the message would start out or the data would start out at the application layer and it's sort of conceptual. Lee travels down the S I model so application, will your protocols have it first, and then it goes down and things happen That presentation session trains weren't so on,
04:13
but each layer as it travels down the S I model, there's header information added,
04:19
and you can see as we go down the S I model the data states the same side. But headers have been added each layer. Now that being said,
04:30
dependent upon the headers that have been placed on the data,
04:34
we can really refer to the data accurately by different names. So a really generic term for data
04:44
and its packaging, regardless of what that packaging is. It's a P D u A protocol data unit, and I'm just going back to the page before because this shows us that it at layer 76 and five. We still call it Dad,
04:58
right? We just still refer to it is data nothing really significant that has changed. That information has been added yet, but when we come down to the transport layer, it now becomes a segment
05:12
at layer three, a packet at layer two frames
05:16
and it layer one bits. And I know there are a couple of other names here, but the names I would prefer you to have segment packet frame bits, segment packet frame bits, the phrase some people fear birthdays s p f being segment, packet frame bits.
05:36
And that's again
05:38
the description of the protocol data unit, the P. D you, which is essentially the data plus whatever headers have been added.
05:47
All right, now you can see the OS I model is a seven layer model starting at the top, going down to the sending computer. The data goes down through the S I model at every layer. Information's added header informations happened and trailer information.
06:04
Now, as it gets the receiving computer,
06:08
the headers air stripped off in the opposite order. You know much like if I mail you a package and wrap it up and do all these things when you get it, you got the reverse order. So we call that de caps elation, and we also talk about this is a peer to peer network because what's added at the network layer
06:26
on the sending computer is needed by network layer devices on the receiving computer.
06:31
You absolutely, positively have no the seven layers of OS I by name as well as by number.
06:40
So you can remember that by all people seem to need data process. All people seem to need data processing
06:50
for Domino's pizza, and there are a 1,000,000 little pneumonic tricks out there.
06:56
If you already know one that works, go with that. My personal favorite from the bottom up. People don't need to see
07:04
Paula Abdul
07:08
make an argument on that for me.
07:10
I'm just throwing it out there,
07:12
okay? No, you're os I models by the layers know them by the numbers, they might say. What is the PT the PTU refer to at layer four? Well, that's the transport layer. So it would be referred to as a segment. And they'll also ask you about what happens at each of these layers
07:30
as well as network attacks, which is exactly what we're getting ready to cover
07:33
in the next lines.

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Instructed By

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Kelly Handerhan
Senior Instructor