OSI Model Introduction and Encapsulation

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Time
7 hours 50 minutes
Difficulty
Beginner
CEU/CPE
8
Video Transcription
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>> Our next topic is going to be the OSI reference model.
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The OSI reference model
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provides us a framework so that we can
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understand the different elements of
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networking and how they work together.
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There are a couple of purposes with
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the OSI reference model to cover first.
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Then we'll get into exactly
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>> what it is and how it works.
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>> It's the idea of interoperability between vendors.
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What we want is an environment where
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different components from
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different vendors all work together.
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We don't want to have to buy everything
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on our network from a single vendor.
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We don't want vendors to have
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any monopoly of services because they can
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then charge whatever they want and we're
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locked into that specific vendor.
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Whereas if instead we have a variety of
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components and I'm connected to a Netgear switch,
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and that fails, I can go out and buy
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a Linksys switch that makes my life easier.
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We prefer standardization.
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Usually when we add vendors
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that have proprietary equipment,
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it may take additional training in
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order to work with those devices.
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There's usually additional costs
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and sometimes additional complexity.
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The OSI model promotes standardization.
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Ultimately, what we have is seven layers of
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the OSI model and standards at each layer.
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If I have a Layer 2 device and I built it to
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the standard of an OSI Layer 2 device,
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then it will be interoperable
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and a standard space environment.
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That's great for me as a vendor
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because standardization sells,
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it doesn't mean proprietary stuff doesn't sell,
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but standardization really is what
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most network administrators are looking for.
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There might be times when we purchase
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a proprietary device or component.
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But for the most part, we like standardization.
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Each layer of this OSI model is going to give us
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a description of certain services based on the standard.
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What we're also going to see with
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the OSI model is an idea of encapsulation.
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When we talk about encapsulation, it is packaging.
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It's placing headers around data,
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or it's putting a protocol in a protocol.
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The best example I can offer is this.
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Let's say my uncle Steve in Seattle has an anniversary.
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In order to help him celebrate,
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I'm going to send him some champagne glasses.
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Technically, I could write his name on
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the champagne glasses and
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say Happy Anniversary Uncle Steve.
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Put a stamp on them and put them in the mailbox.
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But of course, as soon as I do that,
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I'll hear clink, clink, clink.
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What I'm going to do instead is
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wrap the champagne glasses in bubble wrap,
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put them in a small box,
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and put bubble wrap around that box.
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I'm going to gift wrap the box and write
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happy anniversary or best wishes on the box.
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I do want to point out that when I
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write best wishes uncle Steve,
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that the uncle Steve piece is an addressing element.
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It's not worldwide addressing but
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when the package find Steve's house,
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he's going to see that it's for him.
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Some of what we're doing is for protection and padding,
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and some of it is for addressing.
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Ultimately, we start out with
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a very small payload, the champagne glasses.
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As we package and add
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addressing and protection and all those things,
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by the time we're ready to pass this off to FedEx,
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we wind up with a much more sizable package.
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We continue to add.
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We put in a FedEx box,
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we get a FedEx label we handed off to
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the FedEx carrier, that's encapsulation.
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It's not the same as encryption though,
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sometimes people make that mistake.
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I never encrypted the champagne glasses.
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I didn't magically turn them into coffee mugs.
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There still champagne glasses.
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They're just all wrapped up.
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That's our goal with encapsulation.
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Of course, the only piece that I missed in
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the instructions is that if you have pets,
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at some point in time,
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I can guarantee you there will be a cat in
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the packing peanuts or
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perhaps a pug wrapped up in bubble wrap.
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If you need to [inaudible] the animals,
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you can add those steps in here.
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I do it want to mention no animals were
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harmed during the making of this video.
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This encapsulation idea's illustrated here on the right.
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At the top, we have seven layers.
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Application, down a presentation, session,
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transport, network, data link, and finally physical.
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The idea here is data conceptually
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travels down through the OSI model.
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When I say that, what I mean is that
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the sending computer starts with
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an application layer protocol.
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Application layer protocols are
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those protocols that drive user application.
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If you think about a web browser, you got HTTP.
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If you think about mail, you've got POP3
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or IMAP or some of those other mail protocols.
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Data starts at the very top of the OSI model,
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and as it conceptually travels down, headers are added.
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You can see we start with the data
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alone at the application layer.
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But as we move down to presentation,
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we get a presentation header,
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a section header next, a transport header,
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then network cutter, data link cutter,
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all the way down to the point where
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we have the conversion in
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the bits that relay electronic signal on the network.
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At each layer, something is added that will
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help the data get to where it needs to go.
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All the way to the destination host.
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There's formatting,
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there's perhaps receipt of acknowledgments.
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There's IP addressing, there is MAC addressing,
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and all these things we'll talk about as we move on.
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But I just want you to have this idea that at each layer,
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something gets added until the computer is
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ready to put that data out on the network.
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Now, I would definitely
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recommend that you know the OSI model
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top down and know it by number and by name.
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All people seem to need Domino's Pizza is
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a good little trick that a lot of
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folks use from the top down.
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All people seem to need Domino's Pizza.
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That will help you remember.
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My personal favorite from the bottom up,
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people don't need to see Paula Abdul.
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Change my mind, I'm standing by that.
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But at any rate, find some a little trick.
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I do not recommend that you go out and Google
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mnemonic tricks for the OSI reference model.
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There are some things you cannot unsee and
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there are people of questionable intent on the Internet.
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That's all I'm going to say about that.
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What I've probably just done is guaranteed that 2,200
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people go on the Internet and Google
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in mnemonic tricks for the OSI model.
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But at any rate, find some trick
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that helps you remember it and go with that.
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The other thing I asked you to think about is
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that at each layer of the OSI model,
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we refer to data a little bit
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differently based on the headers.
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When data is at
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the application presentation session layer,
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we still refer to it as data but
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once it comes down to the transport layer,
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we have a segment down to the network layer.
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We have a packet down to the data link layer,
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we have a frame, and at the physical layer,
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it's converted into bits.
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Segment, packet, frame, bits.
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Sometimes you'll hear these terms used
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interchangeably and that's okay.
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But I just want you to know
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that really these terms should
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vary based on the headers
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that have been added to the data.
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A packet has more headers than a segment.
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A frame has more headers than a packet,
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and so specifically at the transport layer,
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there is specific transport information
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that's added to the header.
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Next, at the network layer,
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specific information is added.
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Then at the data link layer,
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specific information is added.
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This gives you an idea of
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the broad purpose of the OSI model.
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Just keep in mind, it's a seven-layer model.
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It's designed to promote interoperability
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>> among vendors.
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>> There are standards at each of the layers,
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and really what it does is it helps us describe
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the process of encapsulation and
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the adding of headers so
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that data can get from destination
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to destination with the correct information.
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