WEBVTT

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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.