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I remember hearing
an explanation

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of how the internet worked.

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It went something like this.

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The user, sitting
at their computer

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with their favorite
browser open,

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puts in the name of a website,
presses Enter, and then boom,

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magic happens.

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Well, of course, there's a lot
of additional detail and things

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that are going on to make that,
quote unquote, magic happen.

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And one way of helping to
understand conceptually

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the things that are done
is do some kind of a model

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as a reference point
or an idea regarding

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how something happens.

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And in the world of networking,
one of the very popular and old

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models is referred to as the
open system interconnection

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model, or for short,
the OSI model.

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And here in our infrastructure
of this network,

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we have some computers.

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Let's imagine that this
computer right here, Computer 1,

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is sitting at the top
of this logical stack.

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Or we could also
say this computer

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is at the top of all these
layers in this OSI reference

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model.

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And the seven layers of
the OSI reference model

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can represent logically some of
the processing and events that

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happen when the user on that
computer and their browser

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presses Enter,
including all the steps

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until the actual
signals are sent

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on the physical wires, or in
the case of a wireless device,

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across the wireless network.

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And the good news is this.

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We can use our story of
the tale of two kings,

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along with the king's
staff, as a way

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to understand some of the
functionality that might happen

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when a computer,
like Computer 1,

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is communicating with another
device on the network.

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So for our discussion,
let's go ahead

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and call Computer 1
King A. And King A is

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at the top of the pile here.

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And if this computer,
playing the role of King A,

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needs to access a file server--

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for example, maybe
this one right here--

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or a web server out
on the internet,

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or perhaps it needs
to talk to a printer,

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if those are all
network resources,

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it's the application
layer that's

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providing access, if you
will, to those resources.

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So just like the scribe
provided a writing service

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to write something
down for King A,

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the application layer in
this OSI reference model

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would make available the
printing service or the file

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service or the web
service, depending

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on the type of resource that
King A, or this computer,

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is trying to go after.

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And as part of that service,
if there needs to be some type

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of encryption or
compression or translation,

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it's the 007 feature--

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or in the OSI reference
model, referred to

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as the presentation layer--

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that's going to be logically
providing those functions.

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And then as we continue to
go down this logical model,

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the session layer is playing
the part of the lawyer.

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And then the OSI reference
model is the session layer

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acting as the lawyer
that's managing

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the affairs and the
communications for the king,

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for the application
that's being used.

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And we can consider
these top three

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layers of the OSI
reference model

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sort of like senior management.

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So if the session between
King A, this computer,

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and some other device is agreed
to and allowed to happen,

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the next logical component
is our middle manager,

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which in the OSI reference
model is the transport layer

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of the OSI reference model.

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So just like the middle manager
in our tale of two kings,

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the middle manager
is responsible

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for reliable or
unreliable delivery,

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depending on the requirements
of the application.

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And it could also be
responsible for taking

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fairly large messages and
chunking them up or chopping

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them up into smaller
pieces and then labeling

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those-- for example, 1 of
3 and 2 of 3 and 3 of 3.

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And then when the
middle manager is done,

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it's going to hand its messages
down to the mailroom, which

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is responsible for
addresses-- for example,

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the from address
and the to address.

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And most addresses have
a couple of components,

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including the street name
and the house number.

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And then the OSI reference
model is the network layer

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of the OSI reference model that
would be adding these address

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labels to each of the
messages that are going

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to be sent over the network.

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And after those network
labels are added,

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the messages are passed down to
be put in the proper envelopes.

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Because not every
network is the same.

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We have ethernet networks.

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We have wireless networks.

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We have serial networks.

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And so depending on the type
of medium that we're going over

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and the protocols
that are in use,

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it's layer 2, the
data link layer,

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that's responsible
for putting the data

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into the correct envelopes
or frames in preparation

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to be sent over the network.

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And once the envelope
stuffing is done at layer 2

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at the datalink
layer, then the bits

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are actually
transmitted and sent

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across the network at
the physical layer, which

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in our analogy of the two kings,
we use the FedEx truck doing

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the actual delivery.

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And there's a few questions that
often come up regarding the OSI

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reference model.

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One is do I need to
memorize all the layers.

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And I would say yes.

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It's going to be beneficial
to you to do that.

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And the second
question is, OK, what's

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the easy way to remember that.

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And a popular
option is to create

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a phrase that reminds us of
the first letters of each

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of the layers.

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And one of those is please do
not throw sausage pizza away.

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And just like when you
build a brick wall,

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those are in the order
from the ground up,

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from the physical
layer at layer 1

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all the way through the
application layer at layer 7.

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So it would serve us to take
a few minutes to practice that

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and write it down a few times.

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It would also be a great
idea to jot down next to them

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what each of those layers
logically can provide for us.

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And we can use the
story of the two kings

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with each of the
servants in the kingdoms

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to help us remember there
are logical functions.

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In this Nugget,
we've taken a look

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at the seven logical layers
in the OSI reference model.

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I hope this has been
informative for you,

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and I'd like to thank
you for viewing.

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