I remember hearing an explanation
of how the internet worked.
It went something like this.
The user, sitting at their computer
with their favorite browser open,
puts in the name of a website, presses Enter, and then boom,
magic happens.
Well, of course, there's a lot of additional detail and things
that are going on to make that, quote unquote, magic happen.
And one way of helping to understand conceptually
the things that are done is do some kind of a model
as a reference point or an idea regarding
how something happens.
And in the world of networking, one of the very popular and old
models is referred to as the open system interconnection
model, or for short, the OSI model.
And here in our infrastructure of this network,
we have some computers.
Let's imagine that this computer right here, Computer 1,
is sitting at the top of this logical stack.
Or we could also say this computer
is at the top of all these layers in this OSI reference
model.
And the seven layers of the OSI reference model
can represent logically some of the processing and events that
happen when the user on that computer and their browser
presses Enter, including all the steps
until the actual signals are sent
on the physical wires, or in the case of a wireless device,
across the wireless network.
And the good news is this.
We can use our story of the tale of two kings,
along with the king's staff, as a way
to understand some of the functionality that might happen
when a computer, like Computer 1,
is communicating with another device on the network.
So for our discussion, let's go ahead
and call Computer 1 King A. And King A is
at the top of the pile here.
And if this computer, playing the role of King A,
needs to access a file server--
for example, maybe this one right here--
or a web server out on the internet,
or perhaps it needs to talk to a printer,
if those are all network resources,
it's the application layer that's
providing access, if you will, to those resources.
So just like the scribe provided a writing service
to write something down for King A,
the application layer in this OSI reference model
would make available the printing service or the file
service or the web service, depending
on the type of resource that King A, or this computer,
is trying to go after.
And as part of that service, if there needs to be some type
of encryption or compression or translation,
it's the 007 feature--
or in the OSI reference model, referred to
as the presentation layer--
that's going to be logically providing those functions.
And then as we continue to go down this logical model,
the session layer is playing the part of the lawyer.
And then the OSI reference model is the session layer
acting as the lawyer that's managing
the affairs and the communications for the king,
for the application that's being used.
And we can consider these top three
layers of the OSI reference model
sort of like senior management.
So if the session between King A, this computer,
and some other device is agreed to and allowed to happen,
the next logical component is our middle manager,
which in the OSI reference model is the transport layer
of the OSI reference model.
So just like the middle manager in our tale of two kings,
the middle manager is responsible
for reliable or unreliable delivery,
depending on the requirements of the application.
And it could also be responsible for taking
fairly large messages and chunking them up or chopping
them up into smaller pieces and then labeling
those-- for example, 1 of 3 and 2 of 3 and 3 of 3.
And then when the middle manager is done,
it's going to hand its messages down to the mailroom, which
is responsible for addresses-- for example,
the from address and the to address.
And most addresses have a couple of components,
including the street name and the house number.
And then the OSI reference model is the network layer
of the OSI reference model that would be adding these address
labels to each of the messages that are going
to be sent over the network.
And after those network labels are added,
the messages are passed down to be put in the proper envelopes.
Because not every network is the same.
We have ethernet networks.
We have wireless networks.
We have serial networks.
And so depending on the type of medium that we're going over
and the protocols that are in use,
it's layer 2, the data link layer,
that's responsible for putting the data
into the correct envelopes or frames in preparation
to be sent over the network.
And once the envelope stuffing is done at layer 2
at the datalink layer, then the bits
are actually transmitted and sent
across the network at the physical layer, which
in our analogy of the two kings, we use the FedEx truck doing
the actual delivery.
And there's a few questions that often come up regarding the OSI
reference model.
One is do I need to memorize all the layers.
And I would say yes.
It's going to be beneficial to you to do that.
And the second question is, OK, what's
the easy way to remember that.
And a popular option is to create
a phrase that reminds us of the first letters of each
of the layers.
And one of those is please do not throw sausage pizza away.
And just like when you build a brick wall,
those are in the order from the ground up,
from the physical layer at layer 1
all the way through the application layer at layer 7.
So it would serve us to take a few minutes to practice that
and write it down a few times.
It would also be a great idea to jot down next to them
what each of those layers logically can provide for us.
And we can use the story of the two kings
with each of the servants in the kingdoms
to help us remember there are logical functions.
In this Nugget, we've taken a look
at the seven logical layers in the OSI reference model.
I hope this has been informative for you,
and I'd like to thank you for viewing.