(upbeat music)
- Networks have many pieces in parts.
So it can sometimes be difficult for network professionals
to visualize how we new piece of equipment or protocol
fits into the overall network.
Fortunately, we have a set of categories.
Each representing a different network function.
And this set of categories is called the OSI model.
Where OSI stands for Open Systems Interconnection.
And once we learn what sorts of things happen
in each layer of this model,
that's going to let us better categorize our hardware
and software components
and better visualize how they work together.
So let's take a look at these layers one at a time,
beginning with layer one.
The physical layer.
It's here at the physical layer
where we have things like network cabling
or radio waves being sent from wireless access points.
And the name we give data down here at the physical layer
is bits.
At layer two, we have the data link layer.
This layer is concerned with physical addressing.
Remember our discussion about ethernet switches
and how they make decisions based on
Mac address information.
Well a Mac address, that's a physical address
burned into the network device.
And since switches to make their decisions based on
those Mac addresses,
we say that our ether net switches operate
here at layer two.
The data link layer.
And the name we give data at layer two is frames.
Moving up to layer three we have the network layer.
This layer is concerned with logical addressing
such as IP addressing.
So we say that routers operate here
at layer three of the OSI model.
And the name we give data at this layer is packets.
At layer four, the transport layer.
Which is concerned with the logical connections.
Now these connections, they might be considered reliable
where we confirm that transmitted information was received.
Or these connections could be considered under reliable,
where we send our data and hope
that it reaches its destination.
And two of the main protocols that operate here
at layer four,
are Transmission Control Protocol or TCP for short
and User Datagram Protocol or UDP for short.
Now, TCP that's considered to be one of those reliable
transport protocols.
While UDP is considered to be unreliable.
And here at layer four, we call data segments.
Now the remaining layers, five through seven.
They don't have any special name for data,
but layer five is the session layer.
And it's concerned with establishing, monitoring
and then tearing down communication sessions
between our hosts.
And you might hear the term API's.
And that stands for Application Programming Interfaces.
And API's let one's piece of software on one device
talk to another piece of software on another device.
And those API sessions they give us one example
of something we might find here at layer five.
Layer six is the presentation layer.
And it's all about how we represent data.
We might encrypt data for security reasons.
So some of our encryption protocols,
they live here at layer six.
We might format a picture as a JPEG image.
Well, that's a format of how that picture is represented.
So we say JPEG formatting lives at layer six.
And finally layer seven is the application layer.
And it's the layer users that typically interact with.
If we're surfing the web, we're using a protocol like HTTP.
Which stands for Hypertext Transfer Protocol.
Or HTTPS which is short for secure HTTP.
If we know the name of a website but not its IP address,
what we can do is ask a DNS server.
And that DNS server will take the name that we provide
and it's going to give us back the IP address
corresponding to that name.
And the protocol that makes that possible
is Domain Name System or DNS.
DNS is an example of something that lives here
at layer seven.
Now let's check out some memory aids
to help keep all of these layers straight in our mind.
We can memorize these layers from the bottom up
using the acrostic,
Please Do Not Throw Sausage Pizza Away.
Where the P in please reminds us of the P in physical,
the D and don't reminds us of the D in Datalink and so on.
Alternately, we can go from the top down.
We can memorize, All People Seem To Need Data Processing.
And the names that we gave our data
at the bottom four layers.
Those names are called Protocol Data Units or PDU's.
And a way to remember those different
Protocol Data Unit names from the bottom up.
Is to remember the acrostic,
Bacon Frying Produces Salivation.
Which is standing for bits, frames, packets and segments.
And the best advice I've ever heard about the OSI model,
is to remember it's a reference model.
It's not a reverence model.
It doesn't have to be revered
as something that every network component
must neatly fit into.
That's just not reality.
However, this model can be a tool we can use
to better explain and understand
where different network components reside.
(upbeat music)