- [Narrator] Now let's talk about data encapsulation.
When a host transmits data across a network
to another device,
the data goes through a process called encapsulation,
and is wrapped with protocol information
at each layer of the OSI Model.
Each layer communicates only with its pair layer,
on the receiving device.
To communicate and exchange information,
each layer uses what's called Protocol Data Units,
which is just a name for the data at each layer.
These hold the control information
that's attached to the data at each layer of the model.
There usually attached in front of the data field,
but can be at the trailer, for example, with a frame.
So we attach to the data
by encapsulating on each layer of the OSI Model,
and each has a specific name,
depending on the information provided in the header.
In this graphic here, we can see the PDUs,
and how they attach control information to each layer.
In this graphic here,
we can see the transport layer PDU name is segment.
The network layer is packet,
data link is frame and physical layer is bits.
So after transmitting the device,
the data encapsulation method works like this.
We have a data stream coming down,
handed down to the transport layer.
So user information is converted to data
at the application layer, for transmission on the network.
Data is converted to segments at the transport layer,
and a reliable connection is set up
between the transmitting and receiving hosts.
Segments are converted to packets, or data grams,
and a logical address is placed in the header,
so each packet can be routed to an inter network.
The logical address would either be IPV4 or IPV6,
also referred to as a routed address.
Packets, or data grams, are then converted to frames,
for transmission on the local network,
such as an Ethernet or wireless, or fiber network.
Hardware Ethernet addresses
are used with our Ethernet and wireless networks.
They're used to uniquely identify hosts
on the local network segment.
Frames are then converted to bits,
and a digital encoding and clocking scheme is used.
Before we go further in our discussion,
in this graphic, let's discuss port numbers,
and make sure you understand them.
The transport layer uses port numbers
to define both the virtual circuit,
and the upper layer processes.
When using a connection oriented protocol like TCP,
the transport layer takes the data string,
makes segments out of it,
and establishes a reliable session,
by creating the virtual circuit.
It then sequences, meaning numbers,
each segment and uses acknowledgement and flow control.
If you're using TCP,
the virtual circuit is defined by the source,
and destination port number,
plus the source and destination IP address,
and together, this is called a socket.
Understand that the host just makes this up,
starting at port number 1024,
because zero through 1023 are reserved
for well known port numbers.
Most of us were exposed to hierarchy early in life.
Anyone with older siblings
learned what it was like to be at the bottom of a hierarchy.
Regardless of where you first discovered that concept,
most of us experience it in many aspects of our lives.
It's hierarchy that helps us understand where things belong,
how things fit together, and what functions go where.
It brings order to otherwise complex models.
If you want a pay raise, for instance,
hierarchy dictates that you ask your boss,
not your subordinate,
because that's the person who's role it is to grant
or deny your request.
So basically, understanding this helps us discern
where we should go, to get what we need.
Hierarchy has many of the same benefits in network design
that it does in other areas of life.
When used properly, it makes networks more predictable,
it helps us define which areas
should perform certain functions.
Likewise, you can use tools such as access lists,
at certain levels in hierarchical networks,
and avoid them in others.
The Cisco Three-Layer Model helps us design,
implement, and maintain a scalable, reliable,
cost-effective hierarchical inter network.
We have the core, distribution, and access layers.
And let's take a look at each.
The access layer provides endpoints and users
with direct access to the network.
This gives us device connectivity, port security,
but not access control lists.
Voice and video support,
including power over Ethernet ports.
The distribution layer aggregates access layers and
provides connectivity to services.
This is where we have routing, packet manipulation,
access lists with security, and scalability.
The core layer provides connectivity
between distribution layers for large LAN environments.
The core layer provides 24-hour connectivity,
with high-speed forwarding
between different distribution models.
You should implement as few policies as possible here,
no access control lists, and try to use layer two.
The core layer of the LAN is the most critical part
of the scalable network,
so you want data to transmit as fast as possible.
The distribution layer provides fault domains,
and the core represents 24-hour connectivity between them,
but you don't have to have a core layer.
Depending how your network plays out,
we can use a core distribution and access
in large enterprises, or just a large distribution,
connecting your access layers together.