- [Instructor] The OSI Model has seven hierarchical layers
that were developed to enable different networks
to communicate reliably between disparate systems.
Since this video is centering upon all things CCNA
it's crucial for you to understand the OSI Model
as Cisco sees it, so that's how
I'll be presenting the seven layers to you.
Looking at this graphic, the OSI has seven different layers
divided into two groups.
The top three layers define how the applications
within the end stations will communicate
with each other as well with the users.
The bottom four layers define how the data
is transmitted end to end.
When looking at the graphic, understand that users
interact with the computer at the Application layer
and also that the upper layers are responsible
for applications communicating between hosts.
None of the upper layers knows anything
about the networking or network addresses
because that's the responsibility of the four bottom layers.
You can see that it's these four bottom layers
that define how data is transferred
to the physical media like through wire,
cable, fiber optics, switches, and routers.
These bottom layers also determine
how to rebuild a data stream from a transmitting host
to a destination host application.
The Transport layer segments and reassembles data
into a single data stream.
Services located at this layer take all the various data
received from the upper layer applications
and then combine it the same concise data stream.
These protocols provide end to end
data transport services and can establish
a logical connection between the sending host
and the destination host on a network.
We can see this layer has two well known protocols,
Transmission Control Protocol,
which is a connection-oriented protocol,
and UDP, User Datagram Protocol,
which is a connectionless layer protocol.
For reliable transport to occur
a device that wants to transmit must first establish
a connection-oriented communication session
with a remote device.
It's a peer system, known as a call setup
or three-way handshake.
Once this process is complete the data transfer occurs,
and when it's finished a call termination takes place
to tear down the virtual circuit.
This graphic depicts a typical reliable session
taking place between sending and receiving systems.
In it you can see that both the host's application programs
begin by notifying their individual operating systems
that it a connection is about to be initiated.
The two operating systems communicate by sending messages
over the network confirming that the transfer's approved
and that both sides are ready for it to take place.
After all of this required synchronization takes place,
a connection is fully established
and the data transfer begins.
The Network layer, or layer three,
manages device addressing, tracks the location of devices
on the network and determines the best way to move data.
This means that it's up to the Network layer to transport
traffic between devices that aren't locally attached.
Routers which are layer three devices,
are specified at this layer and provide
the routing services within an internetwork.
To define the Network layer simply,
it's logical addressing and routing through an internetwork.
Logical addressing using IP or IPv6.
Also at this layer we have ICMP
or Internet Control Message Protocol
which provides destination unreachable messages for example,
or gives us diagnostics with Ping and Telnet.
The Data Link layer provides for the physical
transmission of data and handles error notification,
network topology, and flow control.
This means that the Data Link layer will ensure
that messages are delivered to the proper device on a LAN
using hardware addresses or maybe you call them
MAC addresses or physical addresses, doesn't matter.
And will translate messages from the Network layer
into bits for the Physical layer to transmit.
Now the Data Link layer formats the messages
and we're going to call these, frames.
They add a customized header containing
the hardware destination and source address.
This added information forms a sort of capsule
that surrounds the original message in much the same way
that engines, navigational devices, and other tools
were attached to the lunar modules of the Apollo project.
And lastly, at the Physical layer, we define how our
networks are physically laid out.
For example, our star topology is the most common
physical topology today, and this is our ethernet networks
with LAN switches, where the cables come from all directions
and plug into a centralized device.
The Logical Topology is the signal path
through this physical topology, and this is defined
at the Data Link layer in the LAN driver.
So in today's network we have a physical star
and a logical bus.
So the signal runs from end to end
across our physical star topology.
In this section I discussed the OSI Model,
the seven layer model which is used to help
application developers to design applications
that can run in any type of system or network.
Each layer has a special job and selects responsibilities
within that model to ensure that solid,
effective communications do in fact occur.
Although this model is basically legacy
it still provides the foundation of what we need to know
for our internetworking.