Welcome to the ITFreeTraining video on the OSI Model. This video will look at the OSI
model and in particular how it relates to the internet protocol.
The OSI model was first developed in the 1980’s. It is a conceptual model aimed at standardizing
network communication. The model was not implemented in its entirety, but it is still referenced today/
As shown, the OSI model has 7 different layers. Each layer is directly linked to the layer
above and below it if a layer exists. As data travels down the OSI model, each layer adds
additional information. This continues until it reaches the last layer. It is then transmitted
across the network. Once the other side receives the data, the process is reversed. You can
see how each layer requires the layer or layers adjacent to it. Also it is not possible to
skip a layer. All communication either starts at the top layer or is received at the bottom layer.
So what does each layer of the OSI model do? At the top, is the application layer. This
layer provides the application programming interface.
The next layer, presentation, is responsible for data formatting. This includes encoding,
encryption and compression.
The next layer, session, is responsible for connecting and reconnection of the network
connection if it is lost. This also includes authentication of the connection.
The next layer, transport, manages packets size and the resending of data if it was lost
or corrupt. The following layer, network, routes data between different nodes in the
network.
The second last layer, data link, is responsible for flow and error control. For example, ensuring
more data is not sent out then the local network can handle.
The last layer, physical, communicates directly with the network and is responsible for transmitting
data on the network.
As stated, the OSI model is a conceptual model and not directly implemented in modern networking,
but is referenced from time to time.
To demonstrate this, I will now compare it to the TCP/IP model. TCP stands for Transmission
Control Protocol and IP as we know stand for Internet Protocol. A number of protocols are
used to make internet communication possible. These protocols are grouped together under
the name TCP/IP. Nowadays you don’t hear this name so much, generally you will jsut hear
just IPv4 or IPv6 but it is still valid.
As you can see, TCP/IP has 4 different layers as opposed to the 7 layers of the OSI model.
It is shown as an exact fit, however in reality it does not fit the original OSI model exactly.
The application layer maps to the 1st 3rd layers of the OSI model. Transport is a direct
1 to 1 mapping. The internet layer maps directly to the network layer and the last two layers
of the OSI model map to the network access layer.
Even though there is only 4 layers, each layer can be broken down further. As seen here,
at the application layer is a number of different protocols. You can see how software will generally
use protocols to access services on the internet.
The protocols themselves will access the transport layer. At this layer there is a choice of
TCP and UDP. TCP stands for Transmission Control Protocol. TCP gives reliable transmissions.
If a packet is lost in transmission, the protocol will notice it has not arrived and request
a resend.
In contrast, UDP, which stands for User Datagram Protocol, is unreliable transmission. If a
packet goes missing in transit, it is not resent. Think of UDP like mailing a letter.
Once you mail the letter you have no idea if it will arrive at its destination.
The next layer determines if IPv4 or IPv6 is used. At the bottom is the device driver
which communicates with the network. The most common type is Ethernet, however any other
network device is supported.
You can see how TCP/IP is very modular. This makes it easy to add additional protocols
and add additional hardware when they are developed. I will now have a look at how the
OSI model may get referenced today.
The first device I will look at is a layer 1 device. Remember that the OSI model refers
to the first layer as physical and controls the transmissions at the bit level. Examples
of layer 1 devices are repeaters and media convertors.
A repeater essentially extends the limits of a network by boosting or recreating the
network signal. A media convertor like the one shown, convertors a particular network
type to a different type. In this case, the media convertor converts a fibre signal to
an Ethernet signal. Layer 1 devices are very simple devices.
The next layer I will look at is the data link layer. This performs flow and error control
on the physical link. Examples of these are switches and bridges. Switches like the one
shown, allow multiple devices to be connected together on the same network. They differ
from layer 1 devices in that they can perform some management of that data that goes through
them. For example, if a packet is sent to the switch, the packet will only be sent to
the port that contains the device that packet is destined for. Layer 1 devices in contrast
simply repeat the data that comes in from one port and replicate it out the other port.
A bridge is a level 2 device that connects two networks together. For example, connects
wireless to wired network. It is different from a media convertor in that it inspects
data before sending it. For example, data that does not need to be sent to the wireless
won’t be. In contrast, a layer 1 device will repeats everything that is transmitted
to it.
Next I will look at layer 3 devices. These devices work on the network layer of the OSI
model. Layer 3 devices have the ability to route traffic between networks. So routers
are considered layer 3 devices. Also, certain switches have the ability to support routing.
Shown here is an example of such a switch. It looks just like a regular switch except
it has routing ability. The advantage of using such a device is since the routing is performed
in hardware it is quite fast.
The last layers that I will look at are 4 to 7. Usually hardware hardware manufacturers don’t
sell devices marketed under these layers. Which device fits in which layer tends to
get a bit blurry. Devices that are considered layer 4 switching, tend to be layer 3 switching
devices with more features. The term layer 4 switching does not tend to get used that
much. Essentially it means that the devices can route traffic based on what type of traffic
it is. In many cases this is used to prioritize certain traffic over other traffic. For example,
routing web traffic before video traffic.
Layer 7 devices tend to be devices that can inspect protocols. For example, they can inspect
http. For example, some firewalls are considered layer 7 devices since they can inspect packets
at the protocol level for certain types of attacks. Most applications work at layer 7,
for example an e-mail server. As I said, devices don’t tend to be marketed using these levels
so it is more having an understanding of which level the device may fit best.
That covers it for this video of the OSI and TCP models. I hope you have found this video
useful. For more videos please see our YouTube channel or web page. Until the next video,
thanks for watching.