Aussie Open Systems interconnection.
Is a reference model for how applications can communicate over a network.
A reference model is a conceptual framework for understanding relationships.
The purpose of the OSI reference model is to guide vendors and developers so that the digital communication
products and software programs they create will interoperate and to facilitate clear comparisons among
communications tools.
Most vendors involved in the telecommunications make an attempt to describe their products and services
in relation to the OSI model and although useful for guiding discussion and evaluation.
OSI is rarely actually implemented as few network products or standard tools keep all related functions
together in well defined layers as related to the model developed by representatives of major computer
and telecommunication companies beginning in 1983.
OSI was originally intended to be a detailed specification of actual interfaces.
Instead, the committee decided to establish a common reference model for which others could then develop
detailed interfaces, which in turn could become standards.
OSI was officially adopted as the international standard by the International Organization of Standards
ISO.
So what about the ozone layers?
The main concept of ozone is that the process of communication between two end points in a telecommunication
network can be divided into seven distinct groups of related functions or layers.
Each communicating user or program is at a computer that can provide those seven layers of function.
So in a given message between users, there will be a flow of data down through the layers in the source
computer across the network and then up through the layers in the receiving computer.
The seven layers of function are provided by a combination of applications operating systems, network
card device drivers and networking hardware that enable a system to put a signal on a network cable
or out over a Wi-Fi or any other wireless protocol.
The seven Open Systems interconnection layers are.
Layer seven, the application layer.
This is a layer at which communication partners are identified.
Is there someone to talk to?
Network capacity is assessed.
Will the network let me talk to them right now and.
They create a thing to send or opens the thing received.
Please note that this layer is not the application itself.
It is the set of services an application should be able to make use of directly.
Although some applications may perform application layer functions.
Layer six, the presentation layer.
This layer is usually part of an operating system and converts incoming and outgoing data from one presentation
format to another.
For example, from cleartext to encrypted text at one end and back to clear text at the other.
Layer five, the session layer.
This layer sets up coordinates and terminates conversations.
Services include authentication and reconnection.
After an interruption on the Internet Transmission control protocol, TCP and User Datagram Protocol
UDP provide these services for most applications.
Layer four, the transport layer.
This layer manages packetization of data.
Then the delivery of the packets, including checking for errors in the data once it arrives.
On the Internet, TCP and UDP provide these services for most applications as well.
Layer three, the network layer.
This layer handles the addressing and routing of the data, sending it in the right direction to the
right destination on outgoing transmissions and receiving incoming transmissions at the packet level.
IP is a network layer for the internet.
Layer two, The data link layer.
This layer sets up links across the physical network putting packets into network frames.
This layer has two sublayers.
The logical link control layer and the media access control layer.
Ethernet is the main data link layer in use.
Layer one, the physical layer.
This layer conveys the bitstream through the network at the electrical, optical or radio level.
It provides the hardware means of sending and receiving data on a carrier network.
Let's see some of the most famous protocols of the layers.
For ease of use.
It's better to talk about physical layer and data link layer together.
Here we have some protocols and data link layer and some physical media and connection methodologies
in physical layer.
Ethernet and 802.11 Wireless LAN are the most known protocols of the data link layer.
Ethernet is the name of the most commonly used network protocol that controls how data is transmitted
over a LAN, which is a local area network.
You need to have network interface cards in the devices that you want to connect to the network.
A wireless local area network.
W Wlan is a wireless computer network protocol that links two or more devices using wireless communication
within a limited area such as a home school, computer laboratory or office building.
This gives users the ability to move around with a local coverage area and yet still be connected to
the network.
Most modern LANs are based on I triple IEEE 802.11 standards and are marketed under the Wi-Fi brand
name.
IP Internet protocol is responsible for addressing hosts, encapsulating data into transferred packets
and routing packets from a source host to a destination host across one or more IP networks.
The best known transport protocol is the transmission control protocol or TCP.
It's used for connection oriented transmissions, whereas the Connectionless user datagram protocol.
UDP is used for simpler messaging transmissions.
We're going to talk more in depth about these protocols in the next lecture.
Again, for ease of use, the last three layers session layer, presentation, layer and application
layer are thought of together as the application layer.
Let me put another parenthesis here.
We're talking about the OSI reference model here.
In addition, there is another reference model called TCP IP reference model.
And instead of OSI models last three layers, there is only a single application layer in the TCP IP
reference model.
Just keep that in the back of your mind.
And back to our subject.
The application layer.
Protocols are classified according to the protocol they are using in the transport layer, and these
protocols interact with the end user via applications.
Therefore, they are the most known protocols by just about everybody.
Some of the most well known TCP based application layer protocols are.
Http hypertext transfer protocol.
Simply a communications protocol used to send and receive web pages and files on the Internet.
Telnet is one of the simplest ways to exchange data between two computers.
It allows two computers anywhere on a computer network, including the worldwide Internet, to exchange
text and, well, other data in real time.
FTP file Transfer protocol is a communication protocol for the rapid simple transmission of files across
a network.
Smtp simple mail transfer protocol is used to send and relay an email message between email servers.
Note that it is not used to retrieve email messages from a server.
Instead, either iMap or Pop is used to retrieve email messages.
DNS.
The domain name system is a system used to convert a computer's hostname into an IP address on the Internet.
For example, if a computer needs to communicate with a web server nhs.uk, your computer needs the
IP address of the web server.
Nhs.uk.
It is the job of the DNS to convert the hostname to the IP address of the web server.
The DNS uses both the UDP and TCP.
SNMp Simple Network Management Protocol is used in network management systems to monitor status of devices
and also spot problems.
So let's see what happens to a packet inside the network traffic.
Please note the data unit transferred between two endpoints has different names in each layer.
In layers five, six and seven application layers.
It's called data in layer four transport layer.
It's called segment for TCP and Datagram for UDP in layer three, the network layer, it's called packet
in layer two, the data link layer, the data unit is called frame.
Now I usually use packet for the transfer data unit in each layer to make it less complicated for you.
Make sense?
Before looking at the road trip of a DNS query in detail, let's look at the relationship between the
OSI layers and computer systems.
The packets and layer one and layer two are managed by network interfaces of your device, for example,
by your Ethernet card.
And the packets from 3 to 7 are managed by the processing unit in software detail.
The layer three and layer four packets are managed by the operating system of your devices, and the
packets of layer 5 to 7 are managed by the related application or service, for example, a web browser.
When you write a URL in the address bar of your browser and hit enter.
The first thing it sends is a DNS query.
A DNS query is the process of a computer networking device making an inquiry to get an IP address for
a DNS name such as Mail Yahoo.com.
The client computer will send a DNS query to one of their Internet service providers DNS servers.
The DNS server looks in its DNS database to tell whether it can answer the query authoritatively.
If the DNS server can answer authoritatively, the DNS server answers the query and the DNS query process
is complete.
So let's see the road trip of a DNS query from your computer to a DNS server.
When data is transmitted by the source toward a specific destination, it passes through the application
presentation and session layers, and the protocol data unit arrives at the transport layer.
Layer four.
Layers five, six and seven are displayed as a single layer.
To simplify the presentation at this layer, a 20 byte data header is placed in front of the data.
A DNS query can also use TCP, but let's assume that it uses UDP at this time.
The data and the layer four header here, it's UDP header which together form a segment or datagram
is passed down to layer three.
The network layer.
The network layer places its layer three header here, the IP header in front of the received segment,
and this group becomes a packet.
The layer three header contains important fields such as the logical address, the IP address of both
the source and the destination device.
The newly formed packet is then passed down to layer two.
The data link layer creates a new data unit called a Frame by adding the layer two frame header, which
is the Ethernet header here.
Like Layer three.
An addressing structure is also applied in the layer two header.
That is the Mac address.
The frame is then passed down to the physical layer.
Which converts the information into zero and one bits that are sent over the physical media using electrical
signals on a copper link, for instance.
Finally, the data is sent over the wire, using a wide variety of methods such as Ethernet or token
ring.
The headers are a specific form of control information that allows the data to go through the network
properly.
Thus, the data at each layer is encapsulated in the information appropriate for the specific layer,
including addressing and error checking.
The overall size of the information increases as the data travels through the lower layers from layer
four to layer one.
The destination device, which is the DNS server here, receives the data and this additional information
is analyzed and then is removed as the data passes through the higher layers up to the application layer
where the data is capsulated.