As I 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 U.S. 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 OWI model.
And although useful for guiding discussion and evaluation, OAC 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, OAC 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.
As I was officially adopted as the international standard by the International Organization of Standards
ISO.
So what about the OK as I layard's?
The main concept of Ossi 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 program is at a computer that can provide those seven layers a 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 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.
That creates 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 and applications
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 clear text to
encrypted text at one end and back to clear text to 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 Data Graem Protocol UDP provide these services for most applications.
Layer for the transport layer, this layer manages packetized of data, then the delivery of the packet,
including checking for errors and 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 to that datalink layer.
This layer sets up links across the physical network, putting packets into network frames, this layer
has two sub layers the logical link control layer and the media access control layer.
Ethernet is the main data link layer and 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.
Reserve use, it's better to talk about physical layer and data link layer together.
Here we have some protocols and datalink layer and some physical media and connection methodologies
in physical layer Ethernet and Eddo two one one wireless lan are the most known protocols of the datalink
layer.
Ethernet is the name of the most commonly used network protocol that controls how data is transmitted
over a land which is a local area network.
You need to have network interface card in the devices that you want to connect to the network.
A wireless local area network LAN 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 plans are based on, I believe, two point one one standard and are marketed under the Wi-Fi
brand name.
IP Internet Protocol is responsible for addressing host, 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 connection less user data Graham 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 ISI reference model here.
In addition, there is another reference model called TCP IP reference model.
And instead of OSFI 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.
HTP 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 in 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, dot, 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.
SMP 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 end points, has different names in each layer and
layers five, six and seven application layers.
It's called data in layer for transport layer.
It's called segment for TCP and data gram for UDP.
And Layer three, the network layer.
It's called packet and 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.
Makes sense.
Before looking at the road trip of a DNS query in detail, let's look at the relationship between the
oscillators, layers and computer systems.
The packets in layer one and layer two are managed by network interfaces of your device, for example,
by your Ethernet card, and the packets from three to seven 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 five to seven are managed by the related application or service, for example, a Web browser,
when you write a new 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 Dot, Yahoo Dotcom.
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 Sesson 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 IP, but let's assume that it uses UDP at this time.
The data and the layer for header here, its UDP header, which together form a segment or data gram
is passed down to layer three.
The network layer, the network layer places.
It's 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 the layer to the datalink layer creates a new data unit
called a frame by adding the layer to frame header, which is the Ethernet header here.
Like layer three, an addressing structure is also applied in the layer to 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.
That's the data at each layer is encapsulated in the information appropriate for this specific layer,
including a dressing 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 the capsule lated.
The physical address, commonly the Mac address, which is located in a special field in the data link
layer header, changes as the packet passes from one device to another, for example, from the source
PC to a switch to a router to another switch and finally to the destination PC.
However, the original IP source and destination addresses do not change when transiting the network
because the packet is stripped of its layer three header only when it goes beyond a router.
When it stays within the same land, it only passes through switches which related at the layer to heter
containing the Mac address.
As a result, the header changes as the packet is rean capitulated, as does the Mac address Fields'.