Original subtitles

All right, all right, hello, guys, so we've got a great video here because we are now going to learn

the great Osai model.

It sounds like something that belongs in the United Nations or something, but.

I'll tell you, once you get this, you're going to get it all.

So just to break it down.

With the use of computer networks.

Well, the necessity of a common standard has emerged in order to integrate different manufacturers

into each other's networks.

And the past companies developed their own special network systems and presented them to the customers

as a package.

And no, you have to stay within our domain, right?

It was very expensive and it actually didn't do anybody any good because although they could work pretty

well within themselves, it's very difficult or next to impossible, really for them to communicate

with networks outside of themselves.

For example, IBM operating systems were able to communicate with each other using IBM network devices.

Well, that makes sense.

But as a result of the inability of supply to meet the demand in network systems like, you know, worldwide

and the pressure of hardware manufacturers in the network market, it was kind of understood that a

standard model should be established for the functions of network systems.

So for this reason, the OSAI reference model, which is the computer network standard, was introduced

by the ISO.

Now the ISO is just the International Organization for Standardization.

Right.

And that was back in 1978.

So this standard, which was indeed first revealed in 78, was rearranged in 1984, and then it was

published as the Osai or open system interconnection reference model.

So the model has been widely accepted, and it's actually become a guideline for network operation.

So the Aussie model is just that it's a standard.

Now.

Just as in the past, anyone can develop a network communication model on her own.

However, if the U.S. model is not referenced, it's going to be difficult to communicate with other

networks, and different manufacturers won't be able to produce hardware and software to work on that

network system.

So why bother?

Because the U.S. reference model has divided the processes from the communication medium of both wired

and wireless to the user into seven different layers of computer networks.

Now these are the layers, and I will read them to you as they are listed because they are always in

this order.

So the first layer is the physical layer.

Second is the data link layer.

Third is the network layer.

Fourth is the transport layer.

Fifth session, layer six presentation layer and finally seven the application layer.

All right.

So always-I is the established standard model for providing data communications, so this hierarchical

network model makes it easier to design and manage computer networks for everybody all over the world,

no matter what their languages.

So according to the Osai model, data is transmitted through all these layers.

Therefore, it's a model that a network specialist should know very well in order to analyze and best

solve a problem in the entire network system because each layer has a task.

Protocols at work on each layer, and these protocols have tasks.

Yeah.

So why don't we just take a look at these layers right now and get used to what they do?

So we've got layer one here, right?

This is the physical layer, and it's the physical layer that defines how data particles or bits are

transmitted in transmission media and the transmission media is just like a cable.

It can be fiber optics as well as the air.

Right.

Wi-Fi radio signals all that.

Now on the sending side, the physical layer sends of ones and zeros in a way that a transmission medium

can understand it.

So its electrical signals radio signals its light flashes, right?

So on the receiving side, the physical layer converts the signals red from the transmission medium

back to ones and zeros.

So in order for network equipment from different manufacturers to communicate seamlessly with each other,

the outgoing and incoming data bits must mean the exact same thing for different brands of equipment.

So in other words, certain standards need to be established and the same protocols need to be used.

All right, so then what's going on in layer two?

This is the data link layer.

So this layer enables the data sent from the source to be framed and forwarded to the destination address.

So this layer consists of two sub layers.

Wait for it.

Media Access Control or Mac and Logical Link Control, or LLC.

The Mac sub layer uses the 48 bit Mac address uniquely located on each network card.

It ensures correct communication with the source and the destination Mac addresses in the data packet.

Now, the LLC sub layer acts as a transition to a parent layer, so it provides a communication between

the two layers by creating logical ports or snaps, which are service access points, error and conflict

control of the Senate data.

Is this done with the CRC error checking protocol, as well as the CSM CD conflict protocol used in

this where?

So don't get too bogged down with that.

Let's move on to layer three.

This is a network layer, so this layer network address IP address allows the data to reach the destination

from the source.

So this layer of data with the source and destination IP addresses, it's called a packet.

So the IP packet contains information such as the total size, the total, the service type version,

error checking, along with the addresses, so it's the routers and the routing algorithms that direct

data in the network environment, and they work all in this layer.

So then what's in level four?

It's a transmission layer, so this layer divides the data sent by the source into parts or segments

and transmits all of them to the destination.

There is something called flow control between the source and the destination that's all done here in

this layer.

Now, the two important protocols of this layer, you might have heard of TCP, which is transmission

control protocol and UDP, which is user data grand protocol, and both have different uses and purposes.

But you'll find port addresses, which are defined differently for each application, are one of the

most important components of this layer.

So what about layer five?

Well, this is your session layer, so it's this layer that initiates, manages and terminates communication

between the source and the destination.

So each network connection used in the client, for example, an email or web browser, whatever it

is, each application, such as FTP, will open a separate session to prevent data mixing in with each

other because you never want to do that.

OK, so layer six is the presentation layer, so it's all thanks to this layer, the data shared between

PC is in a network environment is meaningful.

And by that, I mean, the shared information to be read by pieces, the data must be converted into

a common format.

So the importance of this function is understandable when you consider that the computers that share

the SharePoint are managed with different software.

Yeah.

So it's possible for different programs to use each other's data.

One of the most important tasks of the presentation layer is the encrypted transmission of shared data

to the other computer.

Now, actually, the presentation layer is not necessarily network related.

Right, it's basically software related, so operating systems today cannot read many created formats.

In other words, if our operating system does not support a data format, it comes to us from another

user.

That information basically means nothing to us.

For example, in order to watch videos in a Devex format shared on the internet today, it's necessary

to have the supporting codec programs on your computer right when one computer opens a Devex file on

another.

The presentation layer doesn't do anything, so it means using software in programs that can decode

the same format.

So then we get to layer number seven lucky layers.

Right?

It's the application layer, so this layer provides tools for users and programs to use a network.

All right.

Since the application layer is this last layer, well, it doesn't provide any service to the other

layers.

It creates network services for running applications.

So Microsoft appears they'll work in this layer.

A programmer who makes a program using Microsoft APIs, for example, takes the ready tool in the API

and uses it in their own program when they need to access a network drive.

It doesn't have to deal with any of the dozens of different processes that take place in the lower layers.

So, yeah, another example is HTP, HTP is not a program, but a protocol with a string of rules and

Internet Explorer that functions according to this protocol connects to other web servers that use the

same protocol.

So in addition, this layer detects whether the computer with which it will communicate is ready for

communication.

Right?

And then it synchronizes that communication.

So let's take a look at the example and will define the task of each layer.

So here you see user wants to FTP a file to a server B.

User A's point of contact with a computer is the FTP program in the application layer.

And the file is transmitted from this layer to the computer.

The file to be sent is packaged in the format or shape of the FTP protocol.

And that's the request in the presentation layer.

So this session later initiates a session for this communication.

Then the transmission layer divides the file to be sent into segments and adds a PDA or protocol data

unit into which it adds the relevant data.

That's the port number, control variables, etc..

So in other words, each segment of the email, which is divided into parts, is transmitted separately

to the lower layer, the network layer.

The network layer adds its own PD for each segment with source and destination IP addresses and other

control variables.

The data link layer adds its own Purdue, which contains the source and the destination Mac address

information and the other control parameters to the packet sent to it.

And sent it to the physical layer.

The physical layer sends the entire packet sent to it as an information string consisting of ones and

zeros to the communication medium it is connected to as electricity radio signals or light.

Right.

So after the switching and routing operations, the information string consisting ones and zeros reaching

the destination is recorded by the application layer FTP program by doing the reverse of all those operations

on the source side.

All right.

I know it's crazy, huh?

But but the visual here is, is is, is really great.

Very helpful.

OK, now another model used other than the U.S. reference model quickly is the TCP IP model.

So this model is just basically a simplified version of the U.S. reference model.

It just consists of four layers as a result of combining some of the layers of the U.S. reference model.

So that might be helpful as well.

All right, so there we go.

You now know about the U.S. model, all of its layers and what they do.

Fantastic.

That's really good.

So if you're not asleep?

I want to see in the next lesson.