Model OSI – Open Systems Interconnection model.
– created in 1970
– adopted in 1984
When the Internet was created, each manufacturer had developed a networked way of communicating.
The Internet – everyone communicates together.
But if everyone has their own language of communication, then it is difficult to communicate with everyone…
The International Organization for Standards (ISO) has therefore decided to create a universal communication model:
The OSI model.
I'm in my room and I want to download the latest HD episode of Game of Trones.
This episode takes 800 MB:
800 Megabytes – 6400 Megabits (1 byte – 8 bits/or 800×8 – 6400)
6400 Megabits – 6,553,600 Kilobits (1024 kb – 1 Mb/or 6400×1024 – 6,553,600)
6,553,600 Kilobit – 6,710,886,400 bits !!! (1024 bits – 1 kilobits/or 6,553,600 x 1024)
That's 6.7 billion bits (0 or 1) that's going to have to go around the earth.
If we have a 0 or 1 state that is changed, the file will not work because it will be corrupted.
What will be added to this data to deliver this data to the right recipient?
The identity of the transmitter (the download server)
The identity of the recipient (you)
-And lots of other things
How do you get these bits through?
Imagine you're with a friend on the phone to pass on this serial number:
How are you going to make sure you both have the same number?
You're going to dictate this character number by character
Your friend will repeat the character every time to make sure you don't make a mistake
This will save time, because you will not have to do it 10 times before you get the right information.
In computer science, it's the same:
– the data is fragmented by small bits
– information will be added at every little bit in order to know how to deliver it
For everyone to understand each other, a communication standard has come out and it is called the OSI model!
The OSI model
Imagine that you need to send this paper document from French Polynesia to France.
What do you need?
From an envelope:
On this envelope, we will add:
– destination address
– the sender's address
– fill in an acknowledgement
– put stamps
– make a customs declaration
In computer science, it's exactly the same thing. We need to add to our data fragments a lot of things so that they are transported in the right conditions, safely and for them to be delivered to the right person.
For mail, we call it the postal system
For it, we call it the OSI model
We have this data to send:
In order to add information to this data for it to be sent over the network, our computer will add bits:
The information that has been added is categorized into 7 layers:
– layer 7: Application
– layer 6: Presentation
– layer 5: Session
– layer 4: Transport
– layer 3: Network
– layer 2: Link
– layer 1: Physics
Small mnemotechnical means: (Ah Little Slut, You Spit La Purée)… You do what you want with it, the fact is that this mnemotechnical means exists, I could not not give it to you …
So how does it work?
Layer 7 adds an L7 header to the data
Layer 6 adds an L6 header to the given group – L7
Layer 5 adds an L5 header to the given group – L7 – L6
Layer 4 adds an L4 header to the given group – L7 – L6 – L5
Layer 3 adds an L3 header to the given group – L7 – L6 – L5 – L4
Layer 2 adds a header and an L2 tail to the given group – L7 – L6 – L5 – L4 – L3
Layer 1 sends it all to the network
Now that we are aware that the OSI model is adding information about each piece of data to transmit it over the network and has 7 layers, we will see what these layers are for.
Layers 7, 6 and 5: Application/Presentation/Session
Who uses the data from layers 5, 6 and 7?
What information is found in these layers?
Who is the information for?
These 0 and 1 I received is for whom? A file? My web browser? For Skype?
Layer 4: Transportation
Who uses the layer 4 data?
The sender and recipient of the message.
What information is in layer 4?
The transport layer is responsible for the proper delivery of packages across the network. We can compare that to the acknowledgement of the post office.
We have a choice between two protocols:
– TCP (Transmission Control Protocol)
– UDP (User Datagram Protocol)
The TCP protocol allows us to have an acknowledgement with each message sent. If the acknowledgment is not received, it will be returned.
Example: File transfers, web pages, emails, etc.
The UDP protocol simply sends the information. If packages get lost along the way, it doesn't matter!
Example: Video conferencing, live streaming, Skype call, etc.
Layer 3: Network
Who uses the layer 3 data?
What information is in layer 3?
The IP addresses of our issuer and recipient.
The network layer allows a package to be delivered through a network.
Layer 2: Link
Layer 2 has two underlayers:
– the SUBlayer LLC
– the MAC underlayer
Who uses the layer 2 data?
What information is in layer 2?
The MAC address of the next equipment and the MAC address of the previous equipment.
A MAC (Media Access Control) address is the physical address of your network card. It is unique in the world.
Your computer has a network card as well as a WiFi card. Each of these cards has a MAC address.
What's the point of this address?
It will be used to communicate on a small network. This address remains local and will never circulate on the Internet.
Layer 1: Physics
The physical layer will adapt the message according to the medium.
It is she who is responsible for sending our 0 and 1.
Some protocols of the physical layer:
– coaxial cable
– twisted pair
– 10BASE-T / 10BASE2 / 10BASE5 / 100BASE-TX / 1000BASE-T
The OSI model is the core of the network administrator business. It is impossible to ignore this issue.
Don't hesitate to ask me comment questions if you don't think a part of it is clear.
Hoping we could help you!
Don't hesitate if you have any questions or if you have any information to bring!!