What are the seven layers of OSI's seven layer model? What is the role of each layer? This article is clear!

Hello , This is the network technology alliance station .

There are fans leaving messages backstage saying that they are currently freshmen , Just learned computer network , Acquire OSI When modeling , What the teacher said is very abstract , I can't understand at all , I want to write an article about OSI Model article , And it can be understood by people who have never studied computer network .

This demand is interesting , But think about it , Everyone reading this article , Can not think 、 Speak fluently OSI Are there any models ？ What's the use of separation ？

I think there are two kinds of people who can really do this ：

• A friend who just finished learning this knowledge point

• Textual research friends 、 Friends who are about to interview fresh students , Carry it every day

Friends who have worked , If you don't deliberately associate memory , I don't think what I said is too complete or familiar .

So today, Rui Ge will first talk in the most straightforward words OSI Model , Then summarize and supplement with more professional terms .

This article only speak OSI Model , Don't speak TCP/IP Model .

What is? OSI Model ？

• OSI English full name ：
  Open Systems Interconnection

• OSI Chinese name ：
  Open communication system interconnection

OSI It is the reference model of computer network communication protocol , International Organization for Standardization  (ISO)  stay  1980  The era created .

Why does this model appear ？

During the development of computer networks, many types of operating systems have emerged , The network interconnection of different systems has become a problem , Just like there are different countries in the world , Different countries have their own languages , If there is no standard language , Then we can't communicate with each other .

In the online world, too , Network devices 、 Systems are developed by different manufacturers , Every manufacturer has its own thinking , So you want to be compatible , That's almost impossible . therefore OSI The standard of model appears , In the final analysis, it is a rule , When all your manufacturers produce equipment , Please take your seat , According to this standard .

OSI Model explanation

OSI  The model has seven layers of protocols ：

Before going into detail about the meaning of each layer , Let me give you an example of goods arriving at your hand by express delivery .

A customer bought a product online , Generally, the following process will be followed from the delivery of goods to the receipt of goods ：

Business side ：

• Prepare goods

• Label the goods

• Packaged goods

• Couriers collect goods

• Goods arrive at the express station

• Goods are transported by truck

• Goods leave the city where the goods are located

Customer side ：

• The goods arrive in the destination city

• Goods are transported by truck

• Goods arrive at the express station

• Couriers deliver goods

• Customers get goods

• The customer tore up the express bill

• Take out the goods

This process and OSI The workflow of the seven layer model is very similar , Now let's talk about it in detail .

Let's first look at the meaning of each layer ：

The physical layer

The physical layer is  OSI  The first or lowest floor of , Defines electrical and mechanical specifications , Such as cable 、 Signals from connectors and media .

Data link layer

The data link layer is located at OSI  The second layer of the model , The data unit of this layer is called “

frame

”.

The data link layer includes two sub layers ：

• The first layer is called
  Logical link control layer
  , Responsible for transmitting data packets to the receiving network layer , Identify the network layer protocol address from the header , And provide flow control .

• The second layer is called
  Media access control layer
  , It is the connection layer between the logical link control layer and the physical layer of the network , This layer is used to transmit data packets over the network .

The network layer

The network layer is located in OSI  The third layer of the model , The network layer has two main functions ：

• One is to decompose segments into network packets , Then reassemble the packet at the receiving end .

• The other is to route packets by finding the best path across the physical network .

The network layer uses IP Address routes packets to the destination node .

Transport layer

The transport layer is located in the OSI  The fourth layer of the model , Responsible for transmitting 、 Error checking 、 Flow control and packet sequencing , The data unit of the transport layer is

Data segment

.

Two classic transport layer protocols ：

• Transmission control protocol  (TCP)： Connection oriented reliable protocol

• User datagram protocol  (UDP)： Connectionless and unreliable protocols

The session layer

The conversation layer is located in OSI  The fifth layer of the model , Allow users of different platforms to establish an active communication session between them , Provide synchronization in conversations between two different applications .

This is equivalent to buying a big commodity , Be split and sent , When you don't deliver it to a delivery point, you have to make sure that the split goods are in place , Always confirm , Until all received .

The presentation layer

The presentation layer is located in OSI  The sixth layer of the model , Present data to users in an easy to understand form , The presentation layer is responsible for syntax , Because the sender and receiver may use different communication modes , You can think of the presentation layer as a translation .

application layer

The application layer is located in OSI  The seventh layer of the model , The application layer is the only one that directly interacts with user data  OSI  layer , This layer provides a protocol , Allow software to send and receive information and present meaningful data to users .

OSI Data unit of each layer

• APDU
  ： Application protocol data unit

• PPDU
  ： Represents a protocol data unit

• SPDU
  ： Session protocol data unit

• TPDU
  ： Transport protocol data unit （ paragraph ）

• Data packets
  ： Network layer data unit

• frame
  ： Data link layer data unit

• position
  ： Physical layer data unit

summary

OSI The seven layer model itself is not difficult , This article briefly introduces OSI What are the seven tier models , What does each layer mean , What are the functions , For reasons of length , Each floor is not open , Start with the next article and expand each layer .

Thank you for reading , If you think the article will help you , Don't forget to like 、 Collection ！ Any questions , Welcome to discuss with me in the comment area below ！！！