Network layer 7 protocol

• The physical layer ：

Solve the problem of how to communicate between two hardware , Common physical media are optical fibers 、 cable 、 Repeater, etc . It mainly defines physical equipment standards , Such as the interface type of network cable 、 Interface type of optical fiber 、 Transmission rate of various transmission media, etc .

Its main function is to transmit bitstreams （ Is the 1、0 It's converted into current to transmit , After arriving at the destination, it is transformed into 1、0, That is to say, we often talk about analog-to-digital conversion and analog-to-digital conversion ）. The data in this layer is called bit .

• Data link layer ：

Because of all kinds of interference in the computer network , Physical links are unreliable . The main function of this layer is ： Through various control protocols , Change the physical channel with errors into error free 、 A data link that can reliably transmit data frames .

Its specific work is to receive data in the form of bit stream from the physical layer , And encapsulate it into frames , Send to the next level ; Again , It will also come from the upper data frame , Data in the form of bitstream is forwarded to the physical layer . The data in this layer is called frames .

• The network layer ：

If there are multiple computers in the computer network , How to find the one to send ？ If there are multiple nodes in the middle , How to choose the path ？ That's what routing does .

The main task of this layer is ： Through routing algorithm , For message （ The data unit of this layer , Packaged from the upper layer of data ） Select the most appropriate path through the communication subnet . This layer defines IP Address , adopt IP Address addressing , That's why IP agreement .

• Transport layer ：

When sending large amounts of data , Packet loss is likely to occur , The other computer wants to tell whether all the packets have been received completely . If there is a lack of , Just tell me which bags are missing , Then send it again , Until all are received .

Simply speaking , The main function of the transport layer is ： Monitor the quality of data transmission services , Ensure the correct transmission of messages .

• The session layer ：

Although it can be implemented to the right computer , Send the correct encapsulated information . But we can't always call the transport layer protocol to package every time , Then call IP Protocol routing , So we need to set up an automatic receiving and contracting system , Automatic addressing function . So the conversation layer appears ： Its role is to establish and manage communication between applications .

• The presentation layer ：

The presentation layer is responsible for data format conversion , Convert the information processed by the application into a format suitable for network transmission , Or convert the data from the next layer into a format that the upper layer can process .

• application layer ：

The application layer is the computer user , And interfaces between various applications and networks , Its function is to provide services directly to users , Complete all kinds of work that users want to accomplish on the network . Front end students must be the most familiar with the application layer .