How does the data link layer transmit data

Data link layer function Overview

The main function of the data link layer is to strengthen the function of the physical layer to transmit the original bit stream . It is responsible for the physical connection provided by the physical layer that may go wrong , Transformed into a logically error free data link , Make it an error free link to the network layer .

The data link layer includes three basic problems ： Package into frames 、 Transparent transmission 、 Error control .

Package into frames

The signal sent in the physical layer itself has no start and end marks , At the same time, the data link layer needs to IP Datagrams are also added to the frame , To sum up, this problem is the problem of packaging into frames .

take IP It's easy to encapsulate datagrams into frames , Just add a frame start character in front of the data part SOH, And add a frame end character after the data part EOT. When receiving data, if you encounter the frame start character, it means that you need to start receiving data , If you don't encounter it, it means that the previous received may be noise, so you don't need to deal with it . Then, the end of the frame indicates the end of the data transmission , Some of the later data are processed differently , The schematic diagram of data encapsulation is as follows ：

Transparent transmission

Transparent transmission means that no matter what kind of bit combination the transmitted data is , Should be able to transmit on the link . When the network layer protocol is encapsulated into frames at the data link layer , Will be added in the head and tail respectively SOH as well as EOT These two special characters , The receiver determines the beginning and end of the frame according to these two characters （ There is ambiguity ）, If the data sent by the upper layer protocol ( That is, the data part of the link layer ) contain EOT, Then the receiver will mistakenly think that the data has ended when parsing this frame , In this case, data transmission will cause problems . therefore , If the link layer has no special treatment for this situation , Then we can understand that the link layer is non transparent transmission ( Because it cannot be transferred EOT This character ).

Usually, this problem can be solved in the data part SOH、EOT Add an escape character before the character ESC, If you receive directly during transmission SOH、EOT The character is still represented as the start and end marks of the frame , But if you receive ESC And then received SOH、EOT, It means that these two characters are not the start and end marks of the frame , Is part of the frame data . This completes the problem of transparent transmission .

Error control

Real communication links are not ideal . That is to say , There may be errors in the transmission of bits :1 Could become 0, and 0 It could become 1.

In order to detect such errors , The data link layer adopts the check code . The data link layer generates the check code and sends it together with the data before transmission , After acceptance, check whether the data is correct through the check code , If the check is wrong, the frame will be discarded directly , If there is no error, save it directly , analysis , And submit the data part to the network layer . The most widely used in use is CRC Cyclic redundancy check .