The most basic function of the data link layer is to provide transparent and reliable basic data transmission services to users in this layer. Transparency means that there are no restrictions on the content, format and coding of the data transmitted on this layer, and there is no need to explain the meaning of the information structure; Reliable transmission saves users from worrying about losing information, interfering with information and incorrect sequence. These situations may occur in the physical layer, and error correction codes must be used in the data link layer to detect and correct errors. The data link layer is to strengthen the function of the physical layer to transmit the original bit stream, transform the physical connection that may be wrong provided by the physical layer into a logically error free data link, and make it behave as an error free line to the network layer.