This application provides a data transmission method and an apparatus. In the method, a first data unit is received from a server or a data network, wherein the first data unit comprises first identification information and second identification information, a target transmission requirement corresponding to the first data unit is obtained based on the first identification information, protocol data encapsulation is performed on the first data unit based on the target transmission requirement, to obtain a second data unit that comprises third identification information related to the second identification information; and the second data unit is sent to an access network device.