In the 5G system, the gNB may use a CU-DU architecture. There may be a plurality of possibilities for a CU-DU function split. FIG. 1 is a schematic diagram of a CU-DU split. As shown in FIG. 1, the CU-DU architecture includes RRC, PDCP, low-RLC, high-MAC, low-MAC, high physical layer (PHY), low-PHY, and radio frequency (RF). Option 1 may be splitting between the RRC and the PDCP. Option 2 is splitting between the PDCP and RLC. Option 3 is splitting the RLC into two parts. A function with a low real-time requirement is placed in the high-RIX, a function with a high real-time requirement is placed in the low-RLC, and the split is performed between the high-RLC and the low-RLC. Option 4 is splitting between the RLC and MAC. Option 5 is splitting between the high-MAC and the low-MAC. Option 6 is splitting between the low-MAC and the high-PHY Option 7 is splitting between the high-PHY and the low-PHY. Option 8 is splitting between the low-PHY and the RF. For example, in Option 2, for downlink RRC message and data processing the CU generates a RRC message or data, and after PDCP layer processing, transmits a PDCP protocol data unit (PDU) (namely, an RLC service data unit (SDU)) to the DU through an interface between the CU and the DU. After corresponding RLC, MAC, and PHY processing, the DU further sends the PHY processed result to a wireless channel through the RF for transmission. For uplink RRC message and data processing, after the DU receives a data packet through a radio frequency apparatus, the data packet successively experiences PHY, MAC, and RLC processing, and then the DU transmits an RLC SDU (namely, a PDCP PDU) to the CU through an interface between the CU and the DU. The CU further obtains a RRC message or data through PDCP layer processing, and correspondingly sends the RRC message or the data to a RRC layer or an application layer. It should be particularly noted that with evolution of communications technologies, a protocol layer function and a naming manner of the base station, and message content and a name thereof may be different from those in LTE. For example, an original RLC layer resequencing function may be moved up to the PDCP layer. For another example, an original RRC connection request message may have another name. In this application, no limitation is imposed on a protocol layer, a protocol layer function, protocol layer naming, and a protocol layer message name of a specific CU or DU. For ease of description, a protocol layer name and a corresponding message name in the LTE are used as examples to describe a corresponding solution.
