A method performed by a first distributed unit (DU) providing a primary cell (PCell) is provided. The method includes receiving, from a second DU providing a secondary cell (SCell), a request message for downlink data through a communication interface between the first DU and the second DU, and transmitting, as a response to the request message, a response message to the second DU through the communication interface, wherein the request message includes a user equipment (UE) identity (ID), a bearer ID, information on a downlink data amount, and information on a data type, wherein the response message includes the UE ID, the bearer ID, an index for indicating the SCell, and the downlink data, and wherein the data type indicates one of a plurality of types including a first type for a radio link control (RLC) protocol data unit (PDU) and a second type for a medium access control (MAC) PDU.