Provided is a data transmission method including obtaining, from a core network (CN), at least one packet to be transmitted to a user equipment via a first cell group or a second cell group, determining a packet to be transmitted via the second cell group, among the at least one packet, transmitting the determined packet to the user equipment via the second cell group, obtaining packet delivery state information of the first cell group and packet delivery state information of the second cell group, determining whether to retransmit the transmitted packet based on the packet delivery state information of the first cell group and the packet delivery state information of the second cell group, and retransmitting the packet determined to be retransmitted to the user equipment via the first cell group.