The disclosure relates to a method and a device therefor, wherein the method is applied to CA between base stations including multiple base stations connected through non-ideal backhaul, has a master RLC pre-assign an RLC SN to a PDCP PDU and transmit the PDCP PDU to a slave RLC of each base station, and has the master RLC take full responsibility for ARQ retransmission management, thereby enabling more efficient use of a network resource and a reduction in the time spent for a retransmission.