A method and device for performing handover by a mobile terminal from a source base station to a target base station, the mobile terminal having a RRC layer, a RLC layer and an PDCP layer. The method includes receiving a handover initiation message; and performing handover from the source base station to the target base station, including reassembling RLC SDUs from at least one PDU when possible and delivering the reassembled RLC SDUs from the RLC layer to an upper layer.