A mobile station is configured to support handover from a source base station to a target base station. The mobile station receives signaling from the target base station that indicates Packet Data Convergence Protocol (PDCP) protocol data units which have been successfully received at the target base station. In response, the mobile station generates further PDCP protocol data units for missing PDCP service data units from corresponding PDCP service data units buffered at the mobile station. The generating comprises using a different header compression state from that which was used for the missing PDCP service data units and ciphering specific to a connection with the target base station. The mobile station transmits the further PDCP protocol data units to the target base station.