A method of communicating signals in a mobile communication system having a first base station, a second base station, and a base station controlling node controlling the first base station and the second base station is presented. The method includes during a handover, receiving, at the first base station, a control message from the second base station including a sequence number indicating at least one data unit to be transmitted to a mobile station or to be received from a mobile station, wherein the first base station receives the control message from the second base station without being controlled by the base station controlling node.