A method and system for carrying out a handover between base stations of a telecommunication network. Each mobile terminal is in communication with a set of base stations including at least a first and a second base stations, the first base station receiving downlink data to be transmitted within data packets bearing a downlink sequence number to a first mobile terminal from an access gateway of the telecommunication network. The method includes determining a downlink sequence number, sending a handover request message to the second base station comprising the determined downlink sequence number and information related to the set of base station, requesting the second base station to receive data to be transmitted to the first mobile terminal from the access gateway, and stopping building data packets if the built data packets would bear a sequence number equal or higher than the determined downlink sequence number.