A wireless communication system has a mobile unit, a UTRAN, and a plurality of established radio bearers. The UTRAN is used for transmitting a first control command to the mobile unit, and the first control command is used for triggering establishment of a new radio bearer between the mobile unit and the UTRAN. Each established radio bearer between the mobile unit and the UTRAN has a corresponding first HFN, which are used for ciphering. The mobile unit generates a first value based on the first HFNs of established radio bearers wherein the first value is at least as great as the x most significant bits (MSB_x) of each first HFN. Each of the mobile unit and the UTRAN sets MSB_x of an initial HFN associated with the new radio bearer equal to the first value, regardless of whether or not the ciphering is "STARTED" or "NOT STARTED".