A wireless communication system comprises an integrity key, a mobile unit comprising
a first integrity count value, and a UTRAN comprising a second integrity count
value. The UTRAN is coupled to the mobile unit for transmitting a control command
to the mobile unit. The control command is used for starting an integrity protection
process for a radio bearer, with the integrity protection process using the integrity
key and the first and second integrity count values for verifying a transmission
integrity between the mobile unit and the UTRAN. The mobile unit sends a START
value for the radio bearer to the UTRAN. The UTRAN sends the control command to
the mobile unit to activate integrity protection along the radio bearer, and sets
the x most significant bits of the second integrity count value of the UTRAN equal
to the START value last received from the mobile unit for the radio bearer. In
response to receiving the control command, the mobile unit sets the x most significant
bits of the first integrity count value of the mobile unit equal to the START value
last transmitted to the UTRAN.
