Patent ID: 7054306

Claim:
A method for performing ciphering in a wireless communications system, the wireless communications system comprising a base station in wireless communications with a mobile unit, the base station capable of transmitting protocol data units (PDUs) to the mobile unit and receiving PDUs from the mobile unit using a packet switched (PS) domain or a circuit switched (CS) domain, the method comprising: providing the mobile unit with a first ciphering status variable for the PS domain, and a second ciphering status variable for the CS domain; the base station transmitting a security mode command to the mobile unit indicating an activation or deactivation of ciphering of the PDUs in the PS domain, or indicating an activation or deactivation of ciphering of the PDUs in the CS domain; and the mobile unit setting the first ciphering status variable according to the security mode command if the security mode command indicates an activation or deactivation of ciphering of the PDUs in the PS domain, and setting the second ciphering status variable according to the security mode command if the security mode command indicates an activation or deactivation of ciphering of the PDUs in the CS domain; wherein after setting the first ciphering status variable or setting the second ciphering status variable, the mobile unit activates or deactivates ciphering of PDUs transmitted in the PS domain according to the first ciphering status variable and activates or deactivates the ciphering of PDUs transmitted in the CS domain according to the second ciphering status variable.