Method and apparatus for providing immediate ciphering after an inter-system UTRAN-GSM handover

A mobile station operable for being handed over from a current system to a target system, comprising an RF transceiver coupled to a data processor, said data processor being responsive to a receipt through said RF transceiver of a current system-specific handover command from the current system for triggering the handover of the mobile station from the current system to the target system, where the target system is one that requires the mobile station to initiate the use of ciphering only in response to receiving a specific command from the target system to initiate the use of ciphering, said data processor operating to immediately start the use of ciphering after handing over to the target system in response to receiving a target system-specific handover command that is sent transparently with the current system-specific handover command, where the target system-specific handover command has an information element set for indicating that ciphering is to be started.

A method for performing an inter-system handover, comprising:

• triggering the handover of the mobile station from a UTRAN system to a GSM system by sending a HANDOVER FROM UTRAN COMMAND message from the UTRAN system;
• and
• for the case where a HANDOVER COMMAND is sent transparently with the HANDOVER FROM UTRAN COMMAND, where the HANDOVER COMMAND has an information element set for indicating that ciphering is to be started, beginning ciphering immediately after handing over to the GSM system such that a first data frame transmitted by the mobile station in the GSM system is a ciphered data frame.

A wireless mobile communications system operable for performing an inter-system handover, comprising:

• wireless network means for triggering the handover of a mobile station from a UTRAN system to a GSM system by sending a predetermined command message from the UTRAN system, the predetermined command comprising a HANDOVER FROM UTRAN COMMAND message with a HANDOVER COMMAND that is sent transparently with the HANDOVER FROM UTRAN COMMAND, the HANDOVER COMMAND comprising an information element set for indicating that ciphering is to begin;
• and
• mobile station means, responsive to a receipt of the predetermined command, for beginning ciphering immediately after handing over to the GSM system.