Section: 6.8.4 Intersystem handover for CS Services – from UTRAN to GSM BSS
Content: If ciphering has been started when an intersystem handover occurs from UTRAN to GSM BSS, the necessary information (e.g. Kc, supported/allowed GSM ciphering algorithms) is transmitted within the system infrastructure before the actual handover is executed to enable the communication to proceed from the old RNC to the new GSM BSS, and to continue the communication in ciphered mode. The RNC may request the MS to send the MS Classmarks 2 and 3 which include information on the GSM ciphering algorithm capabilities of the MS. This is necessary only if the MS Classmarks 2 and 3 were not transmitted from UE to UTRAN during the RRC Connection Establishment. The intersystem handover will imply a change of ciphering algorithm from a UEA to a GSM A5. The GSM BSS includes the selected GSM ciphering mode in the handover command message sent to the MS via the RNC. The integrity protection of signalling messages is stopped at handover to GSM BSS.