There are no impacts on existing security mechanisms for the CS Domain or for IMS as a result of Session Transfers.

TS 33.102 describes the Security Architecture for GSM and UMTS subscribers, SC places no additional requirements upon the CS domain security than those already in the detailed access specific specification e.g., above those described in TS 33.102.

TS 33.203 specifies the security features and mechanisms for secure access to the IM subsystem (IMS). SC places no additional requirements upon the IMS above those described in TS 33.203.

To ensure the completeness and correctness of charging during Session Transfer procedure, and to avoid possible double billing in IMS and CS, the following strategy should be applied:

• Provide cohesive charging records with a complete service continuity history for the whole duration of a SC subscriber multimedia session by the SCC AS.
• For cases of CS origination and CS termination, correlate the charging records generated in CS and IMS for the subscriber multimedia session, to avoid double billing to the subscriber.
• Treat the charging records generated in the transferring-in access network for the call(s)/session(s) established during the Session Transfer as subsequent Access Legs, and therefore do not impact the direction of the initial call(s)/session(s) for the purpose of charging.
• Keep the start of charging in the transferring-in access network align with the stop of charging in the transferring-out access network, to avoid double billing to the subscriber during the Session Transfer.

To avoid online charging correlation in IMS and CS domain, the SC online charging should be performed only in IMS, i.e. prepaid service logic in CS domain should not be invoked for anchored CS origination/termination call and subsequent CS origination call established for performing Session Transfer. In addition, the SCC AS should report information related to the initial multimedia session establishment as well as the information related to the Session Transfer procedure to OCS for correct credit control purpose.

To assist in performing the settlement between operators, the following strategy shall be applied:

• Provide cohesive charging records with a complete service continuity history for the whole duration of a SC subscriber multimedia session by the SCC AS.
• Use the charging records for subsequent Access Legs generated in CS/IMS domain and the charging records generated in MGCF performing CS-IMS interworking, taking the complete service continuity history described above as reference, to perform the settlement between the providers of CS domain and IMS.
• Use the access network information in IMS charging records, taking the complete service continuity history described above as reference, to perform the settlement between the providers of IP-CAN and IMS Core.
• Additional reconciliation for I1 flows can also be applied, due to such flows being uniquely identifiable from other flows over the same bearer (see clause 5.4.1 of TS 23.292 for more information).
• At least in roaming scenarios, the SCC-AS returns the ICID of the original access leg to the MSC-Server for the MSC-Server to include in charging records.