DRVCC Alerting Phase PS to CS Handover via SCC-AS
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Solution Overview
Problem
Current Dual Radio Voice Call Continuity (DRVCC) specifications do not provide a means to handle Packet Switched (PS) to Circuit Switched (CS) handover during the call alerting phase, particularly on the terminating side, leading to errors due to unmatched states between the mobile terminal and the Mobile Switching Centre (MSC).
Innovation Solution
A method where the terminating mobile terminal notifies the Service Centralization and Continuity Application Server (SCC-AS) of the need for a PS to CS transfer during the alerting phase, allowing the SCC-AS to ensure the MSC is in the correct state to receive an answer, by sending a SIP message and subsequently receiving a SIP INVITE from the SCC-AS to the MSC, including the UE's identity and Session Transfer Number (STN).
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If DRVCC handover is implemented during alerting phase, then call continuity is improved, but state synchronization between UE and MSC deteriorates
Solution Approach 1:
The UE sends a SIP message to the SCC-AS before the actual handover occurs, allowing the network to prepare the MSC in advance. The SCC-AS sends a SIP INVITE to the MSC with the STN as calling party identity, which preliminarily configures the MSC to expect an answer from the UE, thus preventing state synchronization errors during the subsequent handover.
2Reliability
If PS to CS handover is performed during alerting phase, then access quality is improved, but protocol compatibility deteriorates
Solution Approach 1:
The SCC-AS acts as an intermediary between the UE and the MSC during the handover process. It receives the SIP message from the UE, processes the handover request, and communicates with the MSC using appropriate signaling (SIP INVITE with STN). This intermediary approach allows the system to handle PS to CS handover during alerting phase without requiring modifications to the MSC or other intermediate nodes, thus maintaining protocol compatibility while improving access quality.
3Reliability
If DRVCC transfer notification is sent to SCC-AS, then handover reliability is improved, but signaling overhead increases
Solution Approach 1:
The SIP message sent from the UE to the SCC-AS serves multiple functions: it notifies the network of the impending handover, provides the STN for routing, and triggers the MSC preparation process. This multi-functional approach improves handover reliability while minimizing signaling overhead by consolidating multiple information elements into a single message exchange.
Data Source
AI summary
Dual Radio Access Transfer of an IMS in an alerting phase for the Packet Switched to Circuit Switched direction includes initiating an IMS media session towards UE and, during a session alerting phase, making a determination that a DRVCC transfer is required from a PS to CS, access, sending a SIP message from the UE to a SCC-AS, over the PS access, informing the SCC-AS of the DRVCC transfer, and responding to receipt of the SIP message at the SCC-AS by sending a SIP INVITE from the SCC-AS towards MSC. The SIP INVITE includes an identity of the UE as a called party identity and the STN as the calling party identity. The Transfer includes sending from the MSC to the UE a CS Setup message, whereupon the MSC is placed in a state in which it expects a CS Answer from the UE.