MSC Relay for CSFB Call Termination in IMS Networks
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Solution Overview
Problem
Current solutions for Mobile Terminating Roaming Forwarding (MTRF) do not effectively handle terminating calls for IMS Centralized Services (ICS) users, particularly when the MSC is enhanced for ICS, due to the lack of congruency between Tracking Areas and Location Areas, leading to failed call terminations in CSFB scenarios.
Innovation Solution
A method and system for controlling the setup of terminating calls from IMS to UE in a CSFB situation, where the MSC supports IMS Centralized Services, involves paging the UE, receiving a Send Identification message from a further MSC, forwarding the call setup request, relaying ongoing call messages, and triggering de-registration from IMS when all calls end, ensuring successful call delivery and registration management.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the UE falls back from E-UTRAN to GERAN or UTRAN in a CSFB situation, then voice and CS domain services can be provided, but the UE may land in a LA not controlled by the original MSC-S, causing call termination failure
Solution Approach 1:
The patent introduces an MSC-S enhanced for ICS as an intermediary that maintains the UE's IMS registration context when the UE falls back to a different MSC area. This enhanced MSC-S acts as a mediator that can relay calls between the UE and the IMS network even when the UE is served by a different MSC-S, thus resolving the call termination failure issue without requiring the UE to remain in the original MSC area.
Solution Approach 2:
The patent segments the call handling function by separating the IMS registration management (handled by the enhanced MSC-S) from the actual radio access (handled by E-UTRAN, GERAN, or UTRAN). This allows the UE to move between different radio access networks and MSC areas while the enhanced MSC-S maintains the IMS registration context, enabling reliable call termination regardless of the current serving MSC.
2Reliability
If the MSC is enhanced for ICS to support seamless CSFB, then call delivery succeeds, but additional signaling and resource usage are required
Solution Approach 1:
The enhanced MSC-S is designed with multi-functionality, serving both as a traditional MSC-S for CS domain services and as an IMS registration manager for ICS subscribers. This universal design allows the same MSC-S to handle both CSFB scenarios and IMS centralized services, reducing the need for separate dedicated infrastructure and minimizing additional complexity.
Solution Approach 2:
The enhanced MSC-S automatically detects when a subscriber is an ICS subscriber and maintains their IMS registration context without requiring external intervention. The system self-manages the registration state and call routing, reducing the need for additional signaling from other network elements and simplifying the overall call setup process.
3Productivity
If the UE maintains IMS registration during CSFB, then seamless service continuation is enabled, but registration management complexity increases
Solution Approach 1:
The enhanced MSC-S acts as an intermediary that maintains the UE's IMS registration context during CSFB operations. Instead of requiring the UE to re-register with IMS when moving between MSC areas, the enhanced MSC-S preserves the registration state and handles call routing, thereby maintaining service continuity while simplifying the registration management burden on the UE and other network elements.
Data Source
AI summary
Techniques and apparatus are described for controlling set up of a terminating call from an IMS to a UE, in a Circuit Switched Fallback (CSFB) situation. A first MSC receives a terminating call from IMS and tries to page the UE. The UE has moved to the second MSC and performs a Location Update to the second MSC. The second MSC informs the first MSC with a Send Identification message comprising an address of the second MSC. The first MSC maintains the IMS subscription of the UE at Cancel Location and forwards the terminating call to the second MSC. The first MSC acts as relay for subsequent call control messages for calls of the UE. The first and second MSCs are supervising the ongoing calls and as soon as all calls are completed the second MSC registers the UE into IMS and the first MSC de-registers the UE from IMS.