Cellular IMS Call Setup Using Silent P-CSCF Fallback
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Solution Overview
Problem
Voice call setup failures in 5G SA IMS networks due to network errors, such as HTTP 500 series internal server errors, result in immediate call drops, degrading user experience.
Innovation Solution
A fallback mechanism using a second Proxy-Call Session Control Function (P-CSCF) address for silent redial attempts, allowing the user equipment device to silently de-register from the first P-CSCF, register on the second P-CSCF, and send a second SIP INVITE request without alerting the user, continuing the call attempt until successful.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the UE uses a single P-CSCF for call setup, then the call setup process is simple, but the call drops immediately when network errors occur
Solution Approach 1:
The network pre-configures multiple P-CSCF addresses in the IMS registration phase. When a call setup fails on the primary P-CSCF, the UE can immediately switch to a pre-configured alternative P-CSCF without additional registration steps, thereby improving call reliability while maintaining relatively simple call setup procedures
Solution Approach 2:
The system prepares backup P-CSCF addresses in advance during IMS registration. This cushioning mechanism ensures that when network errors occur during call setup, the UE has alternative paths available, preventing call drops and improving overall call success rates
2Reliability
If the UE performs silent redial by switching P-CSCFs, then call success rate improves, but the device complexity increases
Solution Approach 1:
The UE autonomously monitors call setup responses and independently executes the fallback mechanism by switching to alternative P-CSCF addresses when errors are detected. This self-service approach improves call success rates without requiring complex network-controlled fallback procedures
Solution Approach 2:
The UE changes the P-CSCF address parameter dynamically based on call setup success or failure. This parameter change mechanism allows the system to improve reliability through simple address switching rather than complex procedural changes
3Loss of information
If the UE alerts the user during fallback attempts, then the user is informed of the issue, but the user experience degrades
Solution Approach 1:
The UE performs the complete fallback mechanism silently without user notification, achieving sufficient error handling through automated retry attempts. This partial action approach maintains seamless user experience while still addressing the information need by attempting multiple fallbacks before potentially notifying the user
Data Source
AI summary
Disclosed is a fallback mechanism and a silent redial technique using a second Proxy-Call Session Control Function (P-CSCF) address and third P-CSCF (or one or more other additional broadcasted P-CSCF addresses) to overcome voice call setup failures due to network error (e.g., an error indicated by a received Hypertext Transfer Protocol (HTTP) 500 series internal server error response). The user equipment device (UE) attempting to use one or more of the additional P-CSCFs as a fallback P-CSCF is performed silently such as not to alert the user. This is accomplished by silently de-registering from the first P-CSCF, registering on the second P-CSCF operating at a same generation of mobile network technology standard as the first P-CSCF, and sending a second SIP INVITE request to the second P-CSCF (i.e., performing a “silent redial”). As part of this “silent redial” process, the call screen continues to be displayed on the UE until the call is successfully established using the first P-CSCF or one of the additional fallback P-CSCFs, or the call attempt is ended, such as when the UE fails to establish the call using any of the additional P-CSCFs.