Alternative MME CS Paging for MT Call Recovery
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Solution Overview
Problem
The 3GPP standardized MSC/VLR initiated SGs restoration procedure for Mobile Terminated calls in cellular systems often results in destruction of ongoing PS service and hanging PDN connections due to communication failures, especially when the serving MME is not reachable, leading to failed CSFB services and service disruptions.
Innovation Solution
A method where an alternative MME fails to forward a paging request to the serving MME and instead pages the UE using a CN domain equal CS paging, utilizing a UE Context Replica's S-TMSI and TAI-list for limited paging scope, and triggers a recover scenario by sending an SGsAP Location Update message to the MSC/VLR, ensuring continued processing of MT calls or SMS even if the serving MME is unavailable.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the MSC/VLR sends a paging request to an alternative MME using PS domain paging, then the UE can be reached, but the ongoing PS service is destroyed and PDN connections are deleted
Solution Approach 1:
The patent changes the paging domain parameter from PS to CS. The alternative MME sends paging requests with domain set to CS instead of PS, which allows the UE to be paged without triggering PS service destruction or PDN connection deletion, thus resolving the contradiction between reliable MT call delivery and PS service continuity
Solution Approach 2:
Instead of using the conventional PS domain paging approach, the patent inverts the approach by using CS domain paging. This inversion allows the paging function to work correctly without the harmful side effects of PS service destruction, as CS paging does not trigger the same service destruction mechanisms
2Object-generated harmful factors
If the alternative MME forwards paging request to serving MME, then PS service is maintained, but the paging fails when serving MME is not reachable
Solution Approach 1:
The alternative MME acts as an intermediary when the serving MME is unreachable. Instead of forwarding the paging request to the unreachable serving MME, the alternative MME directly pages the UE using CS domain paging, mediating between the MSC/VLR and the UE while avoiding the unreachable serving MME
Solution Approach 2:
The patent implements preliminary detection of serving MME reachability. Before attempting to forward paging requests, the alternative MME checks whether the serving MME is reachable, and if not, takes preliminary action by directly paging the UE using CS domain paging to ensure delivery reliability
3Reliability
If IMSI paging is performed with CN domain set to PS, then the UE can be paged, but UEs in connected mode do not handle the paging message and CSFB service cannot be restored
Solution Approach 1:
The patent changes the CN domain parameter in paging messages from PS to CS. When UEs in connected mode receive CS domain paging messages, they properly handle them and can respond with service requests, enabling CSFB service restoration whereas PS domain paging is ignored by connected mode UEs
Data Source
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AI summary
It is presented a method for error handling during a Mobile Terminated, MT, call to a User Equipment, UE (1), in a cellular system. The method is performed by a network device (20) being an alternative Mobility Management Entity, MME (10) of the cellular system. The method comprises the steps of: failing (40) to forward a paging request to a serving MME (11), at which the UE (1) is registered; and paging (41, 42, 43, 45) the UE (1) using a Circuit Switched, CS, paging. A corresponding network device, User Equipment, computer program and computer program product are also presented.