AMF EPS Fallback Configuration for 5GS Race Condition Mitigation
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Solution Overview
Problem
In 5G networks, the EPS fallback procedure faces challenges during call setup, particularly due to race conditions between fallback and QoS establishment processes, leading to resource reservation failures when early PCRF/PCF interaction with IMS is used, especially in scenarios without SIP preconditions.
Innovation Solution
A configuration parameter per Public Land Mobile Network (PLMN) is made available to the Access Mobility Function (AMF) to indicate whether early or late PCRF/PCF interaction with IMS is used, allowing the AMF to determine EPS fallback availability and send an indication to the UE, and the SMF-PGW-C is instructed to wait for EPS fallback completion before establishing a QoS flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If early PCRF/PCF interaction with IMS is used to speed up call setup, then call setup time is reduced, but race conditions occur between fallback and QoS establishment processes causing resource reservation failures
Solution Approach 1:
The AMF determines EPS fallback availability in advance during 5GS registration and notifies the UE before the actual call setup. This preliminary determination allows the UE to prepare appropriate fallback mechanisms, ensuring that when early PCRF/PCF interaction occurs, the necessary fallback procedures are already in place to prevent resource reservation failures.
Solution Approach 2:
The system implements feedback by notifying the UE about EPS fallback availability status during registration. This feedback mechanism enables the UE to adjust its behavior accordingly - when fallback is available, it can proceed with early PCRF/PCF interaction for faster setup; when not available, it can avoid race conditions by using alternative procedures, thus maintaining resource reservation success.
2Reliability
If UE delays sending fallback request until receiving SIP 100 Trying, then race condition between fallback and QoS establishment is avoided, but call setup time increases
Solution Approach 1:
The AMF performs preliminary determination of EPS fallback availability during the registration phase, well before the actual call setup. This advance preparation eliminates the need for the UE to delay its fallback request during call setup, as the availability status is already known and configured. The UE can immediately proceed with fallback procedures when needed, achieving both fast setup and successful calls.
3Productivity
If SMF-PGW-C establishes QoS flow in parallel with EPS fallback, then call setup is faster, but resource reservation fails due to race conditions
Solution Approach 1:
The AMF acts as an intermediary that coordinates between the QoS establishment process and EPS fallback procedure. By determining fallback availability in advance and providing this information to the SMF-PGW-C, the AMF enables the SMF to synchronize QoS flow establishment with the completion of EPS fallback, avoiding race conditions while maintaining efficient parallel processing where possible.
Data Source
AI summary
A method of managing an Evolved Packet System (EPS) fallback procedure in a 5GS network. The method comprises making available to the Access Mobility Function (AMF), a configuration parameter per Public Land Mobile Network (PLMN) indicating if early or late PCRF/PCF interaction with the IMS is used. Then, upon receipt by the network of a 5GS network registration request from a UE, the home PLMN of the UE is identified as is the configuration parameter for that PLMN. The identified parameter is then used to determine for the UE whether or not EPS fallback is available to the UE, and an indication of the determination is sent to the UE.