RRC Inactive Timer Synchronization for Accurate UE Reachability
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Solution Overview
Problem
The coordination between Radio Resource Control (RRC) level and Non-Access-Stratum (NAS) protocol level periodic timers is not adequately addressed, leading to incorrect determination of UE reachability state duration, either keeping the UE wrongly reachable or unreachable by the network.
Innovation Solution
Coordination between the access network (5G RAN) and core network (5G Core, e.g., AMF) to manage the time spent in RRC inactive state, allowing the core network to start a proper NAS level timer when the UE transitions from CM-CONNECTED to CM-IDLE state, using elapsed time values and negotiated timers to determine appropriate actions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the periodic timer coordination between RRC level and NAS protocol level is not properly addressed, then the UE reachability management becomes simple, but the UE is wrongly considered reachable or unreachable by the network
Solution Approach 1:
The access network device performs preliminary action by determining the elapsed time of the periodic timer at the RRC level before the UE transitions to CM-IDLE state, and communicates this information to the core network device in advance. This allows the core network device to accurately calculate the remaining time and determine the correct NAS level timer value, ensuring reliable UE reachability management without complex coordination mechanisms during the transition.
Solution Approach 2:
The access network device acts as an intermediary by calculating and communicating the elapsed time information to the core network device. This intermediary role simplifies the overall system complexity by centralizing the timer coordination function at the access network level, while still ensuring accurate reachability determination at the core network level through the transmitted time information.
2Measurement precision
If the total time for UE to stay in reachable state is calculated incorrectly, then the timer management becomes simple, but the UE is wrongly considered reachable by the network
Solution Approach 1:
The access network device performs preliminary measurement of the elapsed time for the periodic timer at the RRC level before UE state transition. By communicating this pre-measured time information to the core network device, the system achieves precise reachable state duration measurement without requiring complex coordination mechanisms during the actual state transition and timer calculation.
Solution Approach 2:
The timer management function is segmented between the access network device and the core network device. The access network device handles the RRC level periodic timer and measures elapsed time, while the core network device handles the NAS level timer and makes reachability determinations. This segmentation allows each device to focus on specific measurement tasks, improving precision without overall system complexity.
3Productivity
If the periodic timer coordination is not addressed, then the system operation is simple, but unnecessary UE disconnection occurs
Solution Approach 1:
The access network device performs preliminary determination of the periodic timer elapsed time and communicates this information to the core network device before the UE transitions to CM-IDLE state. This preliminary action enables the core network device to accurately calculate the remaining reachable state duration and avoid unnecessary UE disconnections, thereby improving connection maintenance efficiency without requiring complex real-time coordination mechanisms.
Solution Approach 2:
The access network device provides feedback to the core network device about the elapsed time of the RRC level periodic timer. This feedback mechanism allows the core network device to adjust its timer management decisions based on actual time elapsed, preventing premature or incorrect UE disconnections while maintaining simple operational procedures through automated time-based decisions.
Data Source
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AI summary
A method performed by an AMF is provided. The method includes determining an elapsed time value (δRAN). The method further includes determining whether the elapsed time value is less than a negotiated NAS periodic timer (τNAS-per). The AMF is configured such that, as a result of the AMF determining that δRAN < τNAS-per, the AMF transitions the UE to CM-IDLE state and starts a NAS periodic timer with a value equal to τNAS-per - δRAN.