5G Terminal (UE) Reachability in CM-IDLE State

Reachability management in the 5G (NR) system is responsible for detecting whether a UE is reachable and providing the UE's location (i.e., access node) so that the network can easily access the terminal (UE); this can be achieved through paging the UE and (UE) location tracking; UE location tracking includes: registration area tracking (i.e., UE registration area update) and reachability tracking (i.e., UE periodic registration area update); the reachability management function can be located in the 5GC (CM-IDLE state) or NG-RAN (CM-CONNECTED state).

I. CM-IDLE reachability is the result of negotiation between the UE and the AMF during the registration process. UE reachability in the CM-IDLE state is divided into two types:

1. UE Data Transmission Reachability

• The network determines the UE's location based on the tracking area list granularity.
• Applicable to paging procedures.
• Applicable to CM-CONNECTED and CM-IDLE states supporting mobile-initiated data and mobile terminal data.

2. MICO (Mobile Initiated Connection Only) mode:

• Applicable to CM-CONNECTED and CM-IDLE states supporting mobile-initiated data.
• Mobile terminal data is only supported when the UE is in the CM-CONNECTED state.

II.When a UE in the RM-REGISTERED state enters the CM-IDLE state, it starts a periodic registration timer based on the periodic registration timer value received from the AMF during the registration process; during this period,

• the AMF assigns a periodic registration timer value to the UE based on local policies, subscription information, and information provided by the UE. After the periodic registration timer expires, the UE should perform periodic registration. If the UE moves out of network coverage when its periodic registration timer expires, the UE should perform the registration procedure when it returns to coverage.
• The AMF runs a mobile reachability timer for the UE. When the CM state of a UE in the RM-REGISTERED state changes to CM-IDLE, this timer starts with a value greater than the UE's periodic registration timer.
• If the AMF receives elapsed time from the RAN when the RAN initiates UE context release and indicates that the UE is unreachable, the AMF should infer the mobile reachability timer value based on the elapsed time received from the RAN and the normal mobile reachability timer value.
• If the UE CM state in the AMF changes to CM-CONNECTED state, the AMF stops the mobility reachability timer. If the mobility reachability timer expires, the AMF determines that the UE is reachable.

However, the AMF does not know the duration of UE unreachability, so the AMF should not immediately deregister the UE. Instead, after the mobility reachability timer expires, the AMF should clear the PPF (Paging Proceed Flag) and start an implicit deregistration timer, which should have a relatively large value.

III.CM-CONNECTED: If the UE CM state in the AMF changes to CM-CONNECTED state, the AMF should stop the implicit deregistration timer and set the PPF (If the UE CM state in the AMF is CM-IDLE, and the UE is in MICO mode - see Section 5.4.1.3, the AMF considers the UE to be always unreachable).

• If the PPF is not set, the AMF will not page the UE and should reject any requests to send downlink signaling or data to that UE.
• If the implicit deregistration timer expires before the UE contacts the network, the AMF implicitly deregisters the UE.

As part of a specific access (3GPP or non-3GPP) deregistration, the AMF should request the relevant SMF of the UE to release the PDU sessions established on that access.