5.4.1.1 General

Reachability management is responsible for detecting whether the UE is reachable and providing UE location (i.e. access node) for the network to reach the UE. This is done by paging UE and UE location tracking. The UE location tracking includes both UE registration area tracking (i.e. UE registration area update) and UE reachability tracking ((i.e. UE periodic registration area update)). Such functionalities can be either located at 5GC (in the case of CM-IDLE state) or NG-RAN (in the case of CM-CONNECTED state).

The UE and the AMF negotiate UE reachability characteristics for CM-IDLE state during Registration procedures.

Three UE reachability categories are negotiated between UE and AMF for CM-IDLE state:

1. UE reachability allowing Mobile Terminated data while the UE is CM-IDLE state.

• The UE location is known by the network on a Tracking Area List granularity
• Paging procedures apply to this category.
• Mobile originating and mobile terminated data apply in this category for both CM-CONNECTED and CM-IDLE state.

2. Mobile Initiated Connection Only (MICO) mode:

• Mobile originated data applies in this category for both CM-CONNECTED and CM-IDLE state.
• Mobile terminated data is only supported when the UE is in CM-CONNECTED state.

3. UE unreachability due to Unavailability Period:

• Mobile originated data and Mobile terminated data are not transmitted in this category (handling of data by extending buffering may apply).
• Paging procedure is not applicable to this category.

Whenever a UE in RM-REGISTERED state enters CM-IDLE state, it starts a periodic registration timer according to the periodic registration timer value received from the AMF during a Registration procedure.

The AMF allocates a periodic registration timer value to the UE based on local policies, subscription information and information provided by the UE. After the expiry of the periodic registration timer, the UE shall perform a periodic registration. If the UE moves out of network coverage when its periodic registration timer expires, the UE shall perform a Registration procedure when it next returns to the coverage.

The AMF runs a Mobile Reachable timer for the UE. The timer is started with a value longer than the UE's periodic registration timer whenever the CM state for the UE in RM-REGISTERED state changes to CM-IDLE. If the AMF receives an elapsed time from RAN when RAN initiate UE context release indicating UE unreachable, the AMF should deduce a Mobile Reachable timer value based on the elapsed time received from RAN and the normal Mobile Reachable timer value. The AMF stops the Mobile Reachable timer, if the UE CM state in the AMF moves to CM-CONNECTED state. If the Mobile Reachable timer expires, the AMF determines that the UE is not reachable.

However, the AMF does not know for how long the UE remains not reachable, thus the AMF shall not immediately de-register the UE. Instead, after the expiry of the Mobile Reachable timer, the AMF should clear the PPF and shall start an Implicit De-registration timer, with a relatively large value. The AMF shall stop the Implicit De-registration timer and set the PPF if the AMF moves the UE CM state in the AMF to CM-CONNECTED state.

NOTE: If the UE CM state in the AMF is CM-IDLE, then AMF considers the UE always unreachable if the UE is in MICO mode (refer to clause 5.4.1.3).

If the UE indicates an Unavailability Period Duration, then AMF shall consider the UE as unreachable during the unreachability period as described in clause 5.4.1.4.

If the UE is unreachable (i.e. the PPF is not set), the AMF does not page the UE and shall reject any request for delivering DL signalling or data to this UE.

If the Implicit De-registration timer expires before the UE contacts the network, the AMF implicitly de-register the UE.

As part of deregistration for a particular access (3GPP or non-3GPP), the AMF shall request the UE's related SMF to release the PDU Sessions established on that access.