Section: 5.31.7.5 MICO mode and Periodic Registration Timer Control
Content: If the Expected UE Behaviour indicates the absence of DL communication, the AMF may allow MICO mode for the UE and allocate a large periodic registration timer value based on e.g. Network Configuration parameters to the UE so that the UE can maximise power saving between Periodic Registration Updates. If the Expected UE Behaviour indicates scheduled DL communication the AMF should allow MICO mode for the UE and allocate a periodic registration timer value such that the UE performs Periodic Registration Update to renegotiate MICO mode before or at the scheduled DL communication time, if the AMF decides to allow MICO mode for the UE. When UE requests the MICO mode with active time, the UE may also request a periodic registration timer value suitable for the latency/responsiveness of the DL communication service known to UE. If the UE wants to change the periodic registration timer value, e.g. when the conditions are changed in the UE, the UE consequently requests the value it wants in the registration procedure. The AMF takes the UE requested periodic registration time value into consideration when providing the periodic registration timer to UE during Registration procedure as specified in clause 4.2.2.2.2 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. If the UE supports 'Strictly Periodic Registration Timer Indication', the UE indicates its capability of supporting 'Strictly Periodic Registration Timer Indication' in the Registration Request message. If the UE indicates its support of 'Strictly Periodic Registration Timer Indication' in the Registration Request message, the AMF may provide a Strictly Periodic Registration Timer Indication to the UE together with the periodic registration timer value, e.g. based on Expected UE Behaviour. If the indication is provided by the AMF, the UE and the AMF shall start the periodic registration timer after completion of the Registration procedure. The UE and the AMF shall neither stop nor restart the periodic registration timer when the UE enters CM-CONNECTED, and shall keep it running while in CM-CONNECTED state and after returning to CM-IDLE state. If and only when the timer expires and the UE is in CM-IDLE, the UE shall perform a Periodic Registration Update. If the timer expires and the UE is in CM-CONNECTED state, the AMF and the UE restart the periodic registration timer while still applying 'Strictly Periodic Registration Timer Indication'. The AMF may use the UE Configuration Update procedure to trigger the UE to perform Registration procedure, in which the periodic registration timer value and 'Strictly Periodic Registration Timer Indication' can be renegotiated. When the UE and the AMF locally disable MICO mode (e.g. when an emergency service is initiated), the UE and the AMF shall not apply 'Strictly Periodic Registration Timer Indication'. If the periodic registration timer is renegotiated during a Registration procedure, e.g. triggered by UE Configuration Update, and if the periodic registration timer is running, then the periodic registration timer is stopped and restarted using the renegotiated value even when the Strictly Periodic Registration Timer Indication was provided to the UE.