Section: 4.2.5.3 UE Activity Notification procedure
Content: The UE Activity Notification procedure is illustrated in figure 4.2.5.3-1. Figure 4.2.5.3-1: UE Activity Procedure 0. Event has been subscribed in the AMF for UE reachability for DL traffic or for UE reachability status change. 1a. For a UE in CM-IDLE, the AMF receives (N1) NAS signalling implying UE is reachable for DL traffic, e.g. a Registration Request or Service Request message from the UE, the AMF performs step 2; 1b. For a UE in CM-CONNECTED, if the AMF has initiated the N2 Notification procedure in Step 4 of clause 4.2.5.2 and when the AMF receives a (N2) UE Notification (see clause 4.8.3) or a (N2) Path Switch Request (see clause 4.9.1.2) implying UE is reachable for DL traffic from the NG-RAN, the AMF performs step 2. Otherwise, i.e. UE is in CM-CONNECTED and AMF has not initiated N2 Notification procedure, the AMF performs step 2; or 1c. The UE's reachability state changes from reachable to unreachable, then AMF performs step 2. 2a. For event subscription of "UE reachable for DL traffic", if the AMF has an MM context for the UE and the URRP-AMF information flag associated with the subscribing NF is set to report once that the UE is reachable for DL traffic, the AMF initiates the Namf_EventExposure_Notify service operation (SUPI, UE Reachable) message (or Nudm_UECM_Registration service operation when applicable) to the UDM following step 1a or step 1b. The AMF clears the corresponding URRP-AMF information if applicable for the UE. 2a1. When the UDM receives the Namf_EventExposure_Notify service operation (SUPI, UE-Reachable) message or Nudm_UECM_Registration service from AMF for a UE that has URRP-AMF information flag set in the UDM, it triggers appropriate notifications to the service-related entities associated with the URRP-AMF information flag that have subscribed to the UDM for UE Reachability notifications. If SMSF is registered, it also triggers appropriate notifications to the service-related entities associated with the URRP-AMF information flag that have subscribed to the UDM for UE reachability for SMS delivery notification (e.g. SMS-GMSC, HSS). UDM clears the URRP-AMF information for the UE. If no SMSF is registered and there are service-related entities subscribed to the UDM for the UE reachability for SMS delivery notification, the UDM clears the URRP-AMF information for the UE but does not notify any service-related entity. When the UDM receives the Nudm_UECM_Registration service from SMSF for a UE that has service-related entities subscribed to the UDM for the UE reachability for SMS delivery notification and no URRP-AMF flag set in the UDM, the UDM triggers appropriate notifications to the service-related entities that have subscribed to the UDM for UE reachability for SMS delivery notification). NOTE: The UE Reachability Notification is sent by the UDM using different interfaces/services depending on the service-related entity. For example, an SBI capable service-related entity can receive the notification using the Nudm_EventExposure_Notify service operation (if previously subscribed) while an SMS-SC can get the notification as described in TS 23.040[ Technical realization of the Short Message Service (SMS) ]  [7] based on the SC address stored in the MWD list. 2b. If in step 0 the AMF received Namf_EventExposure_Subscribe_service operation directly from an NF authorised to receive direct notifications in the case of UE reachability status change, or the UDM indicated that the notification needs to be sent directly to the NF in the case of UE reachability for DL traffic, the AMF initiates the Namf_EventExposure_Notify service operation (SUPI, UE reachability state) message directly to the NF.