4.15.4 Core Network Internal Event Exposure

23.5023GPPProcedures for the 5G System (5GS)Release 18TS

4.15.4.1 General

The exposure of events internally within the 5GC NFs is explained in the following clauses. Only the event notifications that are independent of the ongoing system procedure are specified in this clause. For the event notifications that are part of the system procedure, see the system procedure descriptions under clause 4.2 to clause 4.14.

4.15.4.2 Exposure of Mobility Events from AMF

The AMF invokes the Namf_EventExposure_Notify to provide mobility related events to NF consumers that have subscribed for the events by invoking Namf_EventExposure_Subscribe, in the following scenarios listed below and after Namf_EventExposure_Subscribe service operation.

– During Registration procedure, Inter NG-RAN node N2 based handover procedure, when there is a change of AMF (within the same AMF Set or across the AMF Set), the new AMF receives all event subscriptions from old AMF or UDSF. For each event subscription:

if the event subscription only applies to the UE, the new AMF allocates a new Subscription Correlation ID and notify the NF consumer of the new Subscription Correlation ID associated with the change of Subscription Correlation ID event.

if the event subscription applies to a group of UE(s) and there is no corresponding subscription for this group (identified by the internal group Id and notification endpoint) at the new AMF, the new AMF shall create corresponding event subscription, allocate a new Subscription Correlation Id and send it to the received notification endpoint, i.e. Notification Target Address (+Notification Correlation Id), associated with the addition of Subscription Correlation ID event. The new AMF does not need to allocate another Subscription Correlation ID for any subsequent registrations of the members of the same group. The initial Maximum number of reports and the remaining number of reports within the Maximum number of reports quota for the UE is transferred from the old AMF.

NOTE: For group monitoring, even if the remaining number of reports is zero, indicating that reporting for the group event has been completed by the old AMF for this UE, the new AMF uses the received information related to initial event subscription to create the corresponding group event subscription.

– During Registration procedure, when there is a change of AMF, the new AMF notifies each NF that has subscribed for UE reachability event about the UE reachability status.

– During Registration, Handover, UE Triggered Service Request procedure in CM-IDLE state, Location Reporting, N2 Notification and AN Release procedures, the AMF determines the UE presence in Area Of Interest (i.e. IN, OUT or UNKNOWN status ) as described in Annex D.1 and notifies the NF Consumers of the UE presence in an Area Of Interest if the NF consumers (e.g. SMF) had subscribed for this Area Of Interest and if the UE presence in Area Of Interest is different from the one reported earlier.

– During Registration and Handover procedure or during Service Area Restriction update by UDM or PCF, if the UE is moving from an Allowed Area to a Non-Allowed Area, then the AMF informs all the NF consumers (e.g. SMF), that have subscribed for UE reachability event, that the UE is reachable only for regulatory prioritized service. The SMF shall explicitly subscribe UE reachability unless the established PDU Session is related to regulatory prioritized service.

– If the AMF had notified an SMF of the UE being reachable only for regulatory prioritized service earlier, the AMF informs the NF consumers (e.g. SMF), that have subscribed for UE reachability event, that the UE is reachable if the UE enters into Allowed Area.

– During Registration procedure and Service Request procedure, if the AMF had notified an SMF earlier of the UE being unreachable and that SMF need not invoke Namf_Communication_N1N2MessageTransfer to the AMF due to DL data notifications, the AMF informs the SMF when the UE becomes reachable.

– During Registration procedure and Service Request procedure, if the AMF had notified an SMF earlier that the UE is unreachable together with an Estimated Maximum Wait time, then the AMF informs the SMF when the UE becomes reachable. When the SMF learns that the UE is reachable and:

– if the SMF performs Extended Buffering for a PDU session, the SMF sends the buffered data to the UPF and invokes the Namf_Communication_N1N2MessageTransfer service operation to the AMF to establish the User Plane(s) for the PDU Sessions, or the buffered data is delivered to the UE as per the procedure in clause 4.24.2 starting from step 2g for a PDU session using Control Plane CIoT 5GS Optimisation;

– if the UPF performs Extended Buffering for a PDU session, the SMF invokes the Namf_Communication_N1N2MessageTransfer service operation to the AMF to establish the User Plane(s) for the PDU Sessions, or the buffered data is delivered to the UE as per the procedure in clause 4.24.2 starting from step 8a for a PDU session using Control Plane CIoT 5GS Optimisation.

– If NEF had subscribed for UE reachability event notification for Extend Buffering, then the AMF informs the NEF when the UE becomes reachable. When the NEF learns that the UE is reachable, it invokes the Nsmf_NIDD_Delivery service operation of the corresponding SMF to deliver the buffered data to the UE as per the procedure in clause 4.25.5 starting from step 2 for a PDU session using Control Plane CIoT 5GS Optimisation.

– During Registration procedure, Handover without Registration procedure and Service Request procedure, if the NF consumers had subscribed for UE reachability status, the AMF notifies the UE reachability status changes.

– If the Mobile Reachable Timer expires the AMF notifies the NF consumers that have subscribed for the corresponding events that the UE is not reachable.

– If the UDM had subscribed for UE reachability event notification either to be reported to the UDM or to an NF consumer directly, then the AMF notifies the UE reachability event to the UDM or to the NF consumer as specified in clause 4.2.5.2.

– If UE’s TAC is already known by the AMF, then, the AMF notifies UE TAC to the NF consumers (e.g. to NWDAF). If UE TAC is unknown, then the AMF notifies the UE TAC when it obtained the UE TAC from the UE.

During Connection, Registration and Mobility Management procedures, the AMF may store and update the UE access behaviour trends specified in Table 4.15.4.2-1 and the UE location trends specified in Table 4.15.4.2-2. Each metrics is updated incrementally, e.g. using exponential moving average. This information is exposed to consumer NFs (e.g. NWDAF) that subscribe for the event ID "UE access behaviour trends" and/or "UE location trends", respectively, by invoking Namf_EventExposure_Subscribe.

Table 4.15.4.2-1: UE access behaviour trends exposed by AMF

Information	Description
UE Identity	SUPI
List of state transitions
> State transition type	State transition identifier: – "Access Type change to 3GPP access", – "Access Type change to non-3GPP access", – "RM state change to RM-DEREGISTERED", – "RM state change to RM-REGISTERED ", – "CM state change to CM-IDLE", – "CM state change to CM-CONNECTED", – "Handover", or – "Mobility Registration Update".
> Spacing	Average and variance of the time interval separating two consecutive occurrences of the state transition.
> Duration	Average and variance of duration in the resulting state.

Table 4.15.4.2-2: UE location trends

Information	Description
UE Identity	SUPI
List of location information (1..N)
> UE Location	TAI, Cell-ID (if available), non-3GPP access identity.
> Spacing	Average and variance of the time interval separating two consecutive arrivals at this location.
> Duration	Average and variance of duration of stay in the location.
> Timestamp	Timestamp of last arrival in the location.
NOTE: The maximum size of the list (N) is defined per configuration and only the N entries with the highest average value of "Duration" are kept in the list. The list is ordered by descending order of "Duration".

4.15.4.3 Exposure of Communication trends from SMF

During Session Management procedures, the SMF may store and update the UE access behaviour trends specified in Table 4.15.4.3-1 and the UE communication trends specified in Table 4.15.4.3-2. Each metrics is updated incrementally, e.g. using exponential moving average. This information is exposed to consumer NFs (e.g. NWDAF) that subscribe for the event ID "UE session behaviour trends" and/or "UE communication trends", respectively, by invoking Nsmf_EventExposure_Subscribe.

Table 4.15.4.3-1: UE session behaviour trends exposed by SMF

Information	Description
UE Identity	SUPI
List of state transitions
> State transition type	State transition identifier: – "PDU Session Establishment", – "PDU Session Release", – "Communication failure", or – "PLMN change".
> Spacing	Average and variance of the time interval separating two consecutive occurrences of the state transition.

Table 4.15.4.3-2: UE communication trends

Information	Description
UE Identity	SUPI
List of communication information (1..N)
> Communication characteristics	S-NSSAI and DNN
> Spacing	Average and variance of the time interval separating two consecutive PDU Session Establishment procedures corresponding to the communication characteristics.
> Duration	Average and variance of duration of PDU Sessions corresponding to the communication characteristics.
> Timestamp	Timestamp of the last PDU Session Establishment procedure corresponding to the communication characteristics.
NOTE: The maximum size of the list (N) is defined per configuration and only the N entries with the highest average value of "Duration" are kept in the list. The list is ordered by descending order of "Duration".

4.15.4.4 Internal Event Exposure Subscription/Unsubscription via UDM

This clause describes an indirect method of event exposure subscription in AMF and SMF via UDM for a UE or group of UEs. This can be used after the removal of UE context in the AMF including event exposure subscriptions, or the creation of new UE context in AMF or SMF. In this case, the UDM is responsible for (re)creating event exposure subscriptions in AMF and SMF.

Figure 4.15.4.4-1: Internal Event exposure subscription/unsubscription in AMF or SMF via UDM

1. A consumer of event exposure for events detected in AMF or SMF (e.g. NWDAF) sends an Nudm_EventExposure Subscribe/Unsubscribe request to the UDM for a UE or group of UEs, including the subscription details (Event ID, Event filters, etc.).

2. UDM examines the event type and subscription details to determine whether one or more events are to be detected by the AMF. In this case, for those applicable events that are detected by the AMF, if an AMF is registered in UDM for the UE (or for a UE that is member of the group of UEs), UDM creates an Namf_EventExposure Subscribe/Unsubscribe request and sends it to the AMF of the UE, including the subscription details.

3. AMF answers with an Namf_EventExposure Subscribe/Unsubscribe response.

4. UDM examines the event type and subscription details to determine whether one or more events are to be detected by the SMF. In this case, for those applicable events that are detected by the SMF, if one or more SMFs are registered in UDM for the UE (or for a UE that is member of the group of UEs), UDM creates an Nsmf_EventExposure Subscribe/Unsubscribe request and sends it to each applicable SMF of the UE, including the subscription details.

5. SMF answers with an Nsmf_EventExposure Subscribe/Unsubscribe response.

6. UDM sends an Nudm_EventExposure Subscribe/Unsubscribe response to the consumer of event exposure.