Content for TS 23.540 Word version: 19.0.0
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5.1.5 Unsuccessful Mobile Terminated short message transfer without SMS Router/IP-SM-GW p. 19
Step 1.
MT SMS interaction between SC and SMS-GMSC follow the current procedure as defined in TS 23.040.
Step 2a.
SMS-GMSC invokes the Nnrf_NFDiscovery to discover and select the UDM instance(s), supporting SMS SBI interfaces, and managing the user subscriptions of the GPSI. The SMS-GMSC may need to retrieve the PLMN ID of the recipients GPSI before the discovery of the UDM instance based on the GPSI-to-Subscription-Network resolution procedure defined in clause 5.1.7.
Step 2b.
If no UDM supporting SMS SBI could be discovered, the NRF indicates so to SMS-GMSC (by not including any UDM instance in the discovery response), and SMS-GMSC shall quit the SBI-based procedure and fallback to legacy (MAP/Diameter) protocol based procedures, as defined in TS 23.040,
or if a UDM supporting SMS SBI is discovered and selected, NRF returns the IP addresses or FQDNs of the serving UDM to provide Nudm_UECM_SendRoutingInfoForSM service to SMS-GMSC.
Step 3.
SMS-GMSC invokes Nudm_UECM_SendRoutingInfoForSM (GPSI) to the UDM to get the routing information of the nodes available for MT SMS delivery, in this case the registered serving SMSF instance for all access types for UE.
Step 4.
The UDM shall check the registration/reachability flags to determine the potential target nodes. If the UDM is failed at this step, e.g. user not found in the UDM, the UDM shall respond to the SMS-GMSC by sending Nudm_UECM_SendRoutingInfoForSM response with error cause. If there is no target node address registered in the UDM, a response with error case indicating absent subscriber for SM is sent to the SMS-GMSC and the procedure continues in step 11.
Step 5.
If successful response is returned in step 4, the SMS-GMSC forwards the SMS message to the SMSF by invoking Nsmsf_SMService_MtForwardSm service operation. If the SMS-GMSC has more than one SMSF address to use for SMS transport towards the UE, then the SMS-GMSC chooses which SMSF address to use first based on operator local policy.
Step 6.
MT SMS over NAS procedure between SMSF, AMF and UE is same as the definition in step 4a to 6b of Figure 4.13.3.6-1 of TS 23.502.
Step 7.
If the AMF informs the SMSF that it cannot deliver the MT-SMS to the UE in step 6, e.g. UE is not reachable, or the SMSF is failed at this step, e.g. memory capacity exceeded, the SMSF shall send the Nsmsf_SMService_MtForwardSm response with error cause to the SMS-GMSC.
If the SMS-GMSC has more than one SMSF address to use for SMS transport towards the UE, then upon receiving MT-SMS failure report, the SMS-GMSC, based on operator local policy, may re-attempt the MT-SMS delivery via the other SMSF. If MT-SMS delivery also fails over the other SMSF, then the SMS-GMSC continues with step 8.
Step 8.
The SMS-GMSC may report the SM-Delivery Status (e.g. UE is not reachable or memory capacity exceeded) to UDM by invoking Nudm_ReportSMDeliveryStatus_Request .
Step 9.
UDM responses Nudm_ReportSMDeliveryStatus_Request response to SMS-GMSC.
Step 10.
The SMS-GMSC sends the failure report to SC as defined in TS 23.040.
Step 11.
The SMS-GMSC subscribes in UDM to be notified when the UE becomes reachable for SMS (i.e. when the UE gets in radio contact with the AMF while an SMSF is actually registered, or when an SMSF gets registered) by using the Nudm_EventExposure_Subscribe service operation for Reachability for SMS event as defined in TS 23.502.
Step 12.
If applicable, the UDM subscribes to UE reachability notification in the AMF(s) using the Namf_EventExposure service and sets the relevant reachability flags. The UDM acknowledges the event subscription created by the SMS-GMSC.
