Content for  TS 23.540  Word version:  18.3.0

MT SMS interaction between SC and SMS-GMSC follow the current procedure as defined in TS 23.040.

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.

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.

SMS-GMSC invokes Nudm_UECM_SendRoutingInfoForSM (GPSI) to the UDM to get the serving node information for UE.

The UDM shall check the registration/reachability flags to determine the potential target nodes, e.g. SMSF. For MT SM transfer via IP-SM-GW, the UDM shall invoke the Nipsmgw_SMService_RoutingInfo to provide the SMSF Instance Id to the IP-SM-GW. The address of the IP-SM-GW to be contacted by the UDM may be configured locally.

If any failure at this step, the IP-SM-GW shall send Nipsmgw_SMService_RoutingInfo response with error cause to the UDM.

If the UDM receives error response from IP-SM-GW in step 5 or UDM is failed after step 3, 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 14.

If successful response is returned in step 6, the SMS-GMSC forwards the SMS message to the IP-SM-GW by invoking Nipsmgw_SMService_MtForwardSm service operation.

The IP-SM-GW performs service authorization and domain selection to determine the domain for delivery of the Short Message as defined in TS 23.204. If the SMSF is selected, the IP-SM-GW forwards the SMS message to the SMSF by invoking Nsmsf_SMService_MtForwardSm service operation. If the IP-SM-GW has more than one SMSF address to use for SMS transport towards the UE, then the IP-SM-GW chooses which SMSF address to use first based on operator local policy.

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.

If the AMF informs the SMSF that it cannot deliver the MT-SMS to the UE in step 9, 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 IP-SM-GW.

If the IP-SM-GW receives error response from SMSF in step 10 and the IP-SM-GW has tried all selectable domains and accesses or the IP-SM-GW is failed after step 7, the IP-SM-GW shall send the Nipsmgw_SMService_MtForwardSm response with error cause to the SMS-GMSC.

The IP-SM-GW may report the SM-Delivery Status (e.g. UE is not reachable or memory capacity exceeded) to UDM by invoking Nudm_ReportSMDeliveryStatus_Request.

The IP-SM-GW subscribes in HSS to be notified when the UE becomes reachable again, and subsequently the HSS subscribes in the UDM to UE reachability for SMS over IP event, as defined in clause 5.5.6 of TS 23.632. 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 SMS-GMSC may report the SM-Delivery Status to UDM by invoking Nudm_ReportSMDeliveryStatus_Request and the UDM shall ignore the information provided in this report.

If the SMS-GMSC receives error response from IP-SM-GW in step 6 or step 11, the SMS-GMSC sends the failure report to SC as defined in TS 23.040.