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.
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 or may be received from HSS via the Nudm_UECM service for IP-SM-GW registration, containing the registration data provided by the IP-SM-GW.
The UDM responds to the SMS-GMSC by sending Nudm_UECM_SendRoutingInfoForSM response, including IP-SM-GW address, the indication for SMSF SMS_SBI support and the indication for IP-SM-GW SMS_SBI support.
The SMS-GMSC forwards the SMS message to the IP-SM-GW, and then 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. 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.
The SMS-GMSC selects protocol based on the indication for SMSF SMS_SBI support and IP-SM-GW SBI support:
If both SMSF and IP-SM-GW indicate support for SMS_SBI, SMS-GMSC discovers the appropriate service instance(s) of the target IP-SM-GW and forwards the SMS message to the IP-SM-GW by invoking Nipsmgw_SMService_MtForwardSm service operation. And then the IP-SM-GW forwards the SMS message to the SMSF by invoking Nsmsf_SMService_MtForwardSm service operation.
If SMSF or IP-SM-GW indicates that it does not support SMS_SBI, SMS-GMSC should forward SMS message to IP-SM-GW by legacy MAP/Diameter protocol. Then IP-SM-GW forwards the SMS message to the SMSF by legacy MAP/Diameter protocol. The following steps follow the procedures for legacy MT SMS message transfer, as illustrated in Figure 15aa of TS 23.040.
The SMS-GMSC may report the SM-Delivery Status to the UDM by invoking Nudm_ReportSMDeliveryStatus_Request and the UDM shall ignore the information provided in this report.