Content for  TS 23.540  Word version:  18.3.0

The SMS-GMSC located in the PLMN of the SMS sender contacts the NRF in the originating PLMN to perform NF/NF service discovery of the UDM instance(s). The discovery request is based on the GPSI of the SMS recipient and includes an indication for the NRF to determine the target PLMN and interface to be used (SBI or legacy interface).

Based on the indication to determine the target PLMN and interface to be used included in the discovery request, if direct routing mechanism is used the NRF in the originating PLMN retrieves the target PLMN ID from the MNPF in the originating PLMN by consuming the SBI services of the MNPF described in clause 6.7. The NRF performs an NP query to the MNPF using SBI for the GPSI of the SMS recipient. If indirect routing mechanism is used, steps 2-3 are skipped and the procedure continues in step 4a.

The MNPF provides in the response the target PLMN information corresponding to the GPSI of the SMS recipient and the procedure continues in step 4.

The NRF may, as an alternative implementation option to steps 2-3, obtain the target PLMN information by other means, e.g., using DNS/ENUM resolution, local configuration in the NRF or direct access to Number Portability (NP) databases via non-SBI, if applicable. The NRF may use the DNS/ENUM translation mechanism to resolve the GPSI of the SMS recipient in E.164 format to a URI as specified in IETF RFC 6116. The NRF performs an ENUM query for the GPSI of the SMS recipient in step 2a. The output of the lookup process in the DNS/ENUM server is a URI that is provided in the ENUM response and points to the originating PLMN or the NRF in the target PLMN with which the originating PLMN has an interconnection agreement using SBI, so that the NRF in the originating PLMN can send an inter-PLMN service discovery request to the NRF in the target PLMN.

Based on the response from the MNPF, the NRF in the originating PLMN determines the target PLMN where to search for UDM instances. If the NRF has obtained the target PLMN information by other means in steps 2a-3a, the NRF determines the target PLMN based on the information from ENUM response, NP databases via non-SBI and/or local configuration. If the GPSI belongs to the originating PLMN (i.e. originating network is the same as subscription network), the NRF searches for UDM instances matching the discovery criteria that can serve the request in the originating PLMN using SBI services and provides the discovery response in step 7. If the GPSI belongs to a different PLMN, the NRF checks whether the originating PLMN has an interconnection agreement using SBI with the subscription PLMN and, in that case, sends an inter-PLMN discovery request including the GPSI to the NRF in the subscription PLMN to retrieve the UDM instances that can serve the SMS recipient, as defined in clause 4.17.5 of TS 23.502 and TS 29.510, and the procedure continues in step 6.

If indirect routing mechanism is used (i.e., no interaction as in steps 2-3 between NRF and MNPF in the originating PLMN), the NRF in the originating PLMN determines the PLMN ID of the number range holder network (e.g. from the recipient's GPSI prefix (CC+NDC) based on local configuration) and sends an inter-PLMN discovery request of UDM instances to the NRF in the number range holder network. In this case, the discovery request across PLMNs shall include the GPSI of the SMS recipient and the indication to determine the target PLMN and interface to be used.

Based on the indication to determine the target PLMN and interface to be used included in the discovery request from the NRF in the originating PLMN, the NRF in the number range holder PLMN applies the behaviour described for the NRF in steps 2-3 (or alternatively, steps 2a-3a) and step 4. The NRF in the number range holder network may perform a query to the MNPF using SBI if NP is required and NP information has not been retrieved previously (e.g., if originating and number range holder PLMNs belong to different countries or portability domains and NP is required in the country of the recipient GPSI). The NRF in the number range holder network obtains the PLMN ID of the subscription network in the response from the MNPF (or alternatively by other means such as ENUM/NP) and, if applicable, sends an inter-PLMN discovery request of UDM instances including the GPSI to the NRF in the subscription network.

The NRF in the subscription network provides the inter-PLMN discovery response including UDM instance(s) matching the discovery criteria or no UDM instance(s) found in the target PLMN, implying that SBI interactions should not be used. When using indirect routing, the NRF in the number range holder network forwards the discovery response from the NRF in the subscription network to the NRF in the originating network.

If the NRF in the originating network finds UDM instances matching the filter criteria in the originating PLMN or receives UDM instances in the response to a discovery request across PLMNs in step 6, the NRF provides the UDM instances in the discovery response to the SMS-GMSC. If no UDM instances can serve the request using SBI, the NRF provides the discovery response indicating the SMS-GMSC to use a non-SBI interface for the next operation request in the procedure.

If the discovery response includes UDM instances that can serve the SMS recipient, the SMS-GMSC sends the operation request using SBI to e.g., retrieve the SMS routing information from the UDM, and the SBI-based MT SMS procedure can be executed as described in clause 5.1.

If no UDM instances are provided in the discovery response, the SMS-GMSC sends the operation request via legacy interface using MAP/Diameter to e.g., retrieve the SMS routing information from the HLR/HSS/UDM.