5.1.5 Unsuccessful Mobile Terminated short message transfer without SMS Router/IP-SM-GW

23.5403GPP5G System: Technical realization of Service Based Short Message ServiceRelease 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

Figure 5.1.5-1: Unsuccessful MT SMS over NAS without SMS Router/ IP-SM-GW

1. MT SMS interaction between SC and SMS-GMSC follow the current procedure as defined in 3GPP TS 23.040 [2].

2a-2b. The SMS-GMSC should query the NRF to find the UDM instance(s), supporting SMS SBI interfaces, and managing that manages the user subscriptions of the GPSI. If there is no available UDM returned from NRF, the SMS-GMSC may send error response to the SC.

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.

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.

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.

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 3GPP TS 23.502 [4].

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.

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.

9. UDM responses Nudm_ReportSMDeliveryStatus_Request response to SMS-GMSC.

10. The SMS-GMSC sends the failure report to SC as defined in TS 23.040 [2].

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 3GPP TS 23.502 [4].

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.