23.2043GPPRelease 17Stage 2Support of Short Message Service (SMS) over generic 3GPP Internet Protocol (IP) accessTS
When a Short Message is received in the IP-SM-GW for delivery to an IMS subscriber, the IP-SM-GW shall verify the registration status of the UE. If the UE is not registered in IMS, or is registered in IMS but does not advertise the Converged IP Messaging, SIMPLE IM or SMSIP capability, the Short Message shall not be interworked; neither at service level nor at transport level. Based on operator policy and user preferences, either the message is sent over CS/PS or an error indication is sent back to the SMS-SC. In the latter case, when the UE registers in IMS advertising the Converged IP Messaging, SIMPLE IM and/or the SMSIP capability at a later time, this information is sent to the SMS-SC and the delivery is attempted at that time, as an Instant Message or an encapsulated Short Message as appropriate.
NOTE: The service level or transport level interworking of a message is prohibited as identified in the above scenario in order to prevent the possibility of the message being deferred in the terminating network.
If the IP-SM-GW detects the UE is reachable for SMS over IP and the UE is registered with support of Converged IP Messaging, SIMPLE IM or SMSIP capability, the IP-SM-GW clears local UNRI and informs HLR/HSS that the UE is available again.
If the HLR/HSS has recorded the MWD with a failure reason that the message failed to be sent by IP‑SM‑GW due to the UE not being available, once the HLR/HSS receives a message from any of the domains indicating that the UE is available again, e.g. IMSI attached, or IMS registered, the HLR/HSS initiates an Alert service centre procedure to request the SMS-SC to re-send the stored message.
The following figure shows an example of how a deferred message is re-transmitted to an IMS UE upon the UE availability.
Figure 6.5b: Alert service centre procedure when UE is available
1) The message is transmitted from SMS‑SC to IP‑SM‑GW for delivery to the subscriber, possibly after transport-level and/or service-level interworking. Prior to this interworking, the IP‑SM‑GW shall check for UE availability. If the UE is not registered in IMS, and delivery over CS/PS is unsuccessful (see clause 6.5a), the IP‑SM‑GW returns an appropriate error response to SMS‑SC. The SMS-SC then informs the HSS/HLR about the unavailability of the UE. After an unsuccessful SM termination procedure due to the UE being unavailable, the HSS records the MWD i.e. the SMS‑SC address which stores the un-delivered message and the failure reason which indicates that the message failed to be sent by IP‑SM‑GW due to the UE not being available, for a subsequent Alert service centre procedure.
At any time after the unsuccessful SM termination procedure, the UE may attach in the PS and or CS domain again, in which case a Ready for SM message from the SGSN, MSC, MME or SMSF is sent to the HLR/HSS as described in TS 23.040 . The HLR/HSS initiates an Alert service centre procedure to the SM‑IWMSC when the user’s MWD is not NULL, and the procedure continues as described in step 3.
2) At any time after the unsuccessful SM termination procedure, the status of the UE may indicate that the UE is available due to, e.g. registration in IMS (step 2a). At that point UE-Not-Reachable-for-IP (UNRI) is updated in HLR/HSS, as described in TS 23.040 . After the IMS registration is finished, the procedure continues as described in step 3.
At any time after the unsuccessful SM termination procedure, the HSS can receive a notification from the transport layer e.g. MME that the UE is reachable again (step 2b). As the IP-SM-GW has subscribed to the event as described in clause 6.5a, the HSS shall notify the IP-SM-GW of the UE being reachable again (step 2c). If the UE is already registered in IMS, the IP-SM-GW shall then send a Ready for SM message to the HLR/HSS (step 2d) and the procedure continues as described in step 3. Otherwise, the IP-SM-GW discards the notification message. The procedure will resume when one of the conditions described in steps 1 and 2 is fulfilled.
3) The HLR/HSS checks the user’s MWD. If MWD is not Null, the HLR/HSS initiates an Alert service centre message to the SMS‑IWMSC.
4) The SMS‑IWMSC forwards the Alert service centre procedure to the responding SMS‑SC.
5) Upon receipt of the Alert service centre message, the SMS‑SC re-attempts to send the stored Short Message. The message is transmitted to IP-SM-GW and thereafter to the UE after appropriate interworking (transport-level and/or service-level interworking) is performed. The UE acknowledges the reception of the message.