23.2043GPPRelease 17Stage 2Support of Short Message Service (SMS) over generic 3GPP Internet Protocol (IP) accessTS
This procedure describes the delivery of an Instant Message to a registered or an un-registered IMS subscriber. For the unregistered case, the S‑CSCF forwards the Instant Message to the IP‑SM‑GW based on the unregistered iFC of the subscriber.
Figure 6.13: Successful IM terminating to SMS procedure with Interworking in the Terminating Side
1) UE submits an Instant Message, destined to another IM user or CPM user in another IMS domain, using an appropriate SIP method. The UE may request to hide its Public User Identity from the recipient within the Instant Message, as described in OMA‑TS‑SIMPLE_IM‑V1_0  and in OMA-TS-CPM_Conv_Fnct-V1_0 .
2) The S-CSCF resolves the destination domain and routes the message towards the S-CSCF in the terminating network ("Terminating S-CSCF").
3) The terminating S-CSCF forwards the Instant Message to the IM AS ("Terminating IM AS") or to the CPM AS ("Terminating CPM AS") based on stored iFC.
NOTE: Depending on iFC configuration, it is possible that the IM AS or CPM AS is not triggered for the unregistered subscribers.
4) The terminating IM AS or CPM AS invokes terminating IM or CPM services as applicable for the destination IM or CPM user.
5a) The IM AS or CPM AS can forward the Instant Message back to the terminating S-CSCF, e.g. the terminating IM user is offline or the CPM user has no terminating interworking user preferences.
5b) In the case of the CPM AS, if terminating interworking user preferences are set, the Instant Message is routed directly to the IP-SM-GW in which case the procedure continues with step 7.
6) The terminating S-CSCF forwards the Instant Message to the IP-SM-GW, e.g. based on stored iFC.
7) If the user is authorized, the IP-SM-GW performs service-level interworking by converting the Instant Message to Short Message. The IP-SM-GW shall obtain the routeing information for the UE from the HLR/HSS and deliver the message to the UE. If the sender of the Instant Message requests to hide its Public User Identity from the recipient and operator policy allows for this, the IP‑SM‑GW shall anonymise the identity of the user to the recipient. Otherwise, if operator policy prohibits this, the IP‑SM‑GW shall return an appropriate error to the user.
8) The IP-SM-GW obtains the routeing information for the destination UE from the HLR/HSS.
9) The IP-SM-GW sends the Forward Short Message message to the target MSC/SGSN/MME/SMSF.
10) The MSC/SGSN/MME/SMSF sends the Short Message to the UE.
11) The UE acknowledges the receipt of the Short Message.
12) The MSC/SGSN/MME/SMSF sends a Delivery report (SMS-DELIVER-REPORT) to the IP-SM-GW.
13) The IP-SM-GW sends OK response to the terminating S-CSCF.
14) The S-CSCF forwards the OK to the terminating IM AS or CPM AS.
15) The terminating IM AS or CPM AS forwards the OK response back to the terminating S-CSCF.
16) The terminating S-CSCF forwards the OK back towards the originating S-CSCF.
17) The originating S-CSCF forwards the OK to the originating UE.