23.2043GPPRelease 16Stage 2Support of Short Message Service (SMS) over generic 3GPP Internet Protocol (IP) accessTS
An IMS registered user with SIMPLE IM service or CPM service sends an Instant Message via service-level interworking as an encapsulated Short Message to an SMSIP UE, which did not indicate support for SIMPLE IM or CPM when registering to IMS.
NOTE 1: Based upon user subscription and depending on network deployment, other Application Servers could be processing the incoming Instant Message before the IP-SM-GW. The behaviour of the IM AS is described in OMA-TS-SIMPLE_IM-V1_0 . The behaviour of the CPM AS is described in OMA-TS-CPM_Conv_Fnct-V1_0 .
Figure 6.11: Successful UE termination Instant Message to encapsulated Short Message procedure
1) IM or CPM UE sends an Instant Message to the S-CSCF#1. 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#1 forwards the Instant Message to the S-CSCF#2.
3) The S-CSCF#2 forwards the Instant Message to the IP-SM-GW based on iFC.
4) Based on user subscription and authorisation for service-level interworking, on operator policy and user preferences, and on UE capability indicated during IMS registration, the IP-SM-GW shall decide whether to perform service-level interworking. If the user is subscribed and authorised for transport-level interworking, and based on UE capability indicated during IMS registration, and on operator policy and user preferences, the message may be delivered as an encapsulated Short Message to the UE over IMS. Otherwise, the Short Message is delivered over CS/PS, as described in clause 6.13. 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.
NOTE 2: If a delivery notification was requested in the message sent by the UE in step 1 as described in IETF IMDN RFC 5438 , the procedure for delivery report described in clause 6.12 applies. Otherwise IP-SM-GW will just acknowledge, at the SIP level, the Delivery report received from the SMSIP UE.
5) The IP-SM-GW forwards the encapsulated Short Message to the S-CSCF#2.
6) The S-CSCF#2 forwards the encapsulated Short Message to the SMSIP UE.
7) The SMSIP UE acknowledges the translated encapsulated Short Message.
8) The S-CSCF forwards the acknowledgement of the translated encapsulated Short Message to the IP-SM-GW.
9-11) The IP-SM-GW forwards the acknowledgement of the translated encapsulated Short Message to the originating IM UE or CPM UE.