23.2043GPPRelease 17Stage 2Support of Short Message Service (SMS) over generic 3GPP Internet Protocol (IP) accessTS
The interaction between transport-level interworking (between SMS over CS/PS and SMS over IMS) and service-level interworking (between Instant Messaging and SMS) depends on the user subscription and authorisation, on the UE capabilities, and on operator policy.
If a user is only subscribed to either transport-level interworking or service-level interworking, only procedures defined for the subscribed interworking may be performed.
If a user is subscribed to both transport-level interworking and service-level interworking, but the user is only authorised for one of the interworking when the message is processed, only the authorised interworking may be performed.
If a user is subscribed to both transport-level interworking and service-level interworking, and is authorised for both, the behaviour of the IP-SM-GW depends on the specific scenario, on the registered capabilities of the UE, and finally is defined by operator policy and user preferences.
For a user subscribed to service-level interworking, two Application Servers in the network are normally called upon to handle an Instant Message:
– the IM AS, defined in OMA-TS-SIMPLE_IM-V1_0  or the CPM AS, defined in OMA-TS-CPM_Conv_Fnct-V1_0 .
– the IP-SM-GW.
The following clauses describe the different interaction scenarios.
6.8.2 IMS Originating
In the originating network, a UE sends a SIP MESSAGE (Encapsulated Short Message or Instant Message). The originating S‑CSCF forwards the SIP MESSAGE to the IP-SM-GW based on the iFC. The subscription of the transport level interworking and the service level interworking applies for different iFC. However, the SIP MESSAGE is forwarded to the IP-SM-GW if the user subscribes to one of the interworking services. If there is no subscription for the interworking service, the S‑CSCF continues with the subsequent iFC check. After all the originating iFC triggers have been handled, the S‑CSCF attempts to route the message to the terminating IMS network. If it fails, an error is returned to the sender.
NOTE 1: if an IM AS or a CPM AS is present in the network, Instant Messages are routed to it before going to the IP-SM-GW.
NOTE 2: An encapsulated Short Message uses the PSI of the SC as the Request-URI. If the user is not subscribed to transport-level interworking and the IP-SM-GW is not invoked, the ENUM query fails, and an error is returned to the sender.
When the IP-SM-GW receives the SIP MESSAGE, it shall decide which interworking should be performed based on the content of the received SIP MESSAGE, as the IP-SM-GW can distinguish between an encapsulated Short Message and an Instant Message. If an encapsulated Short Message is received and if the subscriber is authorised for the service, the IP-SM-GW maps the encapsulated Short Message to a Short Message. Similarly, when an Instant Message is received, the IP-SM-GW considers performing the service-level interworking if the service is authorized: the IP‑SM‑GW shall decide whether to send the SIP MESSAGE via interworking service based on SIP request header (e.g., R‑URI), operator policy, when the Instant Message is not routeable in the IMS or when the IP-SM-GW is selected to handle the message.
Figure 220.127.116.11: Performing interworking service on originating side
6.8.3 IMS Terminating
When the IP-SM-GW receives a Short Message from the legacy network on the terminating side, it performs the domain selection to determine the preferred domain to transfer the short message. If the selected network is IMS, the IP-SM-GW will determine whether the transport level interworking or the service level interworking is to be preformed based on the users’ subscription and authorisation, and on the UE capability as indicated during IMS registration. If the user has subscribed to both services, is authorised for both and the UE has indicated its capability to receive both encapsulated Short Messages and Instant messages, the priority between the transport-level interworking and the service-level interworking is based on operator policy and user preferences.
NOTE 1: If the incoming Short Message is interworked to an Instant Message, the resulting Instant Message could be routed to the IM AS or CPM AS before being sent to the UE.
Figure 18.104.22.168: Performing interworking service on terminating side for an incoming Short Message
When the IP-SM-GW receives an Instant Message, 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 may perform service-level interworking to transform the message format to SMS and deliver the message to the UE. 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.