The UE-A registers to S-CSCF according the IMS registration procedure. Note that I-CSCF and P-CSCF are not shown in this Figure.
UE-A submits the encapsulated Short Message (SMS-SUBMIT, SC Address) to the S-CSCF using an appropriate SIP method. Since recipient does not have MSISDN, the UE-A fills the TP-DA field with value that indicates recipient UE is addressed without MSISDN, and indicates recipient's SIP URI in SIP MESSAGE (e.g. UE-A sets the R-URI to PSI (SMSC address) and the To header with recipient's SIP URI in the MESSAGE). Based on the existing procedure, P-CSCF fills the UE-A's default URI to p-asserted-identity before sending the MESSAGE to S-CSCF.
S-CSCF forwards the encapsulated Short Message (SMS-SUBMIT, SC Address) to IP-SM-GW (AS) based on stored iFC.
IP-SM-GW (AS) acknowledges the SIP message.
SIP message acknowledge is forwarded by S-CSCF to UE-A.
The IP-SM-GW performs service authorization based on the stored subscriber data as described in the clause 6.1
. The IP SM GW shall check whether the subscriber is authorised to use the short message service (e.g. Operator Determined Barring settings), similar to the authorization performed by MSC/SGSN in case the Short Message is delivered via CS or PS domain. In addition, the IP-SM-GW shall also check whether the user is authorised to use the encapsulated Short Message delivery via IMS. If the result of service authorization is negative, the IP-SM-GW shall not forward the message, and shall return the appropriate error information to the UE in a failure report. Otherwise, the IP-SM-GW (AS) extracts the Short Message (SMS-SUBMIT) and constructs the SMS-DELIVER and sends it towards the terminating IMS network.
The terminating IMS network tries to deliver the SM toward the designated recipient (see clause 6.4a
Terminating IMS network response with one of the following:
Terminating IMS network refuses or does not allow SMS delivery with MSISDN-less operation by sending back a permanent error.
Terminating IMS network is not able to deliver the SM to designated recipient and returns a MSISDN-less correlation ID to allow retries at a later time. IP-SM-GW then forwards the SM to SMSC for storage and forward operation (see clause 6.3a.2
Terminating IMS network is successfully deliver the SM toward the designated recipient.
Proper response as determined in step 8a, 8b, or 8c is returned back to UE-A. For 8a, the response indicates to the UE-A that SMS to this recipient is failed permanently. For 8b, the response is the Submit report received from the SMSC. For 8c, the response indicates a successful delivery of the SM to the recipient.