B.2.4 Short Message Service (SMS)
23.2723GPPCircuit Switched (CS) fallback in Evolved Packet System (EPS)Release 17Stage 2TS
B.2.4.1 General
The procedures for SMS in this annex apply only if the UE is 1xRTT CS Registered and the CS access domain is chosen by the UE and/or the home PLMN for delivering short messages.
This clause describes both the mobile originating and mobile terminating SMS over S102 which uses 1xCS procedures in EPS. SMS support is based on the S102 reference point between the MME and the 1xCS IWS, use of RRC Information Transfer message between the UE and the E-UTRAN, and use of S1 cdma2000 Tunnelling message between the E‑UTRAN and the MME.
B.2.4.2 Mobile originating SMS
The following sequence flow shows the delivery of mobile originating SMS sent via the 1xMSC while in E‑UTRAN.
Figure B.2.4.2-1: Mobile originating SMS sent via the 1xMSC while in E-UTRAN
1. The 1xRTT CS Registration procedure as described in clause B.2.1.1 has been performed earlier.
2. A mobile originating short message is triggered. If the UE is in idle state, the UE performs the UE triggered Service Request procedure, which is defined in TS 23.401 [2].
3. The UE builds the SMS message to be sent as defined in 3GPP2 A.S0008 [16] and 3GPP2 A.S0009 [17].
3a. The 1xRTT SMS message is transferred from the UE to the E-UTRAN.
3b. The E-UTRAN forwards the SMS message to the MME.
4. The MME forwards the SMS message to the 1xCS IWS in an S102 Direct Transfer message.
5. The 1xCS IWS acknowledges the message.
6. The 1xCS IWS sends an ADDS Transfer message to the 1xMSC containing the SMS message as defined in 3GPP2 A.S0008 [16] and 3GPP2 A.S0009 [17].
7. The 1xMSC forwards the SMS message to the Message Centre (MC). If an acknowledgement was requested by the UE, the MC responds with an acknowledgement.
8. The 1xMSC forwards the SMS acknowledgement to the 1xCS IWS in an ADDS Page message.
9. The 1xCS IWS forwards the SMS acknowledgement to the MME in an S102 Direct Transfer message.
10. The MME forwards the SMS acknowledgement to the UE.
11. The MME sends an S102 Ack message to the 1xCS IWS. This occurs immediately after step 9 if the MSC has not requested an acknowledgement from the 1xCS IWS.
12. If the MSC requested an acknowledgement, the 1xCS IWS sends an ADDS Page Ack message to the 1xMSC.
B.2.4.3 Mobile terminating SMS
The following sequence flow shows the delivery of mobile terminating SMS sent via the 1xMSC while in E‑UTRAN.
Figure B.2.4.3-1: Mobile terminating SMS sent via the 1xMSC while in E-UTRAN
1. The UE is E-UTRAN attached and registered with 1xRTT CS as defined in clause B.2.1.1.
2. The 1xMSC receives the SMS message from the MC and sends an ADDS Page message to the 1xCS as defined in 3GPP2 A.S0008 [16] and 3GPP2 A.S0009 [17]. The ADDS Page contains the SMS message.
3. The 1xCS IWS sends the SMS message in an S102 Direct Transfer message.
4. If the UE is in idle state, the MME performs the network initiated Service Request procedure to bring the UE to active state prior to tunnelling of the SMS message toward the UE.
5. The MME forwards the SMS message to the UE.
6. The MME sends an S102 Ack message to the 1xCS IWS. This occurs immediately after step 3 if the MSC has not requested an acknowledgement from the 1xCS IWS.
7. If the MSC requested an acknowledgement, the 1xCS IWS sends an ADDS Page Ack message to the 1xMSC.
8. After receiving the SMS message at step 5, the UE sends an SMS acknowledgement toward the MC.
9. The MME forwards the SMS acknowledgement in an S102 Direct Transfer message to the 1xCS IWS.
10. The 1xCS IWS sends an S102 Ack message to the MME.
11. The 1xCS IWS forwards the SMS acknowledgement to the 1xMSC. The 1xMSC then forwards the SMS acknowledgement to the MC.
NOTE: In addition to above MT SMS procedure (Common Channel SMS), 3GPP2 also defines Traffic Channel SMS delivering method in 3GPP2 A.S0008[16] and A.S0009 [17] that can be used in E-UTRAN-1x interworking architecture. In this method, the messages between 1xRTT IWS and 1xMSC are different from those messages specified in this clause. There is no additional functional requirement to EPS.