B.2.3 Mobile Terminating Call

23.2723GPPCircuit Switched (CS) fallback in Evolved Packet System (EPS)Release 17Stage 2TS

This clause describes the mobile terminating call procedures when the UE accepts or rejects CS paging for the CS Fallback to 1xRTT, in the normal case. Clause B.2.3b describes the procedure when the procedure is rejected by the MME.

When the 1x MSC receives a registration from a UE, it makes note of the RAN equipment from which it received the registration. Subsequent paging activities may thus be directed toward that RAN equipment. However, paging activities by the 1xMSC are not limited to the single RAN equipment from which the registration was received. The MSC may choose to page a wider area, including inter‑system paging. If the 1xMSC has direct interfaces to 1xCS IWS, as well as to 1xRTT access, the MSC may choose to do direct paging activities to both E‑UTRAN and 1x RAN equipments in its attempts to contact the UE.

The 1x paging request sent by the 1xMSC to the 1xCS IWS is delivered to the UE via the tunnel. The UE tunes to 1xRTT access, acknowledges the 1x page and performs the 1xCS procedures for mobile terminated call.

The detailed procedure using RRC connection release with redirection to 1xCS is described in figure B.2.3‑1. For enhanced 1xRTT CS Fall Back procedure, see clause B.2.3a.

Figure B.2.3-1: CS MT call using fallback to CDMA 1x RTT network

1. UE is E-UTRAN attached and pre-registered with 1xRTT CS as defined in clause B.2.1.1.

2. 1xMSC sends a paging request to the 1xCS IWS node with Caller Line Identification if available.

If the call is originated by a priority user or an emergency callback from PSAP, the paging request message from the 1xRTT MSC to the IWS contains a priority value or an emergency indicator, respectively, as specified in 3GPP2 specification A.S0008-C v3.0 [39] / A.S0009-C v3.0 [40].

3. 1xCS IWS node forwards the 1x RTT CS paging request via the S102 tunnel to the MME.

If a priority value or emergency indication was received in the previous step, the S102 message also reflects the same.

4. If the UE is in idle state, the MME performs the network initiated Service Request procedure in order to bring the UE to active state prior to tunnelling of the 1x RTT CS paging request toward the UE. The paging is performed using S-TMSI with CN domain indicator set to "PS".

If the S102 message contains a priority value or emergency indication, the MME also sets priority indication in the S1AP paging request message to the E-UTRAN. The E-UTRAN handles the paging process with priority.

5. MME forwards the 1xRTT CS paging request to the UE.

6a. Void.

6b. If the UE accepts CS paging for the CS Fallback to 1xRTT, the UE sends an Extended Service Request for mobile terminating 1xCS fallback to the MME and proceeds with step 7 to step 15 below.

7. MME sends UE Context Modification Request (CS Fallback Indicator) to indicate the E‑UTRAN to move the UE to 1xRTT.

If priority value or emergency indication was received in Step 3, the MME also sets priority indication to the E-UTRAN in UE Context Modification Request. The E-UTRAN provides preferential treatment to this call in the subsequent steps. E-UTRAN responds with UE Context Modification Response.

8. E-UTRAN may optionally solicit a 1xRTT measurement report from the UE to determine the target 1xRTT cell to which the CS Fallback will be performed.

9. E-UTRAN triggers RRC connection release with redirection to 1xCS.

10. E-UTRAN sends an S1 UE Context Release Request (Cause) message to the MME. Cause indicates that the S1 UE Context Release was caused by CS fallback to 1xRTT.

11. The MME deactivates GBR bearers towards S-GW and P-GW(s) by initiating MME-initiated Dedicated Bearer Deactivation procedure as specified in TS 23.401 [2], and starts the preservation and suspension of non-GBR bearers by sending Suspend Notification message to the S-GW. The S-GW releases all eNodeB related information (address and TEIDs) for the UE, and sends Suspend Notification message to the P-GW(s).

12. S-GW and P-GW(s) acknowledges the bearer updates by responding with Suspend Acknowledge. The MME stores in the UE context that UE is in suspended status. All the preserved non-GBR bearers are marked as suspended status in S-GW and P-GW. The P-GW discards downlink data if the UE is marked as suspended.

13. S1 UE Context in the E-UTRAN is released as specified in TS 23.401 [2].

14. UE tunes to 1xRTT and acknowledges the page by transmitting a 1xRTT Paging Response message over the 1x Access Channel.

15. Subsequently UE performs the procedure for mobile terminated call establishment as specified in 3GPP2 A.S0013 [18].

Once CS service ends in the 1xCS domain the UE returns to E-UTRAN by performing reselection. The EPS service is resumed as described in clause 6.5.