B.2.2 Mobile Originating Call in Active Mode

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

This clause describes the mobile originating call procedures for the CS Fallback to 1xRTT in the normal case. For enhanced CS fallback to 1xRTT procedure, see clause B.2.3a. Clause B.2.3b describes the procedure when the procedure is rejected by the MME.

Figure B.2.2-1: CS MO call using fallback to CDMA 1x RTT network

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

2. UE makes a decision to perform a mobile originated CS call.

3. UE sends an Extended Service Request for mobile originating 1xCS fallback to the MME.

4. MME sends UE Context Modification Request (CS Fallback Indicator, priority indicator) to E-UTRAN. CS Fallback Indicator indicates to the E‑UTRAN to move the UE to 1xRTT.

If MME determines the CS Fallback procedure needs priority handling based on the MPS CS priority in the UE’s EPS subscription, it sets priority indication in the S1AP message to the E-UTRAN. E-UTRAN responds with UE Context Modification Response

5. 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.

6a. The E-UTRAN triggers RRC connection release with redirection to 1xCS and continue with step 7.

7. 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.

In case the Cause indicates that RRC was released due to abnormal conditions, e.g. radio link failure, the MME should continue with steps 8 -10.

8. 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).

9. 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.

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

11. UE moves to 1xRTT and performs the procedure for mobile originating call 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.