B.2.3b Mobile Originated or Mobile terminated call rejected by the MME

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

The MME may reject an Extended Service Request either for mobile originated or mobile terminated CSFB. In this case, the following procedure is executed.

Figure B.2.3b-1: 1xCSFB MO or MT call, rejected by MME

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

2. UE makes a decision to perform a mobile originated CS call or accepts CS paging for the CS Fallback to 1xRTT (Step 6a, Clause 5.2.3).

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

4. If the MME decides to reject the Extended Service Request, the MME sends a Service Reject message to the UE.

Steps 5 – 10 are executed when Service Reject is sent with a reason code which results in the UE selecting 1xRTT access, as specified in TS 24.301 [34].

5. The UE selects 1xRTT access without waiting for RRC Release.

6. The MME releases S1 by sending the S1 UE Context Release Command (Cause) message to the eNodeB. Cause value indicates that the release is triggered by CS Fallback procedure.

7. If the RRC connection is not already released, the E-UTRAN sends a RRC Connection Release message to the UE.

8. The E-UTRAN confirms the S1 Release by returning an S1 UE Context Release Complete message to the MME.

9-10. Depending on the reason for rejection, MME may start Suspend Notification:

– Suspend Notification: The S1-U bearers are released and the MME starts the preservation and suspension of non-GBR bearers and the deactivation of GBR bearers towards S-GW and P-GW(s).

– S-GW and P-GW(s) acknowledges the bearer updates Suspend Notification and marks the UE as suspended. The P-GW discards downlink data if the UE is marked as suspended.