B.2.2a Mobile Originating call in Idle Mode
23.2723GPPCircuit Switched (CS) fallback in Evolved Packet System (EPS)Release 17Stage 2TS
Mobile Originating call in Idle Mode procedure is specified by reusing the Mobile Originating Call in Active mode procedure with Extended Service Request for mobile originating 1xCS fallback to the MME where the messages S1-AP UE context Modification Request and Response are replaced by S1-AP Initial UE Context Request and Response. UE is transited to ECM-CONNECTED mode by following the applicable procedures specified in TS 23.401 [2].
If UE has only LIPA PDN connection and the cell accessed by the UE does not link to the L-GW where the UE had the LIPA PDN Connection, the MME shall reject the Extended Service Request with a reason code which results in the UE selecting 1xRTT access as specified in TS 24.301 [34].
If the UE has only SIPTO at the Local Network PDN connections:
(i) in a network with collocated L-GW deployment and the (H)eNB accessed by the UE is not collocated with the L-GW providing the SIPTO at the Local Network connectivity; or
(ii) in a network with stand-alone GW deployment and the (H)eNB accessed by the UE is not connected to the Local Network where the stand-alone GW (with S-GW and L-GW collocated) providing the SIPTO at the Local Network connectivity is residing,
the MME shall reject the Extended Service Request with an appropriate cause value which results in the UE selecting 1xRTT access.
If the UE has only SIPTO at the Local Network PDN connections and LIPA PDN connections, where:
– the SIPTO at the Local Network PDN connections fulfil criterion (i) or (ii) above; and
– the cell accessed by the UE does not link to the L-GW where the UE had the LIPA PDN connection,
the MME shall also reject the Extended Service Request with an appropriate cause value which results in the UE selecting 1xRTT access.
If the UE is a service user with subscription to CS domain priority service, the UE’s USIM shall belong to one of special Access Classes as specified in TS 22.011 [38] and sets the RRC establishment cause to "HighPriorityAccess" as specified in TS 36.331 [33]. If the network supports a priority call handling, the MME determines that the Extended Service Request requires priority handling of CS Fallback based on the "HighPriorityAccess" establishment cause forwarded by eNodeB to the MME and/or MPS CS priority in the UE’s EPS subscription. According to operator policy, the MME may use MPS CS priority to verify the priority handling of the CS Fallback procedure.
If MME decides to perform CS Fallback with priority, it sets priority indication in the S1-AP Initial UE Context Request message to the eNodeB. The eNodeB should allocate radio bearer resources to the UE preferentially compared to other normal calls.