Content for  TS 23.272  Word version:  17.0.0

Mobile Originating call in Idle Mode procedure is specified by reusing the Mobile Originating Call in Active mode procedures as specified in clause 6.2 and clause 6.3 with Extended Service Request for mobile originating CS 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. The LAI is included in the S1-AP Initial UE Context Request message and sent to the eNodeB. The UE is transited to ECM-CONNECTED mode by following the applicable procedures specified in TS 23.401. 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 GERAN or UTRAN as specified in TS 24.301. If the UE has only SIPTO at the Local Network PDN connections:

1. 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
2. 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 GERAN or UTRAN. 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 GERAN or UTRAN. A UE with a USIM assigned to one of Access Classes 11 to 15, e.g. a UE that is a Service User according to TR 22.952 sets the RRC establishment cause to "HighPriorityAccess" based on the access class handling as specified in TS 36.331. If the network supports CSFB priority call handling, the MME determines whether the CSFB request requires CSFB priority call 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 in the UE's EPS subscription to verify the priority handling of the CS Fallback procedure. If the MME determines to perform CS Fallback priority call handling, based on MPS CS Priority in the UE's EPS subscription or because the call is an emergency call, it sets priority indication, i.e. "CSFB High Priority", in the S1-AP Initial UE Context Request message to the eNodeB as specified in TS 36.413. The eNodeB allocates radio bearer resources to the UE preferentially compared to other normal calls. In the case of emergency call, the MME also requests the eNodeB to inhibit roaming and access restrictions via the Additional CS Fallback Indicator as specified in TS 36.413.

Once CS service ends in CS domain, existing mechanisms can be used to move the UE to E-UTRAN, no specific CS Fallback mechanisms are needed. During the release of an RR connection the MSC should indicate to GERAN/UTRAN that the RR connection (for call or CISS or LCS) was established as a result of CS fallback. GERAN and UTRAN may use the indication to determine which of the existing mechanisms should be used to move the UE to E-UTRAN. When configured to support the return to the last used PLMN after CSFB, after a successful setup of an RR connection or during the release of the UTRAN RR connection, the MSC shall further indicate to GERAN/UTRAN the last used LTE PLMN ID. GERAN and UTRAN shall take the last used LTE PLMN ID into account when selecting the target cell in PS handover to E-UTRAN or when selecting the dedicated target frequency list for idle mode mobility to E-UTRAN in RR Connection Release procedure. When the UE moves to E-UTRAN, if the EPS service was suspended during the CS service, it is resumed according to the procedure shown in the Figure 6.5-1 below.

The MME may reject an Extended Service Request either for mobile originated or mobile terminated CSFB. The following procedure covers this scenario.

Steps 5 - 8 are executed when Service Reject is sent with a reason code which results in the UE selecting GERAN or UTRAN, as specified in TS 24.301.