Content for  TS 23.272  Word version:  17.0.0

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.

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.

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. 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. 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 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 and sets the RRC establishment cause to "HighPriorityAccess" as specified in TS 36.331. 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.

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.

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.