Content for  TS 23.272  Word version:  17.0.0

The UE sends an Extended Service Request for mobile originating CS fallback to the MME. Extended Service Request message is encapsulated in RRC and S1 AP messages. The UE only transmits this request if it is attached to CS domain (with a combined EPS/IMSI Attach).

The MME sends an S1 AP UE Context Modification Request (CS Fallback Indicator, LAI) message to eNodeB. This message indicates to the eNodeB that the UE should be moved to UTRAN/GERAN. The registered PLMN for CS domain is identified by the PLMN ID included in the LAI, which is allocated by the MME. If MME determines the CS Fallback procedure needs priority 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 S1AP message to the eNodeB as specified in TS 36.413. 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.

The eNodeB shall reply with S1-AP UE Context Modification Response message.

If configured to support the return to the last used PLMN after CSFB, the MME sends a MO CSFB indication via SGs to the MSC in order to indicate that the CM service request message, as described in step 9 in this clause, is due to the CSFB via CS MO call.

The eNodeB may optionally solicit a measurement report from the UE to determine the target GERAN/UTRAN cell to which the redirection procedure will be performed.

If the UE and network support inter-RAT cell change order to GERAN and the target cell is GERAN: The eNodeB can trigger an inter RAT cell change order (optionally with NACC) to a GERAN neighbour cell by sending an RRC message to the UE. The inter-RAT cell change order may contain a CS Fallback Indicator which indicates to UE that the cell change order is triggered due to a CS fallback request. If the inter-RAT cell change order contains a CS Fallback Indicator and the UE fails to establish connection to the target RAT, then the UE considers that CS fallback has failed. Service Request procedure is considered to be successfully completed when cell change order procedure is completed successfully. The eNodeB selects the target cell considering the PLMN ID and possibly the LAC for CS domain provided by the MME in step 1b for CCO/NACC purpose.

If the UE or the network does not support inter-RAT PS handover from E-UTRAN to GERAN/UTRAN nor inter-RAT cell change order to GERAN or the network does not wish to use these procedures: The eNodeB can trigger RRC connection release with redirection to GERAN or UTRAN. The eNodeB sets the "CS Fallback High Priority" indication in the RRC Release message, if the S1AP message in Step 1b contains the "CSFB High Priority" indication.

If the UE and network support "RRC connection release with redirection and Multi Cell System Information to GERAN/UTRAN": The eNodeB can trigger RRC connection release with redirection to GERAN or UTRAN and include one or more physical cell identities and their associated System Information. In step 3b or step 3c, the eNodeB includes the redirection control information into the RRC Connection Release message based on the PLMN ID for CS domain and the RAT/frequency priority configured in the eNodeB, so that the UE registered PLMN for CS domain can be preferably selected. The eNodeB sets the "CS Fallback High Priority" indication in the RRC Release message, if the S1AP message in Step 1b contains the "CSFB High Priority" indication.

The eNodeB sends an S1-AP UE Context Release Request message to the MME. If the target cell is GERAN and either the target cell or the UE does not support DTM the message includes an indication that the UE is not available for the PS service.

The MME releases the UE Context in the eNodeB as well as all eNodeB related information in the S-GW as specified in TS 23.401. In case the Cause indicates that RRC was released due to abnormal conditions, e.g. radio link failure, the MME suspends the EPS bearers (Step 8).

(Step 6a is performed if step 3a, Cell Change Order to GERAN, was performed) The UE moves to the new cell in GERAN. The UE uses the NACC information and/or the broadcast System Information and when it has all of the necessary information to access the GERAN cell, establishes a radio signalling connection.

(Step 6b is performed if step 3b, RRC release with redirection, was performed). The UE moves to the target RAT, identifies a suitable cell preferably of the same PLMN as received in LAI IE of combined EPS/IMSI Attach/TAU Accept message, receives the broadcast System Information and when it has the necessary information to access GERAN/UTRAN, establishes a radio signalling connection.

(Step 6c is performed if step 3c, RRC connection release with redirection and Multi Cell System Information, was performed). The UE moves to the target RAT and identifies a suitable cell preferably of the same PLMN as received in LAI IE of combined EPS/IMSI Attach/TAU Accept message. The UE uses the Multi Cell System Information and/or the broadcast System Information and when it has all of the necessary information to access GERAN/UTRAN, the UE establishes the radio signalling connection.

When the UE arrives at the target cell, if target RAT is UTRAN: The UE establishes the radio signalling connection by sending an RRC Initial Direct Transfer message as specified in TS 25.331 that contains a NAS message. The CN Domain Indicator is set to "CS" in the Initial Direct Transfer message. If the eNodeB indicated UE to go to GERAN in step 3 but the UE finally camps on an UTRAN cell of the same RA that the UE is registered with and ISR active, the UE shall locally deactivate ISR. If target RAT is GERAN A/Gb mode: The UE establishes a radio signalling connection by using the procedures specified in TS 44.018 (i.e. UE requests and is assigned a dedicated channel where it sends a SABM containing a NAS message to the BSS and the BSS responds by sending a UA). Upon receiving the SABM (containing the NAS message) the BSS sends a COMPLETE LAYER 3 INFORMATION message (containing the NAS message) to the MSC which indicates CS resources have been allocated in the GERAN cell. After the establishment of the main signalling link as described in TS 44.018 the UE enters either Dual Transfer Mode or Dedicated Mode. If the LA of the new cell is different from the one stored in the UE, the UE (which is received as part of Combined Attach/TAU procedure in E-UTRAN) shall initiate a Location Area Update regardless of the different Network Modes of Operation (NMO). The UE shall set the "follow-on request" flag in the LAU Request in order to indicate to the MSC not to release the Iu/A connection after the LAU procedure is complete. The UE shall indicate to the target MSC that this is an originating call establishment as a result of CSFB by including the CSMO flag. Further the UE performs any Routing Area Update procedure as specified by TS 23.060. When different from the serving MSC, the target MSC derives the Last Used LTE PLMN from the MAP Send Identification message received from the serving MSC/VLR during the Location Area Update procedure. In NMO I a CSFB UE shall perform separate LAU with "follow-on request" flag and "CSMO" flag, and RAU procedures instead of a Combined RA/LA Update procedure to speed up the CSFB procedure.

If the target RAT is GERAN and DTM is not supported or the UE does not support DTM, the UE starts the Suspend procedure specified in TS 23.060. This triggers the (serving) SGSN to send a Suspend Request (TLLI, RAI) message to the old CN node identified by the RAI and TLLI. If ISR is not active, the RAI and TLLI refer to an MME. The MME returns a Suspend Response to the SGSN even though the GUTI cannot be derived from the P-TMSI and RAI pair. If ISR is active, the RAI and TLLI refer to the old S4-SGSN, In this case, if the serving SGSN is different from the old SGSN which has ISR association with MME, the old SGSN returns a Suspend Response to the serving SGSN.

If the S1-AP UE Context Release Request message, received from the eNodeB in step 4, indicates that the UE is not available for the PS service in the target cell, 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, and starts the preservation and suspension of non-GBR bearers by sending Suspend Notification message to the S-GW. If ISR is active, the (old) S4-SGSN deactivates GBR bearers towards S GW and P GW(s) by initiating MS-and SGSN initiated Bearer Deactivation procedure as specified in TS 23.060, and starts the preservation and suspension of non-GBR bearers by sending the Suspend Notification message to the S-GW, which is all triggered by the Suspend procedure in step 7. The S-GW sends Suspend Notification message to the P-GW(s) when it receives the Suspend Notification message from MME or S4-SGSN. If the S-GW receives two Suspend Notification messages for the same UE, it ignores the second one except for sending response. The MME stores in the UE context that UE is suspended status. If ISR is active, the (old) S4-SGSN stores in the UE context that UE is in suspended status. All the preserved non-GBR bearers are marked as suspended status in the S-GW and P-GW(s). The P-GW should discard packets if received for the suspended UE.

The UE continues with the MO call setup procedure with sending CM Service Request. The UE shall indicate to the MSC that this is an originating call establishment as a result of CSFB by including the "CSMO" flag.

If the UE is not registered in the MSC serving the 2G/3G cell or the UE is not allowed in the LA, the MSC shall reject the service request, if implicit location update is not performed.

A UE detecting that the MSC rejected the service request shall perform the Location Area Update or a Combined RA/LA procedure according to existing GERAN or UTRAN procedures as specified in TS 23.060 for the different Network Modes of Operation (NMO). When different from the serving MSC, the target MSC derives the Last Used LTE PLMN from the MAP Send Identification message received from the serving MSC/VLR during the Location Area Update procedure.

The UE initiates the CS call establishment procedure and the UE shall include the CSMO flag in the CM Service Request to the MSC.

After the CS voice call is terminated and if the UE is in GERAN and PS services are suspended, then the UE shall resume PS services as specified in TS 23.060. A Gn/Gp -SGSN will follow TS 23.060 to resume the PDP Context(s); in addition, the Gn/Gp SGSN sends a Update PDP Context Request message to the GGSN/P GW, the GGSN/P GW resumes the PDP Context(s). An S4 SGSN will follow TS 23.060 to resume the bearers, and informs the S-GW and P-GW(s) to resume the suspended bearers. If the UE has returned to E-UTRAN after the CS voice call was terminated, then the UE shall resume PS service by sending TAU Request to MME. If PS services resumption was unsuccessful before returning to E-UTRAN and ISR is active, the UE shall locally deactivate ISR before initiating the TAU procedure. The MME will in addition inform S-GW and P-GW(s) to resume the suspended bearers. Resuming the suspended bearers in the S-GW and in the P-GW should be done by implicit resume using the Modify Bearer request message if it is triggered by the procedure in operation, e.g. RAU, TAU or Service Request. The S-GW is aware of the suspend state of the bearers and will forward the Modify Bearer request to the P-GW. Explicit resume using the Resume Notification message should be used in cases when Modify Bearer Request is not triggered by the procedure in operation.