Content for  TS 23.272  Word version:  17.0.0

The MSC receives an incoming voice call and responds by sending a Paging Request (IMSI or TMSI, optional Caller Line Identification and Connection Management information, CS call indicator, eMLPP priority level indication) to the MME over a SGs interface. The MSC only sends a CS Page for an UE that provides location update information using the SGs interface. In active mode the MME has an established S1 connection and if the MME did not return the "SMS-only" indication to the UE during Attach or Combined TA/LA Update procedures, the MME reuses the existing connection to send the CS Service Notification to the UE. If the MME returned the "SMS-only" indication to the UE during Attach or Combined TA/LA Update procedures, the MME shall not send the CS Service Notification to the UE and shall send Paging Reject towards MSC to stop CS Paging procedure, and this CSFB procedure stops. If the X2-based or S1-based handover procedure is ongoing when the MME receives a Paging Request over the SGs interface, the MME shall follow the procedures as specified in clause 5.5.1.1.1 of TS 23.401 and clause 5.5.1.2.1 of TS 23.401 for X2-based handover and S1-based handover respectively. If the Inter RAT handover procedure is ongoing when the MME receives a Paging Request over the SGs interface, the MME shall simply discard this message and no further action is performed by MME. In this case, mobile terminating call can be continued by the MSC retransmitting Paging message as specified in TS 29.118. The eNodeB forwards the CS Service Notification message to the UE. The message contains CN Domain indicator and, if received from the MSC, the Caller Line Identification. The MME sends the SGs Service Request message to the MSC containing an indication that the UE was in connected mode. The MSC uses this connected mode indication to start the Call Forwarding on No Reply timer for that UE and the MSC should send an indication of user alerting to the calling party. Receipt of the SGs Service Request message stops the MSC retransmitting the SGs interface Paging message. If the MME receives paging request message with eMLPP priority level indication from the MSC, then the MME determines based on its configuration whether this priority level requires CSFB priority call handling.

UE sends an Extended Service Request (Reject or Accept) message to the MME for mobile terminating CS fallback. The Extended Service Request message is encapsulated in RRC and S1 AP messages. The UE may decide to reject CSFB based on Caller Line Identification.

Upon receiving the Extended Service Request (Reject) for mobile terminating CS fallback, the MME sends Paging Reject towards MSC to stop CS Paging procedure and this CSFB procedure stops.

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 determined in Step 1a that CSFB priority call handling shall apply, the MME sends S1-AP UE Context Modification Request message to the eNodeB with priority indication, i.e. "CSFB High Priority", as specified in TS 36.413.

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

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

The eNodeB triggers PS handover to a GERAN/UTRAN neighbour cell by sending a Handover Required message to MME. The eNodeB selects the target PS handover cell considering the PLMN ID and possibly the LAC for CS domain provided by the MME in step 1d. If the eNB is a HeNB, the HeNB should perform step 3 through step 6 of clause 7.4 instead of PS HO if the HeNB detects that the UE has only LIPA PDN Connections. CSFB will not be completed successfully when PS HO is performed if the UE has only LIPA PDN Connections as PS HO would result in the MME detaching the UE. In the following an inter-RAT handover from E-UTRAN to UTRAN or GERAN as specified in TS 23.401 begins. The eNodeB indicates in the Source RNC to Target RNC Transparent container that PS handover was triggered due to CSFB. The eNodeB also indicates whether CSFB was triggered for emergency or priority call handling purpose. If the network supports a priority call handling, the eNodeB may forward the priority indication to the target GERAN/UTRAN in the Source to Target Transparent Container, and the target GERAN/UTRAN allocates radio bearer resources taking received priority indication take into account. As part of this handover, the UE receives a HO from E-UTRAN Command and tries to connect to a cell in the target RAT. The HO from E-UTRAN Command may contain a CS Fallback Indicator which indicates to UE that the handover was triggered due to a CS fallback request. If the HO from E-UTRAN Command 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. The UE establishes the signalling connection as described in step 4b.

If the target RAT is GERAN and the UE has entered Dedicated Mode, the UE starts the Suspend procedure (see TS 44.018) unless both the UE and the Target cell support DTM in which case TBF re-establishment may be performed.

A Gn/Gp-SGSN that receives the Suspend message from the UE follows the Suspend procedure specified in clause 16.2.1.1.1 of TS 23.060. An S4-SGSN that receives the Suspend message from the UE follows the Suspend procedure specified in TS 23.060. The 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 Suspend Notification message to the S-GW. The S-GW releases all RNC related information (address and TEIDs) for the UE if Direct Tunnel is established, and sends Suspend Notification message to the P-GW(s). The 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.

If the LA of the new cell is different from the one stored in the UE, the UE shall initiate a Location Area Update procedure as follows:

• If the network is operating in NMO-I (Network Modes of Operation), the UE shall initiate a separate Location Area Update before initiating the RAU procedure instead of a Combined RA/LA Update procedure (to speed up the CSFB procedure); or
• if the network is operating in NMO-II the UE shall initiate a Location Area Update procedure before initiating the RAU procedure required for PS handover.

The UE shall set the "CSMT" flag in the LAU Request. The "CSMT" flag is used to avoid missing MT call in roaming retry case. Further the UE performs any Routing Area Update procedure as specified in TS 23.060. The UE may initiate a Location Area Update procedure immediately when the UE is handed over to the target cell i.e. before the UE receives e.g. LAI or NMO information as part of the RAN Mobility Information. When the MSC receives a LA Update Request, it shall check for pending terminating CS calls and, if the "CSMT" flag is set, maintain the CS signalling connection after the Location Area Update procedure for pending terminating CS calls.

If the UE does not initiate a LAU procedure, it shall respond with a Paging Response message to the MSC as follows:

• If the Target RAT is UTRAN or GERAN Iu mode: The UE establishes a radio signalling connection and responds to the paging by sending an RRC Paging Response as specified in TS 25.331. The CN Domain Indicator is set to "CS" in the Initial Direct Transfer message.
• If the Target RAT is GERAN A/Gb mode: The UE establishes a radio signalling connection and responds to paging 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 Paging Response to the BSS and the BSS responds by sending a UA). Upon receiving the SABM (containing a Paging Response message) the BSS sends a COMPLETE LAYER 3 INFORMATION message (containing a Paging Response message) to the MSC which indicates CS resources have been allocated in the GERAN cell. If both the UE and the target cell support enhanced CS establishment in DTM (indicated by GERAN system information included within the HO from E-UTRAN Command) an RR connection may be established while in packet transfer mode without release of the packet resources, see TS 43.055. After the establishment of the main signalling link as described in TS 44.018 the UE enters either Dual Transfer Mode or Dedicated Mode and the CS call establishment procedure completes.

After performing the LAU procedure or after receiving the Paging Response the MSC shall establish the CS call if the UE is allowed in the LA.

If the UE is not registered in the MSC that receives the Paging Response or the UE is not allowed in the LA, the MSC shall reject the Paging Response message by releasing the A/Iu-CS. The BSC/RNC in turn releases the signalling connection for UTRAN or GERAN CS domain. The signalling connection release shall trigger the UE to obtain the LAI, which causes the initiation of a Location Area Update or a Combined RA/LA procedure as specified in TS 23.060 for the different Network Modes of Operation (NMO). The Location Area Update triggers the Roaming Retry for CS Fallback procedure as defined in clause 7.5.

After performing the LAU procedure the MSC shall establish the CS call if the UE is allowed in the LA.

The UE performs any remaining steps of the inter-RAT handover from E-UTRAN to UTRAN or GERAN as specified in TS 23.401 With the exception of steps 1a and 1c, above, Call Forwarding (see TS 23.082) is performed on the basis of the TS 24.008 signalling received on the GERAN/UTRAN cell. If the UE remains on UTRAN/GERAN after the CS voice call is terminated the UE performs normal mobility management procedures as defined in TS 23.060 and TS 24.008.