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Sep 23, 2013

Circuit Switched Fallback CSFB - Mobile Terminating Call in Active mode, no PS HO

Recently CSFB is biggest thing I'm struggling with, because of that I went through many pages of Spec for Circut Switch Fallback and decided this is a very good topic to cover here. 

CS Fallback enables Circuit Switch (CS) based voice and supplementary services to be offered together with LTE access. To allow subscribers to use existing CS based services such as voice voice, supplementary services and location based services from 3G/2G networks. CSFB temporarily moves subscriber from LTE down to 3G/2G during the call setup process. When the call is finished the UE should reselect the LTE network

Procedure described below is for case where there PS HO is not supported.

Fig. 1. CS Paging over SGs in 4G, Call in 3G/2G




1a. The MSC receives an incoming voice call and responds by sending a Paging Request 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 relay 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 Page to the UE and sends CS Paging Reject towards MSC to stop CS Paging procedure, and this CSFB procedure stops.
The eNodeB forwards the paging message to the UE. The message contains CN Domain indicator and, if received from the MSC, the Caller Line Identification.
The MME immediately 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.
1b. UE sends an Extended Service Request message to the MME. Extended Service Request message is encapsulated in RRC and S1-AP messages. CS Fallback Indicator indicates MME to perform CS Fallback. The UE may decide to reject CSFB based on Caller Line Identification.
1c. Upon receiving the Extended Service Request (CSFB, Reject), the MME sends Paging Reject towards MSC to stop CS Paging procedure and this CSFB procedure stops.
1d. The MME sends an S1-AP UE Context Modification Request 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.
1e. The eNodeB shall reply with S1-AP UE Context Modification Response message.
2. 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.

The network performs one of steps 3a or 3b or 3c.
3a. 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 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 1d for CCO/NACC purpose.
3b. 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:
 The eNodeB can trigger RRC connection release with redirection to GERAN or UTRAN instead of PS HO or NACC.
3c. 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.

4. 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 (Dual Transfer Mode) the message includes an indication that the UE is not available for PS service.
5. The MME releases the UE Context in the eNodeB as well as all eNodeB related information in the S-GW.
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).

The UE performs one of steps 6a or 6b or 6c and THEN performs step 6d.
6a. 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 receives the broadcast System Information and when it has the necessary information to access the GERAN cell, establishes a radio signalling connection.
6b. 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.
6c. 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 NACC information and/or receives the broadcast System Information and when it has the necessary information to access GERAN/UTRAN, the UE establishes the radio signalling connection.
6d. If the LA (Location Area) of the new cell is different from the one stored in the UE, the UE shall initiate a Location Area Update or a Combined RA/LA 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. In NMO I, the UE in GERAN may perform LA update over the RR connection instead of combined RA/LA update over the packet access, unless enhanced CS establishment in DTM is supported. Further the UE performs any Routing Area Update procedure.
In NMO I a CSFB UE should perform LAU (and if it does so, shall set the "CSMT" flag) and RAU procedures instead of a Combined RA/LA Update procedure to speed up the CSFB procedure.
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.
7. If the target RAT is GERAN and DTM is not supported or the UE does not support DTM, the UE starts the Suspend procedure. 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 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 which has ISR association with the MME, In this case, if the serving SGSN is different from the old SGSN, the old SGSN returns a Suspend Response to the serving SGSN.
8. 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 services in the target cell, the MME deactivates GBR bearers towards S-GW and P-GW(s) by initiating MME-initiated Dedicated Bearer Deactivation procedure, 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, 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 the UE is in 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.
9. If the UE does not initiate a LAU procedure, the UE responds to the paging by sending a Paging Response message. When received at the BSS/RNS, the Paging Response is forwarded to the MSC.
9a. If UE is registered in the MSC serving the 2G/3G cell and the UE is allowed in the LA the MSC shall establish the CS call.
9b. 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 by releasing the A/Iu-cs connection. The BSS/RNS in turn releases the signalling connection for CS domain.
9c. 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 for the different Network Modes of Operation (NMO).
The Location Area Update triggers the Roaming Retry for CS Fallback procedure.
After performing the LAU procedure the MSC shall establish the CS call if the UE is allowed in the LA.

With the exception of steps 1a and 1c, above, Call Forwarding is performed on the basis of signalling received on the GERAN/UTRAN cell.
After the CS voice call is terminated and if the UE is still in GERAN and PS services are suspended, then the UE shall resume PS services. A Gn/Gp- SGSN will resume the PDP Context(s), which is done by the Routing Area Update procedure. During the Routing Area Update procedure in which 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 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 to MME. 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 shall 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.
If the UE remains on UTRAN/GERAN after the CS voice call is terminated the UE performs normal mobility management procedures.




Source:
TS 23.272 3GPP