CSFB in LTE

Here we are showing detailed CSFB in ltes how CS fallback takes and Call flow of CSFB messages

The UE sends an Extended Service Request (CS Fallback Indicator) 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 only transmits this request if it is attached to CS domain (with a combined EPS/IMSI Attach) and can not initiate an IMS voice session (because e.g. the UE is not IMS registered or IMS voice services are not supported by the serving IP‑CAN, home PLMN or UE).

1b.  The MME sends an S1‑AP Request message to eNB that includes a CS Fallback Indicator. This message indicates to the eNB that the UE should be moved to UTRAN/GERAN.

1c.  The eNB shall reply with S1-AP 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 (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.

3b.  If the UE or the network does not support inter-RAT 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.

3c.  If the UE and network support "RRC connection release with redirection and Multi Cell System Information to GERAN/UTRAN":