7.5 Roaming Retry for CS fallback

23.2723GPPCircuit Switched (CS) fallback in Evolved Packet System (EPS)Release 17Stage 2TS

The procedure in this clause may be applied for mobile terminated calls where the MSC, to which the UE sends the LAU, is different from the MSC that sent the paging message to the UE. The procedure is based on the "Mobile Terminating Roaming Retry Call" procedure defined in TS 23.018 [5] and there is an only minor adaptation of the procedure to support CS fallback.

Figure 7.5-1: Roaming Retry for CS fallback

There are only 2 differences in this procedure compared to the "Mobile Terminating Roaming Retry Call" procedure defined in TS 23.018 [5].

– The first difference is that the paging message in E-UTRAN triggers the CS fallback which is followed by a location update in the new RAT. This functionality is already supported in the CS fallback flows for terminating calls and no additional functionality is needed.

– The second difference is that the UE includes the "CSMT" flag in the location update request message so that the signalling link is maintained for longer in case the IAM is delayed by the HLR.

NOTE: Roaming Retry for CS fallback procedure is not designed to provide the target MSC/VLR with the information required for allowing UE return to the last used LTE PLMN after CSFB. If this is required for a certain deployment, Roaming Forwarding for CS fallback could be used.