7.5a Roaming Forwarding 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/VLR, to which the UE sends the LAU, is different from the MSC/VLR that sent the paging message to the UE. The procedure is based on the "Mobile Terminating Roaming Forwarding" procedure defined in TS 23.018 [5]; the specific behaviour for CSFB is described in this clause. This procedure avoids impacting all GMSC nodes and can coexist with Mobile Terminating Roaming Retry procedure described in clause 7.5.

NOTE 1: In order to support Mobile Terminating Roaming Forwarding both the MSC controlling the target cell, the MSC that sent the paging message to the UE and the HLR need to support the feature.

NOTE 2: This procedure has smaller call setup delay than Mobile Terminating Roaming Retry procedure especially in roaming cases.

NOTE 3: If the network does not support this procedure, the Mobile Terminating Roaming Retry procedure specified in clause 7.5 can be used.

In order to ensure roaming forwarding can be offered in all scenarios (e.g. in case of IMSI in the LAU Request from UE), HLRs should be updated to support MTRF. In order to permit Mobile Terminating Roaming Forwarding from the old VLR if the HLR is not updated to support MTRF but the visited network does support MTRF, the new VLR may include MTRF supported flag in Send Identification when it receives a Location Updating Request containing the "CSMT" flag.

Figure 7.5a-1: Roaming Forwarding for CSFB

The main differences compared to the "Mobile Terminating Roaming Forwarding" procedure defined in TS 23.018 [5] are described below:

– The paging message in E-UTRAN triggers the CS fallback which is followed by a location update in the new RAT.

NOTE 4: This functionality is already supported in the CS fallback flows for terminating calls and no additional functionality is needed.

– 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 old MSC/VLR.

– If the Location Update Request contains the "CSMT" flag set and a valid TMSI/old LAI, the new MSC/VLR may indicate to the old MSC/VLR that it supports MTRF in the Send Identification message. The new VLR then performs authentication to the Location update and updates the HSS. If the Location Update Request contains the IMSI, only HLR-based MTRF procedure can be used.

– After Cancel Location is received from HSS, if the HLR authorised the MTRF call between the old and the new terminating MSCs or if the HLR did not disallow the MTRF call between the old and the new terminating MSCs but the new MSC/VLR indicated its MTRF support earlier in Send Identification message, the old MSC/VLR stops paging timer and checks roaming and charging pre-configured agreements with regards to call routeing to the new MSC/VLR. If these checks are successful, the old MSC/VLR sends a Provide Roaming Number request (MTRF Indicator, parameters received from the HLR) to the new MSC/VLR.

– In a MT CSFB call, last used LTE PLMN identity may be added in the Provide Roaming Number Request message from old MSC/VLR to new MSC/VLR, based on configuration.