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In this interworking scenario, the provision for making a voice call for a UE registered with an LTE network is facilitated through the reuse of the legacy GERAN/UTRAN. The provision for making a voice call by an LTE/EPS registered UE through the legacy GERAN/UTRAN is known as the CSFB method; refer to TS 23.272 [38]. One such illustration of the interworking of network elements for CSFB arrangement is shown in Figure 6.8. A CSFB feature may be used by:

Figure 6.8 Illustration: legacy and LTE network interworking for CSFB.

 An MO or MT LTE UE that does not have the voice‐over IMS or VoLTE capability

 Other scenarios such as VoLTE‐capable UE that fails to register in the IMS.

Unlike the interworking scenarios for VoLTE call as mentioned in Sections 6.2.1 and 6.2.2, no IMS is deployed and interworked as part of the LTE/EPS network with the CSFB arrangement. In this interworking scenario with CSFB arrangement, the LTE/EPS network provides only data services to subscribers.

During a CSFB of a CS call, the ongoing PS session may be also transferred to the legacy network through PS HO procedure with reduced data rates. For the CSFB purpose, a new logical interface called SGs is configured between the MME and MSC/Visitor Location Register (VLR) to exchange signaling information only. The SGs logical interface also supports mobility management as well as MO/MT call establishment procedures. For more information on the CSFB, a protocol stack, and procedures supported over the SGs, refer to TS 23.272 [38] and TS 29.118 [68]. SGs interface uses the SCTP [17] layer and IP transport network. The SGs interface is similar to the GPRS/UMTS Gs interface that connects the SGSN and the MSC to provide CS domain services to MS engaged in a PS service.

Example 6.2 below describes the status displayed by an LTE/EPS registered UE while making a voice call through the legacy GERAN/UTRAN using the CSFB method.

 How does a CSFB work?

 Indication of CSFB Feature Support

A UE indicates its CSFB feature support capability under the UE Network Capability Mandatory IE in the ATTACH Request message that is sent to the MME. The UE specifies the Attach Type as the combined EPS/IMSI Attach Request under the EPS attach type IE. A combined EPS/IMSI ATTACH Request from the UE enables the MME to update the UE location in the MSC/VLR end also over the SGs interface. Refer to TS 23.272 [38] for further details on the EMM procedures supported over the SGs interface. If the EPC network supports the CSFB feature, the same will be informed to the UE in the ATTACH ACCEPT, TS 24.301 [46], message through the IE Attach Result = combined EPS/IMSI attach and Additional update result optional IE. This IE will carry the value “CS Fallback not preferred” toward the UE, indicating that it can use the CSFB method to initiate a CS call through the legacy 2G/3G network. The same IE indicating the supporting of the CSFB feature can be also sent in a TAU ACCEPT message, TS 24.301 [46], from the MME to UE.

 Initiation of CS Call Through the CSFB Method by the UE

An LTE UE initiates a CS call through the CSFB method by sending the EMM/NAS EXTENDED SERVICE REQUEST message to the MME.

This message can be used to establish an MO or MT CS call through CSFB by indicating the same in the Service type mandatory IE; refer to Section 9.9.3.27, TS 24.301 [46]. Any existing Radio Resource Control (RRC) connection will be released by the E‐UTRAN by sending the RRCConnectionRelease, TS36.331 [94], message to the UE with