6 Voice Transport in LTE
6.2 CSFB – Circuit-Switched FallBack

In case IMS services are not deployed from the start and/or LTE provides only data services, when a voice call is to be initiated or received, it will fall back to the circuit-switched domain by the CSFB (Circuit-Switched FallBack) mechanism. CSFB allows circuit-switched voice calls to be handled via legacy radio access technology for terminals that are camped on LTE.

When using this solution, operators just need to upgrade the MSC instead of deploying the IMS. However, the disadvantage is longer call setup delay.

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Message sequence diagram for CSFB from LTE to UMTS

Figure shows the message flow for a CSFB call from LTE to UMTS, including paging from the MSC via the SGs interface and MME in the case of UE-terminated calls, and the sending of an Extended Service Request message from the UE to the MME to trigger either a handover or redirection to the target radio access technology.