SRVCC

SRVCC, or Single Radio Voice Call Continuity, is a 3GPP mechanism that preserves an in-progress voice call when a user moves from an LTE network carrying VoLTE to a non-LTE circuit-switched network (2G/3G). It enables seamless voice continuity without manual intervention or a dropped call, by transferring the session from the IMS/VoLTE domain to the legacy circuit-switched voice domain.

Operation proceeds when an ongoing VoLTE call detects that the user is moving out of LTE coverage. The network initiates an inter-system handover coordinated by the core and radio access networks, transferring the call from the IP-based IMS path to a circuit-switched voice path. The mobile device remains on the same radio, while the signaling and media are rerouted through the CS network via MSC/VLR and, if needed, a media gateway. On completion, the far end experiences a standard CS voice call, with minimal disruption to the user.

Requirements and scope: SRVCC requires support across the user equipment, the E-UTRAN nodes (eNodeB, MME), and the circuit-switched infrastructure (MSC/VLR) along with the interworking components that enable inter-system handover. It is related to, but distinct from, Circuit-Switched Fallback (CSFB); SRVCC focuses on preserving an ongoing VoLTE call rather than initiating a new CS fallback.

Deployment and status: SRVCC is defined by 3GPP and implemented by many operators to improve voice reliability in mixed network environments. Its adoption varies by region and operator, depending on the presence of CS networks and VoLTE coverage.