Single Radio Voice Call Continuity

Description

Single Radio Voice Call Continuity (SRVCC) is a complex mobility procedure defined by 3GPP to maintain an active voice call when a User Equipment (UE) moves from a packet-switched (PS) LTE or 5G NR access network, which supports voice over IP (VoIP) via IMS (VoLTE/VoNR), to a legacy circuit-switched (CS) network like GERAN, UTRAN, or 1xRTT. The core challenge SRVCC addresses is that the UE has only a single radio transceiver, meaning it cannot simultaneously maintain the PS VoIP call on LTE and perform measurements or signaling on the target CS network. The procedure is orchestrated by the network, requiring tight coordination between the Evolved Packet Core (EPC) or 5G Core (5GC), the IMS, and the CS core network (MSC).

Architecturally, SRVCC introduces several key functional entities and reference points. In the EPC, the MME plays a central role. It receives measurement reports from the eNodeB indicating deteriorating LTE signal strength and the availability of a suitable CS target cell. The MME then initiates the SRVCC procedure by sending an SRVCC PS to CS Request message to the MSC Server (enhanced for SRVCC, often called an MSC Server enhanced for SRVCC or eMSC). This MSC Server interfaces with the IMS network via the Sv interface to the SRVCC Application Server (SRVCC AS) or the Service Centralization and Continuity Application Server (SCC AS) in the IMS. The SCC AS is the anchor point for the voice call session in IMS, enabling it to manipulate the media path.

During the handover execution, the MSC Server performs a CS handover procedure with the target BSS/RNS and establishes a CS bearer path. Concurrently, it signals the SCC AS via the Sv interface (or using ISUP/SIP interworking) to transfer the IMS leg of the call from the PS access to the newly established CS access. The SCC AS updates the remote end's media path to point to the CS network's Media Gateway (MGW). From the UE perspective, it receives a handover command from the LTE network, tunes its single radio to the target CS frequency/channel, and accesses the CS network, where it sends a handover complete message. The voice media is then seamlessly switched from the VoIP packet flow over LTE to the CS voice channel. For the calling party, this transition is nearly imperceptible, with only a very brief potential audio interruption.

SRVCC also encompasses variants like enhanced SRVCC (eSRVCC), introduced in 3GPP Release 10, which significantly reduces handover interruption time by anchoring the call locally in the serving network's MSC and IMS architecture, rather than involving the home network's SCC AS for the media update. Reverse SRVCC (rSRVCC) handles mobility from CS back to LTE/5G. In 5G systems, SRVCC principles are extended for handover from 5G NR (VoNR) to EPS (VoLTE) or to UTRAN/GERAN, with the AMF and N26 interface (when present) playing roles analogous to the MME and Sv interface.

Purpose & Motivation

SRVCC was created to solve a critical business and technical problem during the initial rollout of 4G LTE networks. LTE was designed as a pure packet-switched (PS) network, optimized for high-speed data, with no native circuit-switched (CS) domain for voice telephony. The industry's solution for voice over LTE was to use Voice over IP (VoIP) via the IP Multimedia Subsystem (IMS), known as VoLTE. However, LTE coverage was not ubiquitous at launch and often lagged behind the extensive 2G and 3G CS coverage. Without a continuity mechanism, a VoLTE call would simply drop when a user moved out of LTE coverage, leading to a poor user experience that was unacceptable for a primary telephony service.

The primary motivation was to enable mobile operators to deploy LTE as a data-optimized network while leveraging their existing, widespread CS networks for voice service continuity. This allowed for a phased migration strategy. Operators could launch LTE for high-speed data and VoLTE in dense urban areas, while relying on SRVCC to provide seamless voice service in suburban and rural areas still covered only by 2G/3G. It protected the operator's substantial investment in CS infrastructure and spectrum, while meeting customer expectations for reliable, uninterrupted voice calls.

Prior to SRVCC, dual-radio devices could use Circuit Switched Fallback (CSFB) to place or receive calls by temporarily falling back to a 2G/3G network, but this was not suitable for handover of an active call. SRVCC specifically addressed the active call continuity scenario for single-radio devices, which are the norm due to cost and complexity. It was a foundational enabler for the commercial success of VoLTE, as it guaranteed that the voice service quality and reliability on LTE would be at least as good as on legacy networks, thereby removing a major barrier to LTE adoption for voice.