Dual Radio Voice Call Continuity

Description

Dual Radio Voice Call Continuity (DRVCC) is a mobility solution standardized by 3GPP to ensure uninterrupted voice service when a User Equipment (UE) moves between a 3GPP Packet Switched (PS) network (such as LTE) and a non-3GPP Circuit Switched (CS) network (such as CDMA 1xRTT). Unlike Single Radio Voice Call Continuity (SRVCC), which requires the UE to have a single radio transceiver capable of switching between access technologies, DRVCC is designed for UEs equipped with two independent radio transceivers. This architecture allows one radio to maintain the active voice call on the CS network while the other radio manages the PS data session on the LTE network. The handover process is coordinated by the network, specifically involving the Mobility Management Entity (MME) in the Evolved Packet Core (EPC) and the Interworking Function (IWF) that interfaces with the legacy CS network.

The core technical procedure involves a handover preparation phase triggered when the UE, connected to an LTE network for a Voice over LTE (VoLTE) call, detects that it is moving out of LTE coverage and into a legacy CDMA coverage area. The UE reports these measurements to the eNodeB, which forwards the information to the MME. The MME, recognizing the UE's DRVCC capability, initiates a session transfer procedure towards the IMS network via the Sv interface to the MSC Server enhanced for SRVCC (eMSC). The key difference in DRVCC is that the UE itself uses its second radio to establish a new CS call leg directly with the target CDMA network, in parallel with the ongoing PS bearer in LTE. The network then coordinates the switching of the media path in the IMS core from the PS access leg to the newly established CS access leg, completing the handover.

DRVCC's role is critical in early VoLTE deployments, particularly in regions where LTE coverage was not ubiquitous and operators relied on CDMA 1x networks as a fallback for voice. It provides a standardized, network-controlled handover mechanism that maintains the voice call's quality and continuity without requiring the UE to drop the PS data session. The solution is defined across multiple 3GPP specifications covering stage 1 requirements, stage 2 architecture, and stage 3 protocols for the core network and UE behavior, ensuring interoperability between multi-radio devices and heterogeneous network infrastructures.

Purpose & Motivation

DRVCC was created to solve the critical problem of voice call continuity for multi-radio devices in early 4G LTE deployments. As operators began rolling out LTE networks optimized for high-speed data (PS), voice service initially relied on Circuit Switched Fallback (CSFB) to legacy 2G/3G networks or the eventual deployment of VoLTE over IMS. However, in markets dominated by CDMA technology (which lacks a native CS core compatible with GSM/UMTS), a specific interworking solution was needed for UEs capable of VoLTE. The primary motivation was to enable a seamless user experience where a voice call started on VoLTE could be handed over to the ubiquitous CDMA 1x network as the user moved out of LTE coverage, without dropping the call.

This addressed a significant limitation of the earlier SRVCC solution, which was designed for UEs with a single radio that could only operate on one access technology at a time. Many early LTE smartphones, especially in North America, were equipped with separate LTE and CDMA radios to support simultaneous voice and data. DRVCC provided the standardized network procedures to leverage this dual-radio hardware for seamless mobility. It solved the business and technical challenge of launching VoLTE services without a fully built-out LTE coverage layer, allowing operators to offer high-definition voice on LTE where available while guaranteeing reliability through a robust CDMA fallback, thereby accelerating VoLTE adoption.

Key Features

• Enables handover of a VoLTE call from E-UTRAN to a legacy CDMA 1x Circuit Switched network.
• Designed for User Equipment with dual, independent radio transceivers (LTE and CDMA).
• Maintains the Packet Switched data session on LTE during and after the voice handover.
• Utilizes the Sv interface between the MME and the enhanced MSC (eMSC) for handover coordination.
• Requires IMS-based service continuity with Session Transfer Number for Single Radio (STN-SR) procedures.
• Defines specific UE behaviors for parallel radio operation and access stratum measurement reporting.

Evolution Across Releases

Defining Specifications