Voice over LTE (VoLTE) is the umbrella term covering all methods of transporting voice and other Circuit Switched (CS) services over LTE, but is also the specific term applied to the provision of those services via an IP Multimedia Subsystem (IMS) rather than a legacy CS core network.
The proponents of VoLTE claim, with some justification, that it is the voice solution that LTE was designed to support and that other methods of providing voice and CS services (such as CS Fallback) are simply transitional steps that operators can take on the way to a full VoLTE/IMS solution.
The IMS is 3GPP’s architecture designed to offer converged SIP-initiated and controlled real-time services to users of both mobile and fixed IP-based networks. In this environment, LTE is seen as simply another type of IP-CAN alongside fixed broadband, Wi-Fi, WiMAX, legacy mobile PS services and others.
The key aspect of VoLTE, in comparison with CS Fallback, is that all services are nominally provided via the E-UTRAN and EPC, meaning that UEs will not be required to hand over to legacy access networks to make or receive CS transactions and therefore will not lose contact with any ongoing LTE PS services during their calls (unless E-UTRAN coverage is unavailable, in which case a handover will be required).
The services offered to end users by VoLTE include real-time voice, video and multimedia calling, messaging and supplementary and location-based services, all of which are supplied via IP-based EPS Bearers. VoLTE allows handover to legacy networks; either standard PS HO (Packet Switched Handover) transactions or SRVCC CS handovers (for CS voice and video calls) depending upon the capabilities of the target legacy cell.
The 3GPP specifications that govern the operation of the IMS and therefore of VoLTE are complex and comprehensive. As vendors and operators began to plan the way in which they would deploy IMS-based VoLTE services it quickly became apparent that the complexity of the IMS specifications meant that there were potentially many different ways of configuring services. In an attempt to promote a common set of options and procedures that would allow VoLTE to be interoperable between networks and vendors an industry initiative known as the One Voice Profile was launched in 2009. This was quickly adopted by the GSM Association and is now known as GSMA (Global System for Mobile communications Association) IR.92. It acts as the ‘blueprint’ for inter-operable VoLTE services and ensures that users should be able to access IMS services via any partner LTE network when roaming.