Providing voice service is a key part of any mobile network. In LTE, there are three main ways to support voice calls: using dual-standby phones that switch between LTE and older networks, using CSFB which temporarily moves calls to 2G or 3G, and using IMS which allows voice directly over LTE. This article focuses on the IMS-based solution, which supports VoLTE (Voice over LTE) and lets users make voice calls without leaving the LTE network.
VoLTE Background
1. Voice Solution Based on Dual-Standby UEs
Dual-standby UEs are mobile devices that can work with both LTE (E-UTRAN) and older 2G/3G networks (GERAN/UTRAN) at the same time. These devices use the older networks (GERAN or UTRAN) to make voice calls, while LTE is used only for internet or data services. This setup allows users to enjoy both voice and data services without conflict, but voice still relies on legacy networks instead of LTE.
2. Voice Solution Based on CSFB
In the early stages of LTE network rollout, CS Fallback (CSFB) was used to handle voice calls before IMS was ready. When a user tries to make a voice call while connected to LTE, the network (via the MME) instructs the device to switch over to the older 2G or 3G network (GERAN or UTRAN) to complete the call. LTE is used for data, while voice services fall back to legacy networks. This allows LTE users to still access voice without needing VoLTE.
3. Voice Solution Based on IMS
The IMS (IP Multimedia Subsystem) is a core part of the VoLTE solution and includes various network elements that manage call control and media handling between users. VoLTE allows LTE users to make voice calls directly over the LTE network. However, if LTE coverage is lost during a call, Huawei ensures service continuity using two methods:
- If the 2G/3G network does not support VoIP, the call is handed over to the CS domain using SRVCC.
- If VoIP is supported, a PS handover is used to continue the call.
