When UE supports DTM it can have both voice and data simultaneously. PS data is supported in 2G network only when DTM (Dual Transfer Mode) is supported. When in legacy network (3G/2G) the UE can have PS data sessions. After the call is over UE again move back to the LTE network. The UE use the legacy network to initiate the call and complete the call. Simply speaking, when a UE or mobile phone starts a voice call the network move the UE to a legacy UMTS (3G) or GSM (2G) network. To support voice and other CS services CS Fallback was introduced. VoLTE was on development phase and was not actively rolled out. When LTE was introduced there was practically no option for voice communication. We will check cases for both MO (Mobile Originated) and MT (Mobile Terminated) calls. In this tutorial we will check simple CSFB signalling call flows. The basic reason behind this is the cost to implement VoLTE and IMS. Some operators support all the above options and some support only CS Fallback (CSFB). Similar to VoLTE, VOWiFi uses the IMS to initiate, connect and control voice communications. Though VoWiFis is the commercial name, the service is sometime referred as EPDG, WiFi Offload, etc. Some operators use VoWiFi to offload voice traffic over a WiFi network. I wrote a complete beginners guide on VoLTE which you can find here. The service is heavily dependent on many new core network elements.
It uses the IMS (IP Multimedia Subsystem) to initiate, connect and terminate voice communication. VoLTE is the true solution for voice communication over the all-IP LTE network. Let’s check briefly about the above options for voice communication. To support voice communication operators have many options. LTE is an all-IP network with no CS support.
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