Please explain the difference between VOIP and VOLTE.

Traditional voice services work in circuit-switched (CS) networks, while LTE is a packet-switched (PS) network, so it cannot handle CS voice. As shown in the figure below:

In order to solve the problem of voice service in LTE network, one solution is to reduce the voice to 2G/3G, which is called CSFB(CS fallback). The biggest disadvantage of CSFB is that it relies on CS core network and 2G/3G wireless network.

Another solution is VoLTE, which transmits voice completely on LTE network, independent of 2/3G network.

However, VoLTE needs an additional IMS network. On the LTE network plane, LTE regards voice and related signaling as encapsulated in packet transmission like other data (only with higher priority); IMS network receives and processes these data packets, distinguishes signaling and voice data parts of these data packets, and manages voice control plane (IMS signaling) and user plane (IMS service).

Because LTE is a network that only transmits data, and WiFi also transmits data, some people think, can WiFi access IMS as an access network like VoLTE? Of course.