5G Network Voice Service (VoNR.EPSFB)

Although the demand for data services is the main driving force for the 5G router industry to promote 5G growth and evolution, voice services remain an indispensable part of mobile communication networks.

The new wireless voice (VoNR) function provided by 5G is similar to volte in 4G. VoNR is a voice solution based on IMS under the 5g SA architecture. In VoNR, the UE resides on the 5G network and provides voice and data services through gNB and 5GC on the 5G network. When the UE is not within the NR coverage area, the Inter-RAT handover will be initiated to provide VoLTE within the LTE coverage area.

In the early stage of 5G, if the 5G outdoor market does not deploy the VoNR function on the SA architecture network, or if the network has deployed the NSA Option 3 architecture but lacks 5GC, the network will not support the VoNR function. In NSA mode, as the network has EPC, VoLTE is used to provide voice services of LTE, while data services are provided by NR. However, in SA mode, the network does not support VoLTE.

Therefore, when VoLTE calls are transferred to 5G SA networks, in order to provide continuous voice services, 3GPP defines a voice fallback function. 5G has two types of fallback functions: Evolution Packet System (EPS) fallback and RAT fallback.

In EPS fallback, if the UE attempts to use voice services in a 5G network that does not support VoNR, the gNB will redirect or switch to the LTE network, so the UE can use voice services via VoLTE. After the voice session is terminated, the UE can switch back to the 5G network.

Rat fallback is similar to EPS fallback, with the difference being that UE fallback to the LTE network controlled by 5GC during a voice session.

VoNR is only applicable to NR deployments with 5GC and gNB. Therefore, operators need to utilize VoLTE services and maintain EPCs to provide continuous voice services until VoNR is available. In regions where VoLTE and VoNR are not supported, 5G-SRVCC can serve as an alternative to 5G voice solutions and provide continuous voice services through the UMTS Terrestrial Wireless Access Network (UTRAN) wireless CPE.

Among them, SRVCC (Single Radio Voice Call Continuity) is a VoLTE voice service continuity solution proposed by 3GPP. It is mainly to solve the problem of how to ensure the continuity of voice calls when a single RF UE moves between the LTE/Pre-LTE network and the 2G/3G CS network. The 4G/5G wireless CPE ensures the smooth switching between the VoIP voice controlled by IMS and the voice in the CS domain.