What are the options for voice call establishment in 5G SA mode? (Select 2)

In 5G Standalone (SA) mode, which utilizes the 5G Core (5GC), there are two primary mechanisms for establishing voice calls. The target, long-term solution is Voice over New Radio (VoNR), where the voice call is handled natively over the 5G radio access and IMS core. As an interim solution, especially during initial deployments where VoNR coverage may be incomplete, EPS Fallback is used. In this procedure, the UE, while remaining registered with the 5GC, is redirected to the 4G E-UTRAN radio access to establish the voice call using VoLTE. The data session can be maintained on 5G or moved to 4G.

B. CS Fallback: This mechanism requires interfaces between the core network and legacy Circuit-Switched (CS) domains, which do not exist in the 5G Core (5GC) architecture.
D. It is not possible: This is incorrect as both VoNR and EPS Fallback are standardized and viable methods for providing voice services in 5G SA networks.

1. Nokia White Paper, "5G voice evolution" (2020).

Page 6, Section "5G SA voice options": "For 5G SA, there are two voice options: Voice over New Radio (VoNR) and Evolved Packet System (EPS) fallback... VoNR is the target voice solution for 5G SA... EPS fallback is an interim solution."

Page 7, Table 1: Explicitly states that CS Fallback is "Not applicable" for the 5G SA deployment option.

2. 3GPP TS 23.501, "System architecture for the 5G System (5GS); Stage 2", V17.4.0 (2022-03).

Section 5.16.3, "IMS voice over PS session in 5GS": Details the architecture and principles for providing voice services directly over 5G NR access, which is VoNR.

Section 5.16.4, "Fallback to EPS for IMS voice": Describes the procedure where a UE registered on 5GS is triggered to move to E-UTRAN to establish an IMS voice session.

3. Gupta, A., & Jha, R. K. (2021). "Voice over 5G: A Survey". IEEE Communications Surveys & Tutorials, 23(2), 970-997.

Section IV-A, "Voice over New Radio (VoNR)": "VoNR is the default and native voice solution for the 5G SA network... In VoNR, the voice call is established over the 5G RAN..." (p. 978).

Section IV-B, "EPS Fallback (EPS-FB)": "EPS-FB is an interim voice solution for the 5G SA network... the UE falls back to the 4G network for voice services." (p. 979).

DOI: https://doi.org/10.1109/COMST.2021.3053314