Delivering 5G Voice Services - Part 1

As Standalone 5G deployment continues at a relatively slow pace, a key aspect of the network which must be addressed is the notion of voice services and 5G, termed VoNR (Voice over New Radio). Before 5G, voice was essentially delivered via the CS (Circuit Switched) domain for 2G and 3G (for those operators still using it), and via the CS or PS (Packet Switched) domain for 4G LTE.  

With respect to LTE, if CSFB (Circuit Switched Fallback) is in use, the device will fall back from the LTE network to make or receive a call on 2G/3G. Conversely, if the LTE network supports VoLTE (Voice over LTE), the call will remain on LTE, with call control being supplied by an IMS (IP Multimedia Subsystem). Of course, as 2G and 3G networks are gradually being switched off around the world, so too is the opportunity to use CSFB. With VoNR, the IMS remains the same but the access network obviously changes; note however that CSFB is not supported in 5G. 

When 5G is brought into the mix, a number of voice delivery options can be available, but the choice is dependent on the 5G deployment model. For instance, Figure 2 shows the “Option 3” deployment model based on EN-DC (E-UTRA New Radio – Dual Connectivity). With this approach, the 5G RAN still relies on the LTE EPC (Evolved Packet Core). As such, an end to end voice service based purely on 5G is not available. Instead, since the device is using an LTE eNB as the Master RAN node, CSFB and VoLTE are essentially the only voice options available. Note that strictly speaking, the en-gNB could be used to deliver VoNR (Voice over New Radio), but due to initial patchy 5G coverage this could be a risky strategy.

A high level comparison of CSFB, VoLTE and VoNR is outlined in Figure 1. 

Figure 1 CSFB, VoNR, and VoLTE Voice Services

In the early days of 5G, strictly speaking a number of voice delivery options could have been used, but the choice was dependent on the 5G deployment model. For instance, Figure 2 shows the ever popular “Option 3” or “Non Standalone 5G” deployment model based on EN-DC (E-UTRA New Radio – Dual Connectivity), which most, if not all 5G operators routinely use. With this approach, the 5G RAN still relies on the LTE EPC (Evolved Packet Core). As such, an end to end voice service based purely on 5G is not available. Instead, since the device is using an LTE eNB as the Master RAN node, CSFB and VoLTE are essentially the only voice options available. Note that strictly speaking, the en-gNB could be used to deliver VoNR (Voice over New Radio), but most, operators chose not to take this approach. Therefore, as a general rule, operators who base their 5G network purely on Non Standalone 5G tend not to have a VoNR offering.

Figure 2 “Option 3” 5G Deployment

Once the 5G Core is deployed as part of a Standalone 5G deployment, the use of VoNR (Voice over New Radio) as an end to end voice service option becomes available. This is made significantly more efficient if the service provider has already deployed an IMS network to support their VoLTE service, which in most cases they have. With this approach, the end to end 5G network will be used to support the 5G QoS Flows required for IMS Signalling and any resultant voice packets associated with a call (in much the same way that VoLTE uses EPS bearers).

Figure 3 outlines this concept and also introduces EPS Fallback and RAT Fallback as additional voice options.

Figure 3 "Option 3" 5G Deployment with EPS Fallback and RAT Fallback as additional voice options

Both EPS Fallback and RAT Fallback assume that the device in on 5G and has registered for IMS services. If VoNR is not available in the current 5G cell, EPS Fallback or RAT Fallback may be initiated:

• If an EPS Fallback procedure occurs - when the device indicates to the IMS that it wishes to make a call, the device will be instructed to move from 5G towards LTE, using both the E-UTRAN and EPC for Packet Data Network Connectivity. At this point, a VoLTE call can be initiated or even a CSFB procedure.
• If a RAT Fallback procedure occurs – when the device indicates to the IMS that it wishes to make a call, the device will be instructed to move from 5G towards the LTE E-UTRAN. A 4G cell will then be used to support the call, although voice and signalling will still flow over the 5GC. This approach isn’t particularly popular, since it requires a specific type of eNB that is able to connect to the 5G core.

Conclusion

When this blog was first released, we were talking about how VoNR was in the early days of standardization and also how CSFB would become more and more limited as 2G and 3G began to be switched off. Moreover, there was hardly any SA 5G deployed which meant VoNR simply wasn’t a possibility at the time. Today, the situation has obviously changed – we’re now seeing a lot more SA 5G (albeit not enough) and the GSMA have developed the NG.114 specification for VoNR. It seems however that mobile operators have still been slow to deploy VoNR, even amongst those operators who’ve chosen to deploy SA 5G. Clearly, coverage needs to be very good, which for some early SA 5G deployments is not the case. There’s also one more final point to consider - although a lot of 3G has been switched off, there’s still a significant amount of 2G that’s out there and hence CSFB is still readily used.

For further study, Mpirical offers the following training courses:

• Introduction to VoNR – providing a high level view of the key aspects of 5G voice services, including VoNR concepts and drivers, end to end architecture, initial procedures, call establishment and interworking, including EPS Fallback.
• Delivering 5G Voice – the course explores the delivery of IMS based voice services over a 5G access network, including the architecture required and the fundamental concepts related to SIP, Diameter and IMS SBI’s. Initial registration and call procedures will also be explored, along with interworking with legacy voice networks.
• 5G System Engineering – this course provides an end to end understanding of 5G, including Concepts and Drivers, Architecture, New Radio and NG-RAN, EN-DC, Initial Procedures, Security, PDU Sessions and Mobility.
  
  

This blog was updated on 27 July 2025.