With 5G imminently about to be commercially realised, a key aspect of the network which must be addressed is the notion of voice services and 5G. Currently, voice is essentially delivered via the CS (Circuit Switched) domain for 2G and 3G, 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). A high level comparison of CSFB and VoLTE is outlined in Figure 1.
Figure 1 CSFB and VoLTE Voice Services
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.
Figure 2 “Option 3” 5G Deployment
Once the 5G Core is deployed, additional deployment options identified as: “Option 2”, “Option 4” and “Option 7” can be introduced. In addition, the use of VoNR (Voice over New Radio) as an end to end voice service is also an option., particularly if the service provider has already deployed an IMS network to support their VoLTE service. With this approach, the end to end 5G network will be used to support the 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).
Note however that with the 5G Core, there is no CSFB capability, so this voice option is lost. Figure 3 outlines this concept and also introduces EPS Fallback and RAT Fallback as additional voice options.
Figure 3 “Option 3” 5G Deployment
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.
Although it’s relatively early days for VoNR standardization, the service itself will have very close similarities to VoLTE. Since 2G and 3G network are gradually being phased out, it is a matter of time until VoLTE and subsequently VoNR will become the prevalent voice delivery technologies.