5G NSA uses the 4G LTE core (EPC) for control functions – so voice calls on 5G NSA phones use VoLTE on an LTE bearer. 5G SA uses the 5G Core (5GC) end-to-end, enabling true VoNR. Most UK consumer 5G launched as NSA. 5G SA is being built out now and is required for VoNR.
5G NSA – formally Option 3x in 3GPP architecture options – was the fastest path to 5G deployment. It uses the existing 4G LTE core (EPC) and the 4G eNodeB as an anchor, with a 5G NR secondary node providing additional capacity. The 5G NR radio delivers higher data rates, but the control plane still runs over 4G.
Because the control plane uses the EPC, voice calls on 5G NSA phones use VoLTE on the LTE bearer. The device may show “5G” in the status bar, but when a call is made, the voice session runs over 4G LTE using VoLTE. There is no VoNR on 5G NSA.
5G SA – Option 2 in 3GPP architecture – deploys both the 5G NR radio (gNB) and the 5G Core (5GC) as a complete end-to-end 5G system. The 5GC integrates natively with the IMS via the N5 interface, enabling VoNR. QoS is managed using 5QI flows rather than EPS bearers.
When a VoNR call is in progress and the device moves out of 5G SA coverage, EPS Fallback transfers the call to VoLTE on the LTE bearer. This is the 5G equivalent of SRVCC. The handover is transparent to the user – the call continues on LTE. EPS Fallback means VoNR deployment is resilient even where 5G SA coverage is incomplete.
5G NSA allowed operators to rapidly build out 5G coverage using existing 4G core infrastructure, reducing both cost and deployment time. The high data rates of 5G NR were immediately available to consumers without the complexity and investment of replacing the core. Voice continued to work via VoLTE. For the initial phase of 5G rollout, this was the pragmatic choice.
5G SA requires building or upgrading the core network to the 5GC service-based architecture, integrating the 5GC with the IMS, and deploying new gNB infrastructure that connects to the 5GC rather than the EPC. This is a more significant programme – which is why VoNR availability in the UK remains limited as operators progress their 5G SA builds.