Now that the non-Standalone (NSA) flavor of 5G has been pretty much established around the globe, it is likely that more focus is put on the 5G standalone (SA) flavor of the technology. In effect that means two things: Instead of using LTE as an anchor for a 5G air interface connection, 5G can stand on it’s own feet. This requires a 5G core network (5GC) with a radically new service based architecture. One existing application that needs to be supported over 5G SA and the 5G core network is of course voice. This means that Voice over LTE (VoLTE) needs to evolve to Voice over NR (VoNR). Rohde & Schwarz has recently published great whitepaper on the topic which I liked very much, so I thought I’d say a few words about the topic here.
In their whitepaper, they go into great detail on how the operator based voice service evolves together with the rest of the network to a 5G standalone architecture. This starts with VoLTE over 5G NSA which has been there from day one of 5G NSA deployments, right to the other end of the spectrum to Voice over NR over the 5G SA air interface. Another important topic described in the whitepaper is Voice with EPS fallback, i.e. falling back to LTE for voice service. This functionality is required when running out of 5G SA coverage, or, if the network, or a mobile device, does not yet support a voice bearer over NR.
One particular piece of information that is a bit difficult to find in the 3GPP specifications is how the IP Multimedia Subsystem (IMS) that provides VoLTE and VoWifi service will have to evolve to also provide VoNR. Fortunately, there are only few changes required on the IMS side to make it interact with the 5G core network, as all ‘IMS legacy interfaces’ towards the LTE core network are also supported towards the 5G core network as per 3GPP Release 15. Here’s a quote from the R&S whitepaper on the topic (page 53):
In the initial Release 15 (Rel-15) of 5G there was an attempt to minimize the impact on IMS in order to assist deployment. In fact, from the perspective of IMS, Rel-15 looked essentially like a 4G EPC. The IMS used for VoLTE lays the groundwork for 5G VoNR. However, by Release 16 (Rel-16) the IMS becomes part of the 5GC and Service Based Architecture (SBA).
The GSMA whitepaper on the topic, already published in 2018, makes this even clearer (page 43):
According to the present 3GPP specifications in Rel-15, the AMF, UDM/HSS and PCF(5GC network function) supports the legacy reference-point interface that respectively interworks with MME (EPC network element) via N26 interface and AS/CSCF/P-CSCF (IMS network elements) via Sh/Cx/Rx interface.
The Cx and Sh interfaces are DIAMETER based and connect the IMS to the mobile network’s (LTE) subscriber database, the HSS. Reusing those interfaces means that the corresponding 5G core network services (the UDM / UDR) are also reachable this way. The Rx interface is used between the IMS system and the (LTE) PCRF and (5G) PCF, e.g. to request the establishment a dedicated bearer when a voice call is set up. In other words, IMS support in 5G SA can be straight forward, if the LTE core network and the 5G core network are from the same supplier and components are integrated with each other.
Using legacy DIAMETER based interfaces must have given the architects of the 5G core’s service based architecture sleepless nights before agreeing to this, at least in my vivid imagination. However, it was a necessary and pragmatic step to get the 5G core off the ground as fast as possible. Not reusing these interfaces would have meant that 5G SA for public networks would have had to wait for the IMS system to catch up. And this could have taken quite a bit of extra time, as the evolution from the stateful DIAMETER protocol to http/2 stateless communication is far from straight forward.
This way, however, the IMS subsystem just has to be enhanced to understand 5G network IDs that are part of some messages, and SIP Re-Registration messages when the mobile device moves between LTE and 5G SA. This should not be too hard to implement, as the same modifications had to be made when VoWifi was introduced some years ago. Here, SIP Re-registrations are sent when a device moves between LTE and Wi-Fi access. And in some messages, LTE network information was replaced with Wi-Fi identifiers.
In real live, the devil is in the detail, of course, but it’s clear that the main task to support IMS in 5G SA networks is part of the 5G core network rather than the IMS. At least at the beginning, as 3GPP Release 16 will add service based interfaces for the IMS as well. Another thing for my reading list.