Despite the worldwide COVID-19 pandemic, 5G deployment has continued at a considerable pace, with over 120 5G networks now offering commercial services.
Of that number however, relatively few are full Standalone 5G networks, with the remainder based on the 4G Core via Non-Standalone access. For voice services, it is clear that VoNR (Voice over New Radio), also known as Voice over 5G, is pretty embryonic in terms of commercial rollout, due to the fact that you need a Standalone 5G network to support it. When you consider commercial rollout of VoLTE services, this is hardly a surprise; despite LTE being available pretty much everywhere, only around 25% of commercial LTE networks are operating an IMS (IP Multimedia Subsystem) for voice services. Without an IMS, you can’t deliver voice over LTE or 5G…
Consider Figure 1; the prediction for a sudden rise in IMS based voice services is not new. Indeed, it feels a little like the timeframe just continues to shift backwards. When you consider the cost of running a network, you would assume that the typical service provider would want to switch off 2G and 3G as quickly as possible. Clearly this hasn’t been the case; there are plenty of service providers leaping ahead with 5G whilst still supporting a 2G network. The reason why of course is varied.
Based on the deployment trajectories outlined in Figure 1, there are clearly several factors at play which are limiting the rollout of IMS based voice services (whether 4G or 5G based). Some of those factors are outlined in Figure 2, although the list is not exhaustive.
To deliver a reliable IMS voice service, excellent coverage is essential. There are many service providers for whom 4G is still a relatively new network and as such, their 4G coverage simply isn’t good enough to support VoLTE. If they were to deploy VoLTE with such poor coverage, calls would be continuously handed to the 2G/3G network as soon as 4G coverage dropped. The situation for 5G is ultimately worse; with 5G being a new technology, coverage is potentially even more limited than 4G.
Nobody said deploying an IMS was easy! Both the IMS network itself, plus the standardization around the IMS, is complicated. The IMS is an additional call control architecture which introduces new protocols, new functions, new call flows and new demands on the access network. Moreover, where the 5G core control plane is largely based on APIs (Application Programming Interfaces), IMS as an older technology is not; bearing in mind that IMS was introduced in Release 5 of the 3GPP specifications, it took until Release 16 for APIs to be introduced to IMS (and only then for certain parts of the network).
Although the introduction of virtualization technology to the IMS architecture has driven the price point down, IMS is still an expensive technology to deploy. For service providers who utilize 2G and 3G for voice services, the incentive to spend time and money on rolling out IMS voice simply isn’t there. 2G and 3G voice networks are tested, reliable and crucially, generate revenue. If switching off 2G and 3G is not on the agenda for a service provider, the motivation for deploying IMS probably won’t be particularly high on that same agenda. Figure 5 outlines some of the advantages of both approaches to voice; at some point, the benefits of IMS voice must ultimately outweigh the cost.
There are numerous different interworking scenarios that must be considered to make an IMS voice deployment successful. Different network types, network architectures, handsets, client software versions, protocols and finally, different service operation all contribute towards making interworking a major hurdle in deployment success.
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