The IP Multimedia Subsystem (IMS) is not perfect by any means, but it is widely deployed and used by billions of customers. Now that the technology is mature, it is time to radically simplify the deployment and operations model for IMS core networks.

In mobile networks, IMS-based voice over LTE (VoLTE) is the main mode of mobile telephony. It counts over 3 billion connections globally, according to the GSM Association, and is forecast to increase to 5 billion 4G/5G connections by 2025.

In advanced markets, 2G/3G switch-off is now the major driver for VoLTE. US operators have led the way in showing how VoLTE can work reliably at scale. They are rapidly closing down 2G/3G networks and reaping the benefits by refarming spectrum for 4G/5G. European operators have not been quite as forward-thinking. They were arguably too slow to mandate VoLTE on 4G devices and are now paying for it by having to support legacy phones and maintain older networks for longer.* In hindsight, they should have been more ambitious, but they are now moving quickly.

*An important side note: IMS device support is one of the reasons some inbound roamers to the US now find they cannot make carrier voice calls, which has serious and urgent implications for emergency calling.

In the meantime, we are seeing the first voice over 5G New Radio (VoNR) services go live. There are 16 operators that have publicly announced they are investing in VoNR, according to the Global mobile Suppliers Association (GSA, October 2021). And this number is set to increase, particularly in countries where low band spectrum is dedicated to 5G. The key point is that VoNR uses the same IMS core as VoLTE, meaning this infrastructure will be around for a long time. In less mature markets, there are still hundreds of operators and billions of subscribers that are reliant on 2G/3G voice. In time, these operators will also need to migrate, with a cost model scaled to their market context and revenue per user.

All these points underline that IMS voice is an essential part of the mobile service offer. Even if you use them infrequently, you would not want a phone that does not make calls or texts.

The challenge, however, is that voice generates little direct revenue – it is a cost center. Even the most basic pay monthly plans or data passes you buy at a kiosk or online now often come with unlimited voice and text included.

For a service that does not generate much revenue but remains essential, the obvious analysis is that operators need a much lower cost IMS core infrastructure.

Achieving this will take some fresh thinking. This IMS architecture was designed in the early 2000s and is over-complicated – with too many discrete functions and interfaces to be easy and simple to manage (yes, we knew this at the time; back in 2004, I published this report). Now that the technology is mature, the time has come to simplify IMS and optimize it for cost.

There are two main options:

And who knows, by making IMS core cloud native, perhaps we can once again start to think about open interfaces and making carrier voice a part of the developer toolkit.

— Gabriel Brown, Senior Principal Analyst – Mobile Networks & 5G, Heavy Reading

This blog is sponsored by Nokia Networks.