Non classé

White Paper

VoLTE: the next wave for MVNO

Executive Summary

Focused on 5G rollout, US mobile operators have already announced the decommissioning of their UMTS, and even 2G, networks. In other countries, 2G and 3G services will also be reduced because HD voice, on 4G and 5G, and simply mobile handset renewal are going to increase VoLTE usage and reduce 2G and 3G interest. By 2023, the worldwide traffic on VoLTE is expected to exceed voice traffic on 2G or 3G.

MVNO, especially full MVNO, should prepare for the coming shift in technology. They should evolve their solutions from Camel or SIP applications towards a standardized VoLTE roaming architecture based on IMS. These MVNO VoLTE architectures, LBO and S8HR, are gaining in maturity while MNO are deploying the same solutions for their VoLTE roaming agreements.

Of course, a MVNO implementing VoLTE should integrate an IMS solution in its network, and this integration should be done with the radio network, at the service level also, and even possibly with its legacy circuit switched service. As a provider of scalable IMS technology, and with its strong history of agility, Cirpack provides such versatile VoLTE and Vo5G solution

Towards VoLTE

According to the GSM association, at the beginning of 2021, 754 mobile operators have launched LTE (Long Term Evolution) service, also known as 4G, covering 86% of the world population. While a part of the global population is still covered by old fashioned 2G or 3G networks, the most advanced operators have begun turning off 3G, as they speed up their evolution with 5G rollout.

5G is seen by many either as a revolution, or at least as the rise of Internet of Things, autonomous vehicles and much more. Beyond connecting humans, mobile operators are going to connect at extremely low latency, or higher than ever speeds, billions of probes, lamps, electricity energy meters… IoT things.

However, there is irony behind this situation: migration to 4G is not completed yet. Not only in terms of coverage, but for the most basic application operators offer: voice. 2G was launched with its telephony systems based on Mobile Switching Centers, but the new 4G telephony systems are often not yet deployed.

In this article, we will see first what 4G Voice implies, why this is a revolution, and why it is not completed. The 4G voice revolution has not happened yet for MVNOs, although they are the most exposed actors of this change in mobile technology. We will then see the rationale and benefits for them to migrate their telephony solutions. Finally, three structural implementation alternatives facing MVNO will be presented.

The coming of VoLTE

When it came to 4G definition, one of the most disruptive choices of 3GPP, the standardization body defining the mobile network protocols and architectures, was VoLTE (Voice over Long-Term Evolution), a. k. a. voice over 4G. Indeed, the 4G specifications do not include a traditional circuit switched service with dedicated equipments and protocols. The 4th generation core network is based on a general purpose IP network handling Voice as a regular data service. Of course, it should provide the end-users with a similar experience to the previous circuit-based telephony services available in 2G and 3G. It should also bring better performances, and extended features: better voice quality, HD voice, the ability to make HD video calls to name the most visible ones.

VoLTE and IMS technologies

The functions implemented by VoLTE could be summarized (and simplified) by user authentication and authorization, call control, including codec negotiation, supplementary services, billing, routing and security. A crucial and transversal requirement of VoLTE is interoperability, not only between terminals and networks but also between mobile networks themselves, and that is what makes it different from existing over-the-top IP telephony applications. This is what allows mobile operators to offer a carrier grade telephony service, fulfilling also national regulations, roaming, and critical services such as emergency calls. The VoLTE service is based on the IMS (IP Multimedia Subsystem) architecture, defined by 3GPP, and based on the IETF SIP protocol. It has been also consolidated and enriched by GSMA standards, in order to select the most appropriate modes of operations with a special interest for interconnections and roaming.

The IMS defines a chain of several functions providing call control. The Proxy Call and Session Control Functions (P-CSCF) is the first function that is interfaced with the terminals, and handles end user authentication, authorization and call requests.

Then, the Serving Call and Session Control Functions (S-CSCF) implements registrar function -where the end user is attached in the network- and receives and routes the signaling to the others IMS functions, in particular the Telephony Application Servers.

The main Application Server is called MMTel AS (Multimedia Application Server) and implements the traditional telephony supplementary services such as call forwards and barrings.

The BGCF (Break out gateway control function) routes the calls and the IBCF (Interconnection Border Control Function) implements boundary control to other IMS networks, operated by other operators.

All these functions implement the SIP protocol, and SIP messages that are usually first sent from a mobile handset are then processed and routed by the CSCF functions. IMS also relies on the IETF Diameter protocol for call and user data management among the different functions.

The first advantages of the VoLTE/IMS architecture are the consequence of this migration to IP:

  • No more voice infrastructure in RAN
  • More efficient use of spectrum
  • Faster calling and better experience, including richer codec list

Then, the intrinsic qualities of IMS can be considered:

  • Interoperability between operators and between solution suppliers, sometimes at the cost of complexity,
  • flexibility of an architecture which can evolve to support new services such as encryption, VoWifi (Voice over Wifi), ViLTE (Video over LTE), and RCS (Rich Communication Service).

Beyond VoLTE: new IMS services


Beyond regular telephony, VoLTE and IMS set up a foundation for new services. First of all, SMS service is already a feature of VoLTE, as IMS routes short messages as regular voice communications. But new capabilities can be supported by the IMS infrastructure, the first example being VoWifi service (Voice over Wifi). VoWifi is indeed a complementary service for a seamless handling of telephony using any internet connected Wifi hotspot. With VoWifi, weak public mobile coverage is then mitigated by simple Wifi as can be found at home, in the office, without any additional configuration of the phone or the Wifi hotspot. In particular, handovers between Wifi and LTE remove the hassling of connections loss, and fuse both VoLTE and VoWifi together in a transparent way.

To benefit from this service, the 4G Evolved Packet Core handling the data connection in 4G network must be completed by a centralized internet access point. On the IMS core side, the VoLTE infrastructure is almost VoWifi ready, as the VoLTE Voice Call Continuity feature preserving established calls for terminals changing their coverage, from 4G to 3G for instance, can also handle continuity from and to 4G and Wifi.


Another smooth upgrade for a VoLTE network is ViLTE (Video telephony over LTE). The IMS functions to be used are the same, the difference being marginal, as a video call is just a IMS call with video codec instead of voice codec. Of course, launch of ViLTE requires upgrade of the network in terms of capacity, and also additional support of some specific scenario, especially routing procedures, for a complete service with video mailbox, video call forwards and restrictions.

Universal Profile for Advanced Messaging

The GSMA Universal Profile is the industry standard for mobile Rich Communication Suite, a SIP based service unifying telephony with chat, audio, video, geographical location messaging, file transfer, and live sketching. It can be considered as the mobile operators answer to over-the-top applications already available on mobile phones.

The Universal Profile service can be deployed with or without VoLTE, but it is reuses the IMS infrastructure. Although it guarantees interoperability over networks and terminals, it is directly in competition with well-known over-the-top applications, and may have lost the momentum it once had.

Today’s situation: incomplete VoLTE adoption

As of beginning of 2021, years after the first 4G rollouts, VoLTE and VoWifi traffic is estimated at less than a third of the worldwide public mobile voice traffic. Some countries and operators have been early adopters of VoLTE, such as new operators that started their operation from scratch with 4G only. But out of the 754 LTE operators mentioned previously, only around 200 operators in 90 countries have launched VoLTE with HD voice.

This divergence has been self-reinforcing, as it was understood from beginning of 4G that the shift in telephony infrastructure would be gradual. Especially, the migration can not be instantaneous, as existing mobile handsets were not VoLTE capable, and 4G coverage was not often as dense and wide than the 2G or 3G ones. Anticipated by the standardization, the Circuit Switch Fallback (CSFB) feature has been allowing a 4G phone to benefit of 4G data most of the time, and then disconnect from 4G to reconnect in real time on 2G or 3G to place or receive a call. CSFB has been deployed with the first 4G rollouts and continue to be operational for the old and non-VoLTE capable phones. The resulting situation explains the rather slow VoLTE migration figures, as CSFB drawbacks, longer call establishment time to name the first, do not significantly impair the mobile operator service.

5G deployments, 3G shutdowns

Then, one point to consider for scheduling this move to IMS is the evolution of spectrum usage with the launch and maturity of 5G. MNO can refarm spectrum from 3G to 4G or better: 5G. Typically, it is expected that operators are going to shut down 3G while keeping a thin 2G coverage dedicated to special and ancient applications that can not be phase out easily. In this case, the Circuit Switched Fall Back to 3G/2G will cease to be the “good enough solution”. The spectrum refarming from 3G to 5G will force the believer in a wait-and-see strategy to finally migrate to IMS: the 4G VoLTE architecture is also the standardized telephony solution for 5G. As migrations of complete core network solutions take always time, operator should consider the move to IMS in 4G even before the critical mass of 5G subscription is reached, in order to avoid unnecessary last-minute logjam.

IMS, the key to MVNO modernization

Up to now, one type of operator has been less impacted by the mobile telephony evolution towards IMS: the Mobile Virtual Network operators (MVNO).  MVNOs offer almost the same services as “regular” Mobile Network Operators, without owning radio license and radio equipments. Different levels of “virtuality” can be considered. The most virtual MVNO simply resell their HNO service (Hosting Network Operator being the operator providing the radio access to MVNO) and focus solely on the commercial facet of telecom operator’s role: marketing the service and finding the best customer base. On the other hand, the less “virtual” MVNOs retain as much control as possible over provisioning, service operation and communication handling, except of course for the licensed spectrum operation. The latter case is referred to as full MVNO.

Concretely, a full MVNO must provide call control, and this type of MNO used to do it with Gateway Mobile Switching Center, Camel application, or even SIP application server. The move to VoLTE may reshuffle the cards on HNO/MVNO interface, or at least bring questions on the MVNO infrastructure. Is there a reason to migrate the MVNO system to a VoLTE solution, while HNO legacy systems are still in operation, and CSFB is still a frequent scenario? Should the interface between the HNO and MVNO be untouched or should it evolve?

Limit of the modernization

As seen in the beginning of this paper, VoLTE relies specifically on IP technology, allowing MNO and also MVNO, to decommission legacy Circuit switched equipments or functions. Operators can then migrate to more modern and more flexible technologies. For instance, IMS implementations typically integrate newer software middleware or tools than previous systems. However, these modernization gains are also accompanied by costs that must be identified, understood and compared to the simple maintenance of legacy solutions.

VoLTE advantages for MVNO: VoWifi and standard HNO interface

That is why modernization and even the inherent VoLTE benefits are often not enough to launch a VoLTE migration on the MVNO side. Other reasons might be found. Even if VoLTE does not appear as a killer application, specific characteristics of VoLTE, or MVNO specificities of VoLTE features, could encourage MVNO to migrate their systems.

First of all, VoWifi can be a trigger to switch to VoLTE. Indeed, MVNO usually buy wholesale traffic minutes from the HNO and resell them as a retailer to their own customers. VoWifi being a shortcut to the licensed spectrum, the one that defines the fundamental difference between the HNO and the MVNO, it would specifically reduce the MVNO primary cost.

Another reason for MVNOs to adopt VoLTE and IMS lies in the very essence of the IMS by the book, and that comes back at the definition of roaming in VoLTE.

IMS Roaming modes for MVNO

The first IMS concept that makes it a reference solution for full MVNO infrastructure could be called the SIP ubiquity in IMS.

According to the rather heavily standardized IMS framework, all IMS functions, except data management ones, implement the SIP protocol. Some alternative interfaces for service enrichment have been specified or proposed in IMS, but the SIP ubiquity demonstrates that telephony services should be better developed on the general-purpose SIP protocol. Hence SIP comes naturally as the protocol between HNOs and MVNOs, especially when HNO have deployed IMS and SIP for their own VoLTE service. Then, the question can be reduced to which SIP IMS interface would fit better the HNO/MVNO split, and how to reconnect the MVNO concept to the IMS multidomain canonical architecture.

Which IMS interface should be selected between the HNO, and the MVNO has indeed two answers. One could argue the Telephony Application Server delivering the supplementary service could be enough for a MVNO, but a large part of a MVNO responsibility is also to handle the end user authentication and authorization. As a consequence, the complete IMS core, including S-CSCF and the HSS’s IMS User Profile Service Function (UPSF) fall in the scope of the full MVNO domain.

The next question would be whether the rest of the IMS, i.e. the IMS access functions: P-CSCF, ATCF, ECSCF, should also be part of the MVNO domain. The answer here comes from the standardization and the industry.

Implementing an IMS core interfaced with the HNO IMS access functions corresponds to the VoLTE roaming method called LBO (Local Break-Out). LBO consists of having the access part of the IMS architecture, namely the PCSCF, ATCF and ECSCF, located in the visited network, while the “core” part of the IMS, namely the HLR/HSS, Serving CSCF and Application servers are located in the home network. This roaming architecture splits the IMS functions between the visited network and the home network according to their roles. We can recognize here a MVNO as a home network, with the HNO as a particular roaming partner.  

However, a second VoLTE roaming method called S8HR (S8 Home Routing) is currently the industry preferred model, due to its simpler architecture. In this architecture, the full IMS solution and the Packet Gateway of the Evolve Packet Core are implemented by the home network. With this solution, there is no need to deploy IMS equipment in the visited network, which explains its adoption. VoLTE Roaming is now recommended to be based on S8HR by the GSMA. The LBO option has even been removed from a number of GSMA reference documents.

Whatever the choice, LBO or S8HR, these two roaming architectures can be used as references for an HNO/MVNO partnership. The GSMA roaming recommendations make these two VoLTE MVNO solutions highly standardized, which is the interest for fast and affordable implementations.

One can also imagine the MVNOs to replicate more easily their roaming partnerships with several MNOs and benefit quickly and without excessive overhead from a larger coverage. In conclusion, MVNO can benefit from VoLTE particularly because it reduces their costs, because of VoWifi, and because of the streamlining of their infrastructure with standard and replicable HNO interfaces. Implementation alternatives for MVNO VOLTE.

Implementation alternatives for MVNO VoLTE

Now that the rationale for deploying VoLTE and IMS at the MVNOs, has been discussed, the implementations of IMS may anyway vary due to its richness, to the legacy of a MVNO, and the desired efficiency of the VoLTE solution. Three major VoLTE design options are presented below.

Voice call continuity

Already mentioned while presenting VoWifi, Voice Call Continuity and more precisely Single radio Voice call continuity (SRVCC) refers to the preservation of calls when a handset changes of network connection, typically from 4G to Wifi and vice versa, or even from 4G to 2G/3G. SRVCC consists of handling the handover with a handset connected to one cellular network generation at a time. This feature implementation is split between the IMS access part, typically in the Access SBC and an application server in the IMS core.

Considered as mandatory by early adopter of VoLTE, because of the incompleteness of LTE coverage at the early stages, it is now considered as of less importance thanks to the of increase of the 4G coverage. Especially, it has shown drawbacks in S8HR roaming configuration. The SRVCC mechanism suffers from additional complexity in S8HR, due to the additional signaling hops of between the HNO and the MVNO, especially in case of international roaming. It adds more delays in the process and impacts negatively the handover success rate.

On contrary, in case of LBO roaming, it is the Access SBC and a MSC of the same HNO which perform the enhanced call continuity between 4G and 2G/3G. Enhanced Voice Call continuity is then simpler to set up than in S8HR, with better performances.

User Data management

A point also to consider for VoLTE deployments is how to store the end user’s telephony subscription data. These supplementary service data list typically call forward numbers, the end user restrictions or call barrings etc. The choice about where to store the data depends on the environment of the MVNO.

The VoLTE Telephony Application Server can be deployed either in standalone or be coupled with the “HSS repository data” of the operator HLR (Home Location Register). In the latter case, the IMS Telephony Application Server accesses the data located in the HLR/HSS using Diameter interface, and the operator may benefit from the legacy HLR features and the existing systems interfaced with it. For instance, the provisioning and monitoring of service data, already in place with previous network generations could be preserved, and the service data are converted or shared between remaining circuit switched services and the VoLTE service.

On the other hand, the option with a standalone Telephony Application Server storing its own data could be seen as an optimization in case of 4G only service and is less interesting in case of a mobile service evolving from 3G to 4G and 5G.

IMS Centralization Service for migration simplification

The optimization of data repository in the IMS core network can also be adopted in case of IMS Centralized Service (ICS). ICS is defined by GSMA and 3GPP as the VoLTE service controlling calls in the Circuit switched access (i. e. covered by 2G or 3G). There, terminals should have circuit calls and should normally be handled by regular Mobile Service Center Server. In order to force the control of these Circuit Switched calls by the IMS, standardization has defined either ICS capable terminals, or MSC Server enhanced for ICS, where the behaviors of the handsets or MSC Servers are changed to delegate the supplementary service execution to the Home network’s IMS.

In today’s network, these alternative behaviors are not implemented, but more conventional ways to implement ICS using Camel services and routing number retrieval from the visited MSC allow the IMS to anchor the calls, and to execute the appropriate service logic. This way, the MVNO can benefit from one unique telephony service: there is no more need for operating a telephony infrastructure dedicated to circuit switched access in parallel to the VoLTE service. The need to integrate the IMS Application server with the HLR for service consistency on the several access networks can be removed, allowing a faster migration to VoLTE.

 White Paper by Alban Couturier

Alban Couturier is Cirpack Core Network Product Manager holds a MSc from Ecole Centrale de Lyon (96). Passionate about astronomy, history and new technologies, he has been working at Cirpack for almost twelve years. Expert in his field, he is happy to share with us his vision on IMS technology and what are the remaining paths to success…

CEO’s words – Patrick Bergougnou

Cirpack has been founded twenty years ago to help alternative Telcos grow their business by using this new and very disruptive Voice Over IP technology. They became the world leaders. It’s now time, twenty years later, to move to the next step. This is the main reason that led me, when taking over the company four years ago, to set up the next step of this strategy by going to « Full IP » & « Full Software ».  Here we are! Right now! Ready to efficiently serve our customers and prospects on their way to the next twenty years ultimate « communication over IP » technology. With this IMS technology they will better serve their customers by developing new multimedia services based on mobile and landline convergent communication tools.

Cirpack has been present alongside its customers for the past twenty years. We carefully listened to them. We better understood them. Using this deep knowledge, we developed the best adequate technology to continue to accompany them for the next twenty years.

Entrepreneur and Business Developer, Patrick Bergougnou led and developed several companies and business units in various environments in France and abroad, in the areas of consulting, software editon, equipment manufacturing, system integration and IT services.

Patrick Bergougnou is graduated Telecom Engineer from Telecom SudParis, graduated MBA from ISG Paris and HEC Executive.