Unleashing VoLTE capabilities. Assessing the Migration from CS Voice to IMS-based Voice over LTE (VoLTE)

Table of Contents

1 Introduction

2 Migrating to IMS-based VoLTE: Initial considerations
2.1 Benefits of IMS-based VoLTE
2.2 Providing voice over LTE
2.2.1 Circuit-switched Fallback (CSFB)
2.2.2 Voice over LTE (VoLTE)
2.3 VoLTE Logical Architecture
2.3.1 4G LTE network architecture
2.3.2 VoLTE Functional Node Description
2.4 Assessing the Voice Quality Performance
2.4.1 Leveraging the Operators’ Assets
2.4.2 Development of audio testing
2.4.3 PESQ/POLQA methodology

3 VoLTE Parameter Optimization
3.1 Robust Header Compression (RoHC)
3.2 Transmission Time Interval (TTI) Bundling
3.3 Discontinuous Reception (DRX)
3.4 Dedicated Bearers
3.5 Semi-Persistent Scheduling (SPS)

4 VoLTE Testing
4.1 Primary Implications
4.1.1 Voice Quality of VoLTE versus OTT Voice Services
4.1.2 Mobile Network Test: Testing Framework
4.2 Mobile Network Test 2014: Connect Magazine
4.2.1 Organizational aspects
4.2.2 Test methodology
4.2.3 Germany: Telephony Test Results
4.2.4 Germany: Data Test Results
4.2.5 Austria: Telephony Test Results
4.2.6 Austria: Data Test Results
4.2.7 Switzerland: Telephony Test Results
4.2.8 Switzerland: Data Test Results
4.2.9 Mobile Network Test: Overall Results DACH Region
4.2.10 Austria: VoLTE Friendly User Tests
4.3 Mobile Network Test 2014: CHIP Magazine
4.3.1 Test methodology
4.3.2 Germany: Telephony Test Results
4.3.3 Germany: Data Test Results

5 Conclusion

Bibliography

List of Figures

List of Tables

List of Abbreviations

Abstract

Operator-provided voice services will gradually migrate from today’s circuit-switched (CS) voice networks to packet-switched IP networks, using Voice over LTE (VoLTE) as the foundation to provide telecom-grade telephony services. This paper presents the first implications of introducing IMS-based VoLTE in Germany, Austria and Switzerland (DACH) from a mobile operator’s perspective. Further VoLTE performance aspects such as parameter optimization and HD voice are discussed, thus serving as a basis to then analyze the DACH mobile network test results for 2014 in terms of telephony and data performance. Overall results for a DACH VoLTE trial conducted in the third quarter 2014 complement the analysis.

Keywords: VoLTE, IMS-based VoLTE, HD voice with VoLTE

1 Introduction

The increasing gap among capacity and demand poses an urgent call for novel network technologies to allow mobile operators to improve performance on a cost-effective basis. In fact, Voice over Long Term Evolution (VoLTE) is a key component for an innovative set of services determined for all-IP networks: the goal relies on making these novel applications as available as voice and messaging are nowadays, while also offering a flexible interaction with Internet applications.

Indeed, LTE focus on a (rather flat) all-IP access technology aiming at delivering a bandwidth-efficient approach for carrying several types of subscriber traffic at the same time. In other words, the capability of transporting Voice over IP (VoIP) services along with the provision of high-rate data throughputs, characterizes one of the critical drivers for the development to LTE. As of December 2014, six countries have deployed VoLTE¹ services and still 56 others are currently preparing its implementation [30].

VoLTE denotes a GSMA standards’ profile for the provision of applications offered nowadays through Circuit Switched (CS) networks over the Packet Switched (PS) network of LTE. For many operators, VoLTE embodies a future-proof approach to rich media services, thus leaving no other reasonable alternative than to opt for its swift implementation. In this sense, the VoLTE deployment has underlined the importance of realizing the IP Multimedia Subsystem (IMS) and its related Session Initiation Protocol (SIP) within a wireless setting. Certainly, IMS and SIP are crucial in enabling VoIP services like VoLTE in a LTE environment.

For instance, IMS delivers the gateway functionality and interconnection supporting the communication between VoIP and non-VoIP devices². For its part, SIP determines the required signaling for authentication, call establishment and the delivery of complementary services³. Undeniably, to deliver ‘telecom-grade’ voice services (at least better than the legacy CS ones) is of outstanding relevance to VoLTE. In this regard, the VoIP implementation in a LTE setting clearly requires both technologies (SIP/IMS) along with LTE radio access network (RAN) features, which altogether enable VoLTE to outperform over-the-top (OTT) voice services [28].

Generally speaking, these OTT applications involve the provision of media (audio/video) to a subscriber’s handset and rely on the specific Internet service provider (ISP) only as a carrier of bits and bytes. Definitely, OTT denotes not only streaming services⁴, it also comprises VoIP speech applications.

Within this context, VoLTE⁵ is predestined to turn into the key technology for IP-based communications. What is more, VoLTE enables even better high definition (HD) voice quality, bringing forward the mobile network operator’s (MNO) competitive capabilities with regard to OTT VoIP providers. In the same manner, the MNO’s strategy for voice as well as spectrum availability will ultimately shape the VoLTE’s deployment. As operators define their implementation plans, the Circuit Switched Fallback (CSFB) might represent a first evolutionary approach, followed by the Single Radio Voice Call Continuity (SRVCC) as soon as VoLTE is launched⁶ [35]. Alternatively, other MNOs would await eagerly for ubiquitous LTE coverage before their VoLTE service offering. In fact, the MNO’s options will be affected by specific business goals, its technology architecture as well as strategic competences.

Rich Communications Services (RCS) will (analogous to VoLTE) benefit from IMS control capabilities, thus embracing an enriched multimedia portfolio. For instance, starting with the first VoLTE launch in 2012 in South Korea, a comprehensive variety of VoLTE-capable devices has been developed, leading many operators to launch it during 2014 (Asia and North America [30].

Furthermore, the numerical data throughout presented in this document uses a comma "," as decimal mark. Moreover, this paper is divided in four parts, the first of which describes some key implications of the migration to IMS-based VoLTE (chapter two). Undeniably, deploying VoLTE requires a number of optimization steps to gain the full benefit of the technology’s potential. This parameter optimization is the focus of chapter three. The fourth chapter presents a high level analysis of two mobile network tests published in 2014 that builds the basis for further findings regarding telephony and data (access) performance. Particularly, this sheds light not only on the voice test methodology, but also on the importance of VoLTE Friendly User Tests to better align the operator’s voice capabilities (over LTE) with the demanding user’s expectations. Finally, some conclusions are drawn based on the relevant aspects discussed, outlining the upcoming challenges operators will cope with in the near term.

2 Migrating to IMS-based VoLTE: Initial considerations

The need for bandwidth derived from devices and subscribers has been constantly increasing for many years. Actually, the data volume transferred by mobile networks is doubling approximately every year and the quantity of connected machine-to-machine (M2M) devices is estimated to exceed 50 billion by 2020 [25]. Nevertheless, the compound annual growth rate (CAGR) is expected to decline in the years to come, from 4% per annum for 2008 – 2014 to a moderate 3,1% p.a. till 2020 [30].

In this sense, to take advantage of mobile-broadband opportunities (while at the same time increasing benefits for business and end-users) remains the focus of every operator’s activities. LTE networks, for their part, are able to carry mobile broadband with huge data capacity and a minor latency level. Though, since there is no circuit-switched voice domain in LTE (Fig. 1), a universally interoperable IP-based voice and video calling solution for LTE is being implemented within the telecom industry: VoLTE [29] and ViLTE⁷, which further facilitate the evolution of innovative communication services.

illustration not visible in this excerpt

Figure 1: Circuit and packet domains. LTE within the Evolved Packet System (EPS) [1]

Furthermore, over-the-top (OTT) communication solutions like Skype (and most recently WhatsApp) have influenced the way users assess a particular service based on VoIP. According to an estimation [26] [30], the business loss derived from users using OTT voice services will amount to USD 386 billion for the period 2012 - 2018. Nonetheless, a completely satisfactory user experience cannot be offered by OTT solutions due to missing QoS measures or the lack of handover mechanisms to the circuit-switched (CS) network. In addition, there is no guaranteed emergency call support or extensive interoperability of services among diverse OTT services. Thus, the readiness of subscribers to use a service that does not offer security, quality, flexibility or even mobile-broadband coverage clearly influence the adoption of OTT services in a negative manner.

Furthermore, VoLTE is expected to be implemented together with CS voice, e.g. parallel to the ongoing evolution towards IMS-based VoLTE: its timing becomes vital for operators to prevent losing out to OTT providers and make the most out of VoLTE capabilities. Operators, therefore, apply the following methods:

- Spectrum refarming⁸ allows to considerably limit the implementation expenses and supports twice as many calls within the same spectrum (e.g. spectral efficiency).
- Deployment expenditures can be substantially decreased in contrast to spectrum acquisition
- Network and planning optimization poses a cost restriction and facilitates future-proof network modernization

The past two years have seen a wave of LTE network deployments around the world [30]. Interestingly, most mobile network operators (MNO) still use a CS system because of their reliability in carrying voice services and also because of user preferences (many still use CS terminals). For instance, many MNOs are currently evaluating a suitable path to ‘align’ their networks with the Internet by migrating the entire services to a (LTE IP) packet-switched environment. For this purpose, the IP Multimedia Subsystem (IMS, [12]), a system standardized by the 3rd Generation Partnership Project (3GPP), has proven to be the commonly chosen solution.

The clear advantage for IMS is its cost-effectiveness when shifting core telecom services (voice, video and messaging) to an IP environment. Additionally, IMS is more network-efficient than current systems and generates supplementary prospects for innovation in the medium term. Particularly, the basic IMS capabilities⁹ can be reused by further services, thus rendering IMS a trend-setting and comprehensive solution in regard to a broader IP-communication approach. Besides, IMS is ‘access-agnostic’, meaning the support for seamless handovers and its independence of the connection method.

Regulatory measures

In general terms, the European Union follows a consumer-oriented as well as interventionist style. In fact, the drop of termination rates, e.g. the fees that one MNO charges another one for terminating calls on its network, has brought a significant weakening of the MNO’s interconnection earnings. In this sense, the European Commission adopted by the end of 2013 a Recommendation [23] to the national regulatory authorities regarding the non-discriminative practices on costing methodologies in order to encourage competition and boost the investment in broadband infrastructure. Still, the European Commission proposed in the same year an EU Regulation targeted at the further development of the single market for electronic communications.

Furthermore, the EU strives to eradicate the roaming fees¹⁰ for consumers within Europe. The ‘Digital Single Market Proposal’, presented by the European Commission two years ago, would involve a decline in roaming fees for messages, data and calls to the particular domestic termination rates level (‘roam like at home’) [23]. Irrespective of the proposal’s result, this sort of harmonization is projected to happen in the medium term, causing returns to further deteriorate.

Moreover, competition authorities impose rigorous controls on telecom players with the aim of guaranteeing the (MNO) compliance with industry specific regulations. It is, therefore, indispensable that European network operators mitigate the negative profit effects owing to price-oriented competition and regulation by concentrating on profitable innovations and (market) sectors. Undoubtedly, finding the appropriate equilibrium among (infrastructure) capital expenditures and cost-efficiency is a crucial element to gain and maintain momentum.

Undeniable, a well thought-out (European Union’s) regulatory method can fully benefit of the socio-economic impact derived from the mobile business by facilitating innovation in mobile connectivity and applications. So, the prospect of a ‘connected’ Europe is better accomplished by promoting investments as well as end-user confidence in novel mobile technologies.

2.1 Benefits of IMS-based VoLTE

In the medium term, VoLTE operators have a sustainable competitive positioning: they can deliver superior HD voice, implement video/messaging as well as converge with the web via Web Real Time Communications (WebRTC). Moreover, (VoLTE) operators can work in partnership with application providers and supply the best user experience on a low cost basis, because they can translate application developers’ solutions onto innovative communication services. Also, a quick customization of features for strategic activities (as the mobile healthcare industry could be) becomes feasible and effective.

illustration not visible in this excerpt

Figure 2: VoLTE’s strategic value for operators. Adapted from [14]

Needless to say, VoLTE plays an important role in empowering all-IP communications in the 4G LTE network. As a result, VoLTE will enable operators to:

- Generate attractive communication services by merging mobile voice with video, the web and social networking
- Provide faster call setup times
- Improve customer experience by delivering data and better HD voice simultaneously (which also contributes to offload legacy infrastructure). In addition, the current ‘fragmented’ communications landscape that relies on various (rich) media approaches becomes more harmonized
- Migrate from dual radio CDMA (only in the USA) and LTE devices to LTE-only devices

Furthermore, the user value proposition for IP-communications¹¹ relies indeed in the MNO’s traditional assets: while each service (RCS, VoLTE) delivers some real benefits compared to other approaches, the intrinsically user value proposition of IP-communications is not based on pioneering service enrichments, but rather on a superior user experience centered on reliability and interconnection as well (refer to Fig. 3).

illustration not visible in this excerpt

Figure 3: Call setup time measurements in comparison [36]

For sure, VoLTE is not the only method by which all operators will initially launch voice service in the short run. Yet, it embodies the ideal approach if competitive advantages and their related (business) risks are considered.

2.2 Providing voice over LTE

Despite the overwhelming enthusiasm on just exploiting LTE for data, several MNOs consider the provision of voice services as LTE is being implemented into the network. In an initial stage, some approaches involve the deployment of LTE to complement the already existent High Speed Packet Access HSPA/HSPA+ and EDGE coverage (within data-intensive areas for example). Additionally, the phased deployment of LTE throughout the whole network forces operators to safeguard the continuity of voice services (with slight service interruptions) as long as subscribers roam among 2G/3G and LTE networks. In the end, since that there is no other well-specified, wide accepted or well-supported solution, operators will have no other choice than adopting VoLTE.

For its part, 3GPP provides two standards for voice service provision in 4G LTE [3] [5]:

- VoLTE: Here, the LTE network (and IMS) care for the delivery of voice calls
- Circuit Switched Fallback (CSFB): Given an established LTE connection, the User Equipment (UE) has to fall back to 2G/3G the moment a call origination/termination occurs

Particularly, the CS Fallback (CSFB) and the dual-radio methods on which CDMA operators rely (in some cases known as Simultaneous Voice-and-LTE, SVLTE) are convenient to the extent that current telephony services are reused. However, these are not indeed LTE solutions and, as a result, are not subject to suitable development. Likewise, OTT methods are attractive since they run over IP and support, on the one hand, richer experiences. On the other hand, important features like interoperability and 2G/3G integration/handover are

(with the OTT approach) definitely at stake (see Table 1).

illustration not visible in this excerpt

Table 1: VoLTE compared to other approaches for voice in LTE devices. Adapted from [16]

The development path towards VoLTE, as shown in Fig. 4¹², may follow different approaches: without a doubt, some operators will opt for a blend of CS voice and LTE data (on LTE devices). Sooner or later, more profitable prospects derived from enriched voice services for LTE smartphones will persuade most MNOs to fully adopt IMS-based VoLTE. By delivering voice and data over the same network at the same time, a superior customer experience can be accomplished.

It should be noted that the features influencing the migration alternatives (e.g. rollout, coverage as well as the implementation speed) are not only determined by the present mobile technology or market conditions, but ultimately by the available spectrum in a particular site.

illustration not visible in this excerpt

Figure 4: Voice Strategies: Providing voice over LTE [35]

2.2.1 Circuit-switched Fallback (CSFB)

CSFB embodies an initial stage in empowering LTE devices with the size, cost-effectiveness as well as power consumption benefits of single-radio approaches (in terms of 2G/3G voice combined with LTE data). In fact, CSFB reuses the legacy networks in order to deliver voice services for LTE. For this purpose, the handset¹³ is required to fall back to the legacy network prior to the call setup origination/termination. In fact, CSFB provides feature transparency and a comprehensive service while facilitating operators to exploit their 2G/3G networks for the CS service provision.

At first glance, some benefits and drawbacks derived from CSFB are outlined in Table 2

illustration not visible in this excerpt

Table 2: Assessing the CSFB approach

Nevertheless, operators who temporarily choose CSFB should critically reflect on the invested resources due to:

- Communication experience for end-users might be at stake: during voice calls CSFB subscribers would be downgraded from 4G LTE data service to 3G HSPA+ or 2G, losing data services completely.
- Boundaries are imposed on innovation: New services, which work on all-IP networks and are based on high quality video codecs, cannot be realized (such as video calling)
- In the medium term, CSFB has to be supported for devices lacking VoLTE capabilities and for inbound roaming subscribers

Another aspect to consider is the supply of devices via non-operator sales channels (‘vanilla’¹⁴ handsets), which may negatively affect the supported voice features. Even if carrier-controlled handsets will predominate, complex configuration steps would be necessary for those not provided by the MNOs.

2.2.2 Voice over LTE (VoLTE)

This approach relies on the IMS call control as stated by 3GPP TS 23.228 [12] for LTE voice application carriage. In fact, IMS delivers value-added innovative multimedia services by facilitating media additions and removals at whatever time throughout a call. Likewise, it is estimated that VoLTE will be extensively implemented in view of the wide-ranging LTE coverage, regardless of the visited network.

Furthermore, end-to-end IP real-time voice and rich media services are allowed by VoLTE executed with HSPA+ and LTE. Indeed, the benefit of this configuration is given by the seamless mobility among LTE and UMTS by means of packet-switched (PS) handovers/handbacks. For instance, VoLTE users would enjoy steady voice services independent of their attachment to a 2G/3G or LTE network. This is guaranteed by the IMS Centralized Services (ICS), in the case of 2G/3G CS service continuity, by linking the IMS call control to the GSM/UMTS access network. Despite these advantages, most operators do not follow the HSPA approach since it is less efficient for voice and QoS enforcement (e.g. delay).

Single Radio Voice Call Continuity (SRVCC)

Normally, a conversation would be dropped once the LTE signal get lost and no handover (towards 2G/3G) takes place. SRVCC and enhanced SRVCC¹⁵ [2], standardized by the 3GPP, characterize in this context the VoLTE technology responsible for this handover as a way to guarantee the call continuity as soon as the end-user finds himself outside the LTE coverage. In other words, SRVCC stands for the call continuity among IMS (over PS/CS access) for conversations attached to IMS given that the handset is only capable of communicating on one of those access networks [17]. What is more, SRVCC performs network-controlled handovers, thus removing the necessity for the UE to connect to two access networks at the same time.

From the abovementioned, the mobile device is not able to attach to more than one radio access networks simultaneously: this implies a cutting off from the LTE network and a reconnection to 2G/3G. Should this arise during a chat, the SRVCC determines the actions to be undertaken (e.g. SRVCC is not valid outside of a conversation). Another relevant factor is given by the IMS call control in the home network of the end-user, which implies that the (VoIP or CS-based) conversation is camped in the IMS domain.

Based on Fig. 5, the general SRVCC mechanism comprises an active VoLTE conversation represented by step 1, followed by a SRVCC handover (after abandoning LTE coverage): the Evolved Packet Core (EPC) ‘talks’ to the Mobile Switching Center (MSC), which initiates the transfer from LTE towards 2G/3G (step 2). Additionally, the MSC prompts the IMS to run a session handover, thus launching a new access route with the MSC. In the last third step, the IMS-controlled conversation is reinitiated over the CS core.

illustration not visible in this excerpt

Figure 5: Common SRVCC mechanism [17]

Moreover, the SRVCC procedure implies not only a handset having a SRVCC client, but also requires the involvement of the Mobility Management Entity (MME), LTE’s radio part, Service Centralization and Continuity Application Server (SCC-AS), the Home Subscriber Server (HSS) as well as SRVCC-upgraded MSCs). These components are further discussed in section 2.3.

IMS Centralized Services (ICS)

For the past years, IMS mobile telephony has been implemented as a response to the growing subscriber demands and the advent of novel 4G technologies. This is because IMS facilitates fast developing and comprehensive services above and beyond speech (video for example). Since LTE¹⁶ is not yet available throughout the world, roaming for mobile IMS rollouts still embodies a challenge. Fortunately, 3GPP and GSMA have promptly react by outlining methods (e.g. IMS Centralized Services [15]) for the provision of worldwide accessibility of IMS-controlled speech applications¹⁷. Nevertheless, no general method has been defined, which implies an explicit mechanism for a particular network type.

The term service consistency (SC), in the ICS context, points out the access reliability of speech applications through 2G/3G and LTE networks. Particularly, SC comprises a wide range of calls, even those terminated/originated from an IMS subscriber for the duration of 2G/3G network coverage. For the sake of clarification, Fig. 6 illustrates the difference between SC (the field of ICS) and service continuity.

illustration not visible in this excerpt

Figure 6: Service continuity compared to service consistency [15]

Furthermore, IMS-capable networks¹⁸ (or those roaming to a capable one) enable IMS operators to offer separated (value-added) services to their subscribers. This may be defined as the home control pattern. However, many networks neither provide IMS VoIP (due to the absence of QoS controls) nor are IMS-capable, leaving one alternative for the provision of voice: the application of 2G/3G circuit-mode MSCs. In this case, a defined home-network pattern applies.

To address the kind of incompatibility between the two settings (patterns), two options appear reasonable: the first one involves the selection of voice services that can be reliable provided via 2G/3G networks, while still offering superior speech applications through LTE. On the other hand, a set of features offering IMS home control to subscribers assisted by MSCs (3GPP ICS, [15]) may apply as the second alternative. Interestingly, most MNOs are opting for the first alternative as no reasonable method has been identified for the second one.

[...]

---

¹ Additionally, 125 mobile network operators (MNO) in 77 countries have commercially deployed HD voice services by the end of the first quarter 2015 [30]

² non-wireless devices also included

³ e.g. call retention or three-way calling

⁴ Hulu, WhatsApp, Netflix, etc.

⁵ together with a complementary conversational video service defined by GSMA

⁶ prior to ubiquitous LTE coverage

⁷ A conversational video application founded on the IP Multimedia Subsystem (IMS) and defined by GSMA IR.94 [29]

⁸ It can be considered as a process involving any basic change in the terms of frequency usage for a specific part of the radio spectrum

⁹ authentication, authorization, charging and routing

¹⁰ a plan currently under discussion

¹¹ which is comprised of VoLTE and IP-messaging or Rich Communication Services (RCS)

¹² VoLGA is not included in the graph since the standards have yet been accepted by 3GPP. Hence, it is outdated

¹³ usually attached to the LTE network for data services

¹⁴ e.g. without any (operator-side) customization

¹⁵ This applies for international roaming. SRVCC is defined by the 3GPP standards TS 24.237 [7], TS 23.237 [5] and TS 23.216 [4]

¹⁶ employed to transport IMS

¹⁷ even in CS networks

¹⁸ An application server (AS) located in the home network performs service logic. Exceptions of the service logic apply for example for emergency services