What are the options for voice over LTE, if IMS is not ready in time?

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1 Perspective What are the options for voice over LTE, if IMS is not ready in time? By David Martin (Analysys Mason Associate) and Terry Norman (Principal Analyst) November 2009 Mobile subscribers are consuming more and more data and LTE is the technology that operators are deploying to support this rising demand. Although LTE is a fully IP packet-switched technology that is very efficient at delivering data, it was not designed to support circuit-switched voice (CSV) and SMS, which continue to be the biggest revenue streams for MNOs. Provision of support for CSV and SMS after the introduction of LTE is a growing problem for MNOs. There are two alternatives: MNOs can continue to support voice on a GSM or UMTS platform, reserving LTE for data, or they can migrate voice to the LTE platform in an approach known as voice over LTE or VoLTE. The IP Multimedia Subsystem (IMS: a mechanism for controlling multimedia sessions in the packet-switched domain) is generally accepted to be the most efficient means of providing VoLTE. Using common Internetbased protocols, IMS delivers various types of multimedia service to end users, regardless of whether the users are fixed or mobile. The efficient and cost-effective delivery of VoLTE will require operators to implement IMS, or key elements of it. The problem is that MNOs are not expected to have migrated completely from the present generation of circuit-switched MSCs to IMS-based service and control layers until 2012, or even later if the development of IMS continues to stall. The first deployments of LTE are expected in 2010 and the majority will happen in MNOs have been reluctant to be specific about their deployment schedules for LTE and IMS, but there is potentially a gap of up to two years during which LTE networks will be operational without the IMS needed to deliver VoLTE. Vigorous competition is developing between the various technologies that are evolving to fill this gap. In this paper, we focus on the arguments for and against VoLTE and on the different ways of implementing it. We examine the commercial and operational aspects of VoLTE and discuss the long-term consequences of the choices currently being made.

2 What are the options for voice over LTE, if IMS is not ready in time? 2 What are the interim solutions for VoLTE? Obviously, if the gap between the deployment of LTE and that of IMS were reduced sufficiently, there would be no need for any interim VoLTE solution. This appears to be the aim of a significant new industry initiative, known as One Voice, described next. The One Voice initiative aims to standardise and accelerate VoLTE over IMS In November 2009, a new set of specifications for VoLTE via IMS was published by a group of operators, equipment vendors and handset manufacturers calling itself One Voice. This group comprises Alcatel-Lucent, AT&T, Ericsson, Nokia Siemens Networks, Nokia, Orange, Samsung, Sony Ericsson, Telefónica, TeliaSonera, Verizon and Vodafone. The new specifications are intended to constitute the optimal combination drawn from all the various 3GPP standards relating to VoLTE, SMS and IN-telephony services via IMS. They define the migration path to VoLTE while at the same time maintaining fundamental services, such as roaming. The One Voice group aims to encourage the development of IMS and LTE in order to speed up the deployment of LTE and IMS infrastructure and the introduction of LTE services, including voice. If One Voice is successful, then VoLTE via IMS may come about soon enough, and it may be unnecessary to adopt an interim VoLTE solution. Partly for this reason, the One Voice initiative has not directly addressed the debate about the need for an interim VoLTE solution. Although One Voice has submitted its recommendations to fora including the GSM Association and the 3GPP with the aim of getting them adopted as a global standard, its recommendations have not achieved industry-wide recognition and a great deal of uncertainty still surrounds the timing, costs and complexity of migration to IMS. Therefore, it is important to consider interim solutions for VoLTE. Key strategic considerations when choosing an interim solution for VoLTE Now that LTE is soon to be deployed, operators must decide on their strategies for SMS and voice, whether or not they need an interim VoLTE solution and, if so, which one. Several important factors influence that decision. LTE and IMS deployment schedules. Many operators plan to deploy LTE before completing the migration of their voice services to an SAE and IMS core. Consequently, they are more likely to need an interim VoLTE solution to deliver voice via the circuit-switched (CS), especially if LTE handsets become available before IMS is deployed. Service mix. Before LTE-enabled smartphones become available, operators are expected to introduce LTE data services onto USB dongles and intermediate devices with built-in LTE capability (such as MIDs and PDAs). This has led some operators to conclude that it will not be necessary to have a commercially ready LTE voice solution immediately, because only data will need to be supported initially. However, even for users of such devices and applications, many operators regard it as crucial to have at least the capability to deliver VoIP over their networks in order to limit users adoption of over-the-top VoIP services such as Skype or Google Voice. In addition, the importance of continuing to support SMS should not be underestimated. Apart from being a major revenue source, SMS continues to be an important service for users of mobile broadband packages. It is also used to deliver SIM-configuration instructions as part of remote devicemanagement systems.

3 What are the options for voice over LTE, if IMS is not ready in time? 3 Availability of LTE clients and handsets. Although the first commercial handsets are expected to be launched from 2011, it is unlikely that LTE handsets will occur in commercial numbers before By 2013, many of the leading operators expect to have completed their IMS deployments. Delaying the introduction of VoLTE until IMS roll-outs are more extensive could avoid the need for an interim VoLTE solution. Such a strategy would, however, fail to capitalise on the benefits of migrating all operators voice traffic over to LTE at the earliest feasible opportunity, which could bring about significant cost and operational-efficiency savings owing to LTE s much more economical utilisation of spectrum and the cost reductions that could be achieved from running only one access network. Alternatively, if the One Voice initiative succeeds in bringing forward the introduction of VoLTE via IMS, then LTE handsets may become widely available sooner than initially projected. Financial constraints. In the current economic climate, operators are focused on the need to make short-term opex and capex savings, which often means that they continue to use existing assets for as long as possible. This may increase the appeal of interim solutions that allow operators to support VoLTE via their existing CS core networks and perhaps to delay full-scale IMS deployment until the financial environment favours longterm investments. Also, operators will not wish to risk their basic voice and SMS revenue by migrating to IMS before they are completely confident that the full range of existing CS services can be supported adequately. What are the interim options for VoLTE? Four interim solutions have emerged to bridge the gap between LTE service launches and completed migrations to IMS. These are listed below. Circuit-switched fall-back (CSFB). This is the official 3GPP-specified standard for delivering voice services to LTE devices until full-scale IMS deployments are complete. CSFB routes voice calls and SMS messages via the existing CS core and 2G and 3G access networks. To set up or answer a telephone call, the user must interrupt a data session over LTE. As the voice calls and SMS are carried over the 2G or 3G RAN, this is not technically voice over LTE Voice over LTE generic access (VoLGA). This prospective standard is supported by a number of vendors, but by only one operator, T-Mobile. VoLGA uses the existing UMA/generic access network (GAN) technology standard to convert CS voice via an access gateway into a VoIP tunnel that can then be transmitted over LTE, but using GSM/UMTS signalling, rather than SIP. This is the same method that is used to deliver VoIP over Wi-Fi to public WLAN hotspots and domestic WLANs. Fast Track VoLTE. This is a proprietary hybrid mobile switching centre (MSC)/VoIP solution from Nokia Siemens Networks. It allows existing Nokia Siemens Networks SIP-enabled MSCs to deliver VoIP over the LTE radio access network (RAN) by means of a software upgrade. The same equipment can later be reused as a VoIP server in an IMS framework. Combined Mavenir mone/acme Packet session border controller (SBC) solution. This is another hybrid MSC/VoIP solution, similar Fast Track VoLTE, but instead using equipment supplied by Mavenir, an independent vendor. As such, it can be implemented by operators that use any SIP/VoIP-enabled MSC, not just that made by Nokia Siemens Networks. The solution combines Mavenir s mone converged IP-voice and messaging platform with the existing MSC and other media servers (if required) to provide a suite of IMS application servers that convert CS voice into SIP-based VoIP, which is delivered to the LTE RAN via a

4 What are the options for voice over LTE, if IMS is not ready in time? 4 gateway. Acme Packet s SBCs manage service delivery on multiple access networks to any SIP-enabled end point, not just via LTE. How do these solutions compare? Circuit-switched fall-back (CSFB) This option is most appropriate when the interval between the commercial availability of LTE handsets and the migration of voice services to IMS is too short to justify a more-expensive interim solution, like VoLGA. This point was made by Orange s Executive VP, Networks, Carriers, Platforms and Infrastructure, who recently said that CSFB would be Orange s interim solution if one was needed, If LTE handsets were available before IMS deployment, the interim solution would be to fall back on the 3G network for voice, as standardized by 3GPP. 1 However, CSFB has considerable disadvantages in terms of quality of service and technology and should be considered only as a stopgap. CSFB allows operators to deliver voice and SMS to users with LTE-enabled handsets before migrating to IMS without having to deploy an interim VoLTE solution. CSFB requires only a software modification to the MSC, and all voice and SMS traffic is sent over the existing 2G or 3G network, which can offer near-100% coverage immediately. No new investment in hardware is needed. The CSFB technique has a number of disadvantages. Whenever voice calls are initiated or terminated via CSFB, the fall-back procedure interrupts the packet-data session. This increases call set-up times, perhaps by as much as seconds. Furthermore, at the instant the call is transferred, the GSM, UMTS/HSPA or LTE network packets may be lost, causing the packet-error rate to rise substantially. The customer experience is likely to be made worse by enduring a markedly longer wait for call connection and a reduction in quality of service due to lost packets. This does not support the marketing message that LTE is the next-generation in mobile technology. Furthermore, CS voice is not compatible with next-generation services, such as rich communication suite-based (RCS-based) presence and multimedia-content sharing, which represent an important revenue stream for LTE operators. CSFB is generally regarded as a step backwards, in terms of technology and services. VoLGA This option is supported by several of the equipment and device manufacturers behind the One Voice initiative, but, of the operators, only T-Mobile has so far lent its support to VoLGA. This makes doubtful VoLGA s adoption as a global standard, which would be necessary to procure some of its key benefits, such as roaming interoperability. VoLGA is best suited to operators like T-Mobile and Orange, which have made substantial investments in UMA/GAN to support both hotspots and convergent cellular/wi-fi services across their 1 Vivek Badrinath, LTE/EPC : addressing the broadening of the mobile data market paper presented at Orange meeting, 29 June 2009.

5 What are the options for voice over LTE, if IMS is not ready in time? 5 European footprints. However, Orange has not been vocal in supporting VoLGA, and instead has backed the One Voice IMS-only initiative. It will be interesting to see whether the stance of either carrier will change now that the merger of their UK operations has gone ahead. VoLGA uses the existing UMA/generic access network (GAN) standard and CS core infrastructure to deliver VoIP over LTE. It requires no modifications to the MSC and preserves existing interconnect and roaming models. It is a relatively inexpensive solution, with little operational risk. It does, however, require software modifications to the GAN VoIP gateway (and the purchase of such a gateway, if operators do not already use this technology) to enable LTE to function as a circuit-switched channel, and to ensure seamless hand-over between LTE and existing 2G or 3G networks. Unlike CSFB, VoLGA supports simultaneous voice, SMS and data sessions over LTE. Although VoLGA enables concurrent VoIP, SMS and SIP-based data sessions, it uses GSM or UMTS signalling, so that dual clients are required to support SIP sessions alongside voice or SMS. VoLGA retains the existing CS core network for voice and SMS and is, therefore, only intended as a transitional solution on the path to IMS. In addition, use of VoLGA requires the use of multi-mode handsets in order to support calls delivered via VoLGA, Wi-Fi and GSM/UMTS. It is not clear if vendors will perceive the development of such devices as being commercially sensible, given that this is an interim solution with a limited product lifecycle. VoLGA is also compatible only with GSM-derived technologies and is, therefore, not a solution for CDMA and CDMA operators. Nokia Siemens Networks Fast Track VoLTE This solution is most likely to appeal to operators that have already deployed 3GPP Release 4-compliant Nokia Siemens Networks MSCs, which include support for SIP-based VoIP. It is not really an option for operators that have not deployed this equipment or even for multi-national operators that have done so, because the latter are unlikely to have installed the appropriate MSCs in all their operations. A solution that is compatible with equipment from multiple vendors would be better, as it would avoid complications with interoperability and roaming. The main advantage of this solution is the ease of implementation for operators using the company s MSCs. The vendor claims that only a minor software upgrade to the MSC is required, but the MSCs must be the latest 3GPP Release 4, SIP-enabled version, which function as a VoIP server and gateway. According to its own figures, the vendor has 220 MSC customers. Many of these will not have the requisite version of MSC and will first have to upgrade their MSCs to support VoIP-server functionality. This solution is proprietary and can be used only with MSCs made by Nokia Siemens Networks, so the benefits are limited to its customers. Furthermore, because the vendor has a small share of the global MSC market and is almost totally absent from North America, Fast Track VoLTE is not a global solution. It is unlikely to be straightforward to implement software and hardware upgrades to the MSC, and this could be a costly and risky process. Like VoLGA, Fast Track VoLTE, does not offer integrated support for mobile telephony and RCS,

6 What are the options for voice over LTE, if IMS is not ready in time? 6 although the Nokia Siemens Networks MSC can be subsequently adapted to function as a fully integrated SIP/VoIP server within an IMS core. Mavenir mone/acme Packet SBC This represents an attractive migration path to full-scale IMS and LTE, as it provides many of the definitive elements of IMS core and access network systems, while also offering compatibility with, and continuity of, existing CS voice and SMS. However, although this solution offers a flexible path towards IMS, many operators are now likely to wait until they have evaluated the implications for their IMS strategies of the One Voice proposals, and their decisions will be also influenced by the response of industry fora, such as GSMA and 3GPP. This may increase the attraction of the quick-fix VoLGA and Fast Track VoLTE solutions. The combined Mavenir and Acme Packet solution is a convergent voice and messaging platform that is compatible with existing CS core or evolved packet core (EPC) networks, and is compliant with IMS standards. The Mavenir platform is fully interoperable with MSCs from any vendor and no modifications are required to the MSC. It does not require the GAN standard to provide VoIP gateways. Also, dual-band LTE and 3G phones will be sufficient, as it does not require an additional handset client. Unlike other VoLTE solutions, the Mavenir/Acme Packet solution allows voice and SMS to be fully blended with RCS services. The platform also offers built-in support for the IMS-specified Single Radio Voice Call Continuity (SRVCC) procedure, which regulates hand-over between LTE and 2G or 3G networks, as specified in the One Voice formula for IMS/LTE and, incidentally, as supported by VoLGA and Fast Track VoLTE. Unlike VoLGA and the Fast Track VoLTE solution, the combined Mavenir/Acme Packet formula has already been deployed by one US and one pan-european mobile operator, although only for fixed services as yet. Neither of these operators has yet deployed LTE, but they are planning to do so using the Mavenir/Acme Packet solution. Mavenir is a modular software solution, which can incorporate a number of different functions, such as RCS servers, core IMS functions and MSC telephony application servers (TASs) for converting SIP to MSC call control. Acme Packet brings experience from about 100 IMS deployments worldwide, mostly in the fixed network domain. In addition, by incorporating both the Proxy Call Session Control Function (P-CSCF) signalling and IMS-access gateway (AGW) media functions of an IMS system, the Acme Packet SBC provides extensive quality-of-service, security and bandwidth management functions. It should be noted that these are vital functions for any SIP/VoIP or IMS solution, which Acme Packet s technology is designed to provide within a multi-vendor environment, including for Nokia Siemens Networks MSCs, not just Mavenir s platform. The Mavenir/Acme Packet solution also guarantees the delivery of mobile calls to all SIP-enabled end points, which resemble mobile end points from the point of view of the MSC. This includes fixed end points. The major disadvantage of the Mavenir/Acme Packet option is that it potentially limits operators choice of IMS platform more than the other interim solutions. This is precisely because unlike the other interim solutions it incorporates key foundational elements of a full-scale IMS deployment. It could be argued that there is no point in investing in a whole voice and messaging convergence platform (Mavenir mone) and a comprehensive Acme Packet SBC solution merely as a temporary measure. On the other hand, Mavenir and Acme Packet are keen to emphasise the modular and flexible nature of their technology: they say that it is not necessary to deploy only a

7 What are the options for voice over LTE, if IMS is not ready in time? 7 full Mavenir/Acme Packet solution, as components of it can interoperate with other vendors CS or IMS and EPC cores. Initial VoLTE deployments are a short-term response with long-term consequences A number of factors are contributing to operators uncertainty about delivering voice and SMS during the rollout of LTE. These include: the uncertain time frames for the deployment of LTE and IMS, and for the availability of LTE handsets the technological complexity of migration to full-scale IMS, particularly for smaller operators and mobileonly providers, as opposed to fixed and mobile players, for which the benefits of technological and service convergence are more obvious the financial costs and operational risks involved in the deployment of LTE and IMS. The operational risks are particularly important, as they affect operators largest revenue streams, circuit-switched voice and SMS. Integrated planning for both interim VoLTE solutions and definitive LTE architecture is needed now, to ensure short-term service continuity and long-term profitability There is much uncertainty about VoLTE, particularly about the timetable for launch of LTE, IMS and LTEcompatible handsets, the lack of a definitive standard IMS model for VoLTE and even whether comprehensive IMS architecture is necessary or feasible, given other financial and operational constraints, particularly for mobile-only providers. It is possible that no set of IMS standards for VoLTE or corresponding ecosystem of technology and devices will be ready in time to support VoLTE. Operators should, therefore, take the following steps: define their own timelines for the deployment of LTE carry out a cost benefit analysis for migrating voice to LTE, considering the deployment of an interim VoLTE solution as part of their planning for LTE and IMS deploy an interim solution that minimises the cost of upgrading to a full IMS architecture or to the combination of IMS and CS elements that is adopted for the longer term. The CSFB option will serve as a stopgap, but is not a long-term solution because of its many disadvantages. For most operators, neither VoLGA nor Fast Track VoLTE appears promising. A generic solution, such as the Mavenir/Acme Packet and any similar IMS solutions that may come along, is probably the best path to explore, as it enables some IMS elements to be implemented and combined with the existing CS core and GSM/UMTS RAN and is also interoperable with IMS solutions from multiple vendors. However, operators need to keep a watchful eye on the development of industry standards that will inevitably shape and restrict their choices, but will help to create more confidence about the timescale and technological options. The One Voice initiative is a highly positive step in this regard.