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Voicing the Future of LTE

23 марта 2010

You may have heard of the VoLTE initiative, and may even be aware that it is a GSMA-led programme to define a common way to carry voice over LTE using an IMS. But how did the industry arrive at this level of co-operation, what is the difference between VoLTE, CSFB, VoLGA, GAN, CSoPS (and other abbreviations), and why is it now important to have a common approach to voice over LTE?

In 2010, we will get a much clearer idea of the potential of Long-Term Evolution (LTE) networks to deliver fast mobile data services to large numbers of people. The world's first commercial LTE networks were launched recently in Sweden and Norway by TeliaSonera, with Verizon Wireless in the US, NTT DoCoMo in Japan and China Telecom set to follow suit this year. But while we all get excited about what LTE will mean for apps, augmented reality, social networking, video-on-demand, Internet browsing and many other multimedia services, we shouldn't forget that voice services still generate close to three-quarters of operators' revenues worldwide.

Soaring demand for the menagerie of multimedia services may be driving the testing and deployment of LTE networks in Europe and elsewhere, but it is crucial that voice services aren't an afterthought. Not only do LTE networks need to support high-quality voice calls, it is also vital that the operators running these LTE networks implement voice services in a consistent way, as they did with GSM.

A consistent approach to implementing voice services, together with the coordinated allocation of spectrum, has underpinned the extraordinary success of GSM both in Europe and worldwide. GSM and related technologies, such as HSPA, thrive because manufacturers can produce mobile phones that will work in many different countries, enabling them to achieve the scale necessary to create a very broad range of models at very low cost. Moreover, people with GSM phones remain contactable all over the planet because more than 700 mobile operators in more than 200 countries and territories have all implemented voice services in a consistent manner.

Going outside the standards

But this fundamental principle seemed to have been forgotten, at least temporarily, when it came to defining a voice implementation for LTE. When LTE was first conceived one of the early decisions taken by the standards body 3GPP was that LTE would be the first technology in the GSM family not to support circuit-switched connections. That decision is easy to understand: a single IP-based transport system is key to ensuring that LTE networks are efficient enough to carry large volumes of traffic cost-effectively.

Unfortunately, that decision also left a vacuum. Instead of an explicit statement of what should replace the circuit-switched domain for voice, there was just an implicit assumption that operators would provide voice services using an IP Multimedia Subsystem (IMS) and the related Multimedia Telephony Service (MMtel) standard - a framework within which specific applications for person-to-person communication, including voice services, could be defined.

But when MMtel was conceived in 2006, many operators were lukewarm in their support for IMS and there was some pressure to define an alternative means of implementing voice services on LTE networks. So, 3GPP began to define a circuit switched fall back (CSFB) approach as an ‘interim' or ‘migratory' solution, which operators could use until they were ready to adopt IMS. Unfortunately, this compromise also opened the door to other approaches and further fragmentation ensued. 3GPP also began to study a further interim or migratory approach, known as circuit-switched over packet-switched system (CS over PS).

But the emergence of these different solutions raised some uncomfortable questions. Wouldn't these "migratory" solutions end up being used for a long time? (A mobile network needs to support a migratory solution as long as there are handsets in circulation that only support that solution.) In the meantime, the cherished GSM principle of interoperability might be jeopardised. Could a CSFB device roam on a CS over PS network, or vice versa? Could either roam on an IMS network? After considering such issues, 3GPP halted the CS over PS work. But soon afterwards the VoLGA Forum was established to continue the definition of CS over PS outside of 3GPP, and to support the provision of voice over LTE using the GAN (Generic Access Network) standard.

Getting back on the IMS track

Meanwhile, the industry still lacked a clear target for the IMS-based implementation that everyone now accepted would be the end goal. So, the One Voice group was formed to flesh out an IMS-based solution. The work of that group has now become the basis of the GSMA's Voice over LTE initiative (VoLTE), which is aiming to accelerate the launch of IMS-based voice services, curtailing the period in which migratory solutions will be needed.

IMS has many merits: it can support all standard voice call service features such as call waiting, call hold and call barring, and is capable of serving large numbers of subscribers. IMS can also be used to integrate voice calls with enhanced, rich features such as presence, instant messaging and video content, across networks run by different mobile operators. Here at the GSMA, we are coordinating the development of the specifications that will enable interconnection and international roaming between LTE networks with the goal of completing that work by the first quarter of next year.

The GSMA is also working with mobile operators to ensure that LTE spectrum is aligned internationally, as far as possible, and that the VoLTE solution will also be fully-compatible with voice services offered by fixed-line operators.

The VoLTE initiative has the backing of more than 40 organisations from across the mobile ecosystem, including many of the world's leading mobile operators, handset manufacturers and equipment vendors. The European mobile operators supporting the initiative include 3 Group, Deutsche Telekom/T-Mobile, mobilkom austria, Orange, Telecom Italia, Telefónica, Telenor, TeliaSonera and Vodafone.

These operators recognise the importance of maintaining the high level of global interconnection and roaming inherent in today's 2G and 3G networks. Telefonica CTO Vicente San Miguel, for example, said in February that his company "strongly supports this initiative to drive a common voice and messaging solution for the mobile industry, as it is a key enabler for the success of LTE."

With this level of support for VoLTE, the mobile industry should be able to ensure that the interoperability and global reach that characterises GSM-based voice services continues in an all-IP world. High-quality voice services that work everywhere are fundamental to the mobile industry's raison d'être and we mustn't forget that.

by Dan Warren, Director of Technology, GSMA


Источник: Mobile Europe

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