Voice over LTE: VoLTE

This edition first published 2012

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Library of Congress Cataloging-in-Publication Data

Poikselkä, Miikka.

Voice over LTE (VoLTE) / Miikka Poikselkä, Harri Holma, Jukka Hongisto, Juha Kallio and Antti Toskala.

pages cm

Includes bibliographical references and index.

ISBN 978-1-119-95168-1

1. Long-Term Evolution (Telecommunications) 2. Internet telephony. I. Holma, Harri, 1970- II. Hongisto, Jukka. III. Kallio, Juha. IV. Toskala, Antti. V. Title.

TK5103.48325.P65 2012

621.3845′6–dc23

Preface

Voice communication has become mobile in a massive way and the mobile is the preferred way for voice communication for more than six billion subscribers. The introduction of high speed packet access (HSPA) also allows a large set of data services from smartphones, tablets and laptops to go mobile. The data volumes in mobile networks greatly exceed the voice volumes—from the traffic point of view, networks have already turned from voice dominated to data dominated. The next generation mobile radio system, called long term evolution (LTE), started commercially in 2009 and is designed to boost further the data rates and capacities. LTE radio is primarily optimised for high capacity data but can also support voice efficiently. During 2012, smartphones with the capability to provide a voice service using the LTE network are becoming available. This book describes how the voice service is supported with LTE capable terminals. The voice support in LTE is not as trivial as in second generation (2G) GSM and third generation (3G) WCDMA solutions where circuit switched (CS) voice was used, since LTE is designed only for packet switched (PS) connections. The voice service in LTE uses the voice over Internet protocol (VoIP)—called VoLTE (voice over LTE)—together with the IP multimedia system (IMS). There are also alternative solutions for supporting voice in the initial phase where the voice service runs on legacy 2G/3G networks while only data is carried on LTE. The voice solutions, architectures and required functionalities are described in this book.

The book is structured as follows. Chapter 1 gives the background and Chapter 2 describes the operator VoLTE deployment strategies. The system architecture is presented in Chapter 3 and VoLTE functionalities in Chapter 4. End to end signalling is illustrated in Chapter 5 and IMS centralized services in Chapter 6. The VoLTE radio performance is presented in Chapter 7 and the VoIP over HSPA networks in Chapter 8.

LTE will access a very large global market—not only GSM/WCDMA operators, but also CDMA and WiMAX operators and potentially also fixed network service providers. The large potential market can attract a large number of companies to the market place pushing the economics of scale, which enables wide scale LTE adoption with lower cost. The voice solution is a key part of the system design—every single LTE capable smartphone needs to have a good quality voice solution. This book is particularly designed for chip set and mobile vendors, network vendors, network operators, technology managers and regulators who would like to get a deeper understanding of voice over LTE.

Acknowledgements

We would like to thank the following colleagues for their valuable comments: Jari Välimäki, Peter Leis, Curt Wong and Martin Öttl.

The authors appreciate the fast and smooth editing process provided by John Wiley & Sons Ltd and especially Susan Barclay, Sandra Grayson, Sarah Tilley, Sophia Travis and Mark Hammond.

We are grateful to our families for their patience during the late night and weekend editing sessions.

The authors welcome any comments and suggestions for improvements or changes that could be used to improve future editions of this book. The feedback is welcomed to the authors' email addresses - miikka.poikselka@nsn.com, harri.holma@nsn.com, jukka.hongisto@nsn.com, juha.kallio@nsn.com and antti.toskala@nsn.com.

List of Abbreviations

Chapter 1

Background

At the end of 2004 the third generation partnership project (3GPP) Standardisation Forum started to evaluate a new radio technology as a successor for wideband code division multiple access (WCDMA). This work was called long term evolution (LTE) and is nowadays the radio interface name used in most official publications. Inside 3GPP the newly developed radio access network is called the evolved UMTS terrestrial radio access network (E-UTRAN) to indicate the path from the global system for mobile communications (GSM)/Enhanced data rates for global evolution (EDGE) radio access network (GERAN) via the GSM/general packet radio service (GPRS)/EDGE to UTRAN [WCDMA/high-speed packet access (HSPA)] and finally to E-UTRAN (LTE). In parallel to the work on a new radio interface 3GPP initiated a study to evolve the 2G/3G packet core network (known as the GPRS core) in order to cope with the new demands of LTE. This core network study was called system architecture evolution (SAE) and it was documented in the 3GPP technical report (3GPP TR 23.882). The final outcome of this work was a new packet core design in Release 8 documented in (3GPP TS 23.401) and (3GPP TS 23.402), called the evolved packet core (EPC). 3GPP Release 8 was officially completed in March 2009 and the world's first commercial LTE network was opened in December 2009 by TeliaSonera.

3GPP Release 8 introduces major advances in mobile networks. For the subscriber, it means higher access rates and lower latency on the connection, while for the mobile communication service provider, LTE radio technology provides lower cost per transmitted bit thanks to more efficient use of radio network resources and delivers excellent voice spectral efficiency, as described in Chapter 7. The technology also offers more flexibility in frequency allocation, thanks to the ability to operate LTE networks across a very wide spectrum of frequencies. LTE also minimises the power consumption of terminals that are used ‘always on’. 3GPP Release 8 also introduces major advances in the core network that improve service quality and networking efficiency, leading to a better end user experience. GPRS technology has already introduced the always on concept for subscriber connectivity and 3GPP Release 8 mandates this ability, with at least one default bearer being always available for all subscribers. This allows fast access to services as well as network initiated services such as terminating voice calls and push e-mail. The connection setup time for person to person communication is also minimised with always on bearers.

But considering the fact that LTE is an all-internet protocol (IP) technology we can get to the conclusion that the voice service will have to be delivered in a different way as circuit switched voice will not be possible. So there is a need for a voice solution on top of LTE. Voice in this IP world, would be implemented as voice over Internet protocol (VoIP). The 3GPP-specified way to support VoIP is the IP multimedia subsystem (IMS). It is an access-independent and standard-based IP connectivity and service control architecture that enables various types of multimedia services to end users using common Internet-based protocols. 3GPP has worked with the IMS since 2000 and there exist thousands of pages, in different specifications, that cover IMS related functionalities. In the meantime, a sophisticated architecture and feature set has been developed. Moreover, 3GPP has specified multiple, different ways to complete single functions (e.g. authentication, session setup, supplementary service execution, bearer setup) which increases complexity of the IMS.

While 3GPP has specified all of the ‘ingredients’ needed to implement IMS-based voice over long term evolution (VoLTE)—such as session initiation protocol (SIP) registration, signalling compression, call set up and supplementary services—it has left it up to communication service providers and vendors to decide which of the numerous alternative implementation options to use. This is frankly a recipe for a chaotic and fragmented rollout of IMS-based VoLTE since there is no way to guarantee that different industry players will opt for the same ‘ingredients’ that their competitors' choose for their own implementations. It goes without saying it was not a model for success.

Unsurprisingly, in the absence of a clear-cut approach to VoLTE, alternatives emerged, most notably 3GPP specified circuit switched fallback (CSFB), in which an communication service provider uses its legacy 2G/3G network to handle voice calls. In this scenario, when an LTE terminal initiates a voice call or receives one from the legacy circuit-switched network, it downgrades any ongoing LTE data session to 3G or HSPA speeds for the duration of the voice call. If the voice call ‘falls back’ to a 2G network, the LTE data session will likely be suspended altogether, as 2G data speeds are not sufficient for broadband data applications. In either case, the impact on customer experience can be obvious.

Another emerged alternative for IMS-based VoLTE was VoLTE via generic access (VoLGA) promoted by the VOLGA Forum.

All major network and handset vendors compete aggressively for the biggest possible slice of network communication service providers' business. But at the same time, their business is an interconnected business, where equipment interoperability, especially between handset and network is the key to ensuring that they can all play together. For communication service providers voice have been the killer application and it is going to be big source of revenue for years to come. When there were three different voice solutions (IMS-based VoIP, CSFB, VoLGA) there was rightfully serious concerns whether LTE would come with voice anytime soon. So the situation in year 2009 was equally challenging for both communication service providers and vendors. This is why, from time to time, serious rivals come together to agree on technical cooperation that is designed to help smooth the way forward for the common good of the telecoms market. On 4 November 2009 the One Voice initiative was published by AT&T, Orange, Telefonica, TeliaSonera, Verizon, Vodafone, Alcatel-Lucent, Ericsson, Nokia Siemens Networks, Nokia, Samsung and Sony Ericsson. These 12 companies announced that they have concluded that the IMS-based solution, as defined by 3GPP, is the most applicable approach to meet their consumers' expectations for service quality, reliability and availability when moving from existing circuit switched telephony services to IP-based LTE services. The companies in One Voice then set about to create a solid foundation for securing the smooth introduction of standards-based VoLTE. They evaluated the different alternative ‘ingredients’ specified by 3GPP in order to settle on a minimum set of essential handset and network functionalities and features that communication service providers would need to implement basic, interoperable VoLTE service. These agreed mandatory set of functionalities for the user equipment (UE), the LTE access network, the EPC network and the IMS functionalities are contained in the ‘technical profile’ published by One Voice, available for use by anyone in the industry. In a sense, the technical profile gives all industry stakeholders a level playing field on which to enhance their VoLTE service as they see fit, but most importantly a level playing field that enables the basic working, and interworking, of VoLTE across the entire industry landscape.

15 February 2010 marks the second important milestone in VoLTE ecosystem development. On that date, the Global System for Mobile Association (GSMA) announced it has adopted the work of the One Voice initiative to drive the global mobile industry towards a standard way of delivering voice and messaging services for LTE (GSMA) and Next Generation Mobile Networks alliance (NGMN) delivered communication service providers' agreement to ensure roaming for VoLTE by recommending to support CSFB in all LTE voice devices and networks (NGMN). The GSMA's VoLTE initiative was supported by more than 40 organisations from across the mobile ecosystem, including many of the world's leading mobile communication service providers, handset manufacturers and equipment vendors, all of whom support the principle of a single, IMS-based voice solution for next-generation mobile broadband networks. This announcement was also supported by 3GPP, NGMN and the International Multimedia Teleconferencing Consortium (IMTC). Following the announcement, work progressed very quickly; and already in March 2010 the GSMA permanent reference document (IR.92) on IMS profile for voice and short message service (SMS) was published containing an improved version of the One Voice profile. In September 2010 GSMA agreed to freeze the content of the permanent reference document (IR.92). A global baseline for commercial VoLTE deployments was finally stabilised.

Chapter 2

VoLTE Deployment Strategies

Global mobile networks have been driven by voice service since the beginning and voice has been the ‘killer’ application for many years. It can be said that there would be no mobile networks without voice services. Today, we have a new situation where data traffic is heavily growing in mobile networks and all deployments are going to be driven by data increase. Also from a standards point of view a data-driven approach has been selected where future mobile networks are based on the internet protocol (IP) technology and mobile services are built on top of the ‘data layer’.

The third generation partnership project (3GPP) long term evolution (LTE) is the main technology for further mobile networks and it can be said already now that it is the fastest developing mobile system technology ever. One reason is that LTE first deployments are for data with dongles and data cards including short message service (SMS) support, but services like voice will follow later. From a LTE deployment perspective the LTE voice service can be regarded as just one data application, but with specific requirements for real-time traffic, quality of service and interoperation with existing voice infrastructure [circuit switched (CS) core networks]. It is also self-evident that voice remains as a mandatory service in LTE.

The LTE/evolved packet core (EPC) standards have been designed as mobile technology based purely on the IP. The LTE/EPC networks will be driven by mobile broadband services in which the voice and SMS traffic amounts are a smaller portion of the total traffic amount but still retain their importance for both subscribers and operator business. From a technology point of view the LTE/EPC network is designed to carry all applications and it has excellent support for voice service with low latency and high capacity, which means savings because in the end the operator does not have to maintain parallel networks for voice and data.

The voice service over LTE is done with an IP multimedia subsystem (IMS) as specified by 3GPP. LTE radio access does not support direct connectivity to CS core