SALKINTZIS LAYOUT 1/19/09 2:46 PM Page 46 LTE — 3GPP RELEASE 8 Voice Call Handover Mechanisms in Next-Generation 3GPP Systems Apostolis K. Salkintzis, Motorola Mike Hammer, Cisco Itsuma Tanaka, NTT DOCOMO Curt Wong, Nokia Siemens Networks ABSTRACT path, and progressively scale network capacity and service innovation in a economically effi- The evolved 3GPP system is a hybrid mobile cient way. The integrated system is characterized network architecture supporting several radio by a heterogeneous architecture (i.e., supporting access technologies and several mobility mecha- diverse radio access networks) capable of provid- nisms. In this article we briefly review the archi- ing mobile broadband services in strategic geo- tecture and key components of this system, with graphic areas and ensuring the “best connection” particular emphasis on how it can support voice of users at any place, anytime. However, such call mobility in several deployment scenarios. integration requires us to address a vast range of First, we present the so-called single-radio voice interoperability and migration issues that arise call continuity mechanisms that enable mid-call from the need to support seamless mobility handover of VoIP calls from E-UTRAN access across new and legacy radio access technologies, to the legacy UTRAN/GERAN or 1xRTT and migrate the legacy services to new radio access. Then we focus on deployment scenarios accesses. As an example, consider a user that ini- that do not support voice services on E-UTRAN tiates a voice call inside a 4G hotspot. This call and present the so-called fallback mechanisms is carried out over the 4G access network with that enable handover from E-UTRAN to voice over IP (VoIP) technologies; but as the UTRAN/GERAN or 1xRTT at the beginning of user goes out of 4G coverage, the call needs to a voice call. Finally, we address the application- be sustained and seamlessly handed over to the layer voice call handover mechanisms enabled by “umbrella” 2G/3G access network, which typical- the IP multimedia subsystem. Our conclusion is ly provides a much wider radio footprint. that the next generation of 3GPP systems are It may also happen that voice services are not highly sophisticated mobile communication sys- initially supported over 4G access (e.g., in order tems that support extended voice call mobility to eliminate the cost of deploying VoIP-based mechanisms, capable of addressing all commer- services), in which case the user would have to cial deployment needs. be handed over to the overlay 2G/3G access right after the call request is made. In this case INTRODUCTION the 2G/3G access is used as a fallback access that supports the legacy services when they are not As the wireless industry makes its way to the yet available in the 4G access. All these require- next generation of mobile communication sys- ments create the need for voice call handover tems, it is important to engineer solutions that mechanisms from the emerging 4G access tech- enable seamless integration of the emerging nologies to the legacy 2G/3G access technolo- fourth-generation (4G) technologies within the gies. currently deployed 2G/3G infrastructures. This is In this article we focus on such voice call important because, in most cases, the second- handover mechanisms and address in particular and third-generation (2G/3G) systems provide the voice call handover mechanisms in the next- the solid ground on which the next-generation generation Third Generation Partnership Pro- systems will be built, and will continue providing gram (3GPP) networks, which typically take the main revenue stream for operators for sever- place between the evolved 3GPP radio access al years along the migration path to 4G. The network (evolved UMTS terrestrial radio access integration of emerging access and core tech- network [E-UTRAN] [1]) and the legacy nologies within the existing 2G/3G networks 3GPP/3GPP2 radio access networks, (e.g., code-division multiple access [CDMA], UTRAN/GERAN and CDMA2000 1xRTT. In Global System for Mobile Communications this context we present and explain the technical [GSM], General Packet Radio Service [GPRS], details of three different voice call handover and Universal Mobile Telecommunications Sys- mechanisms, which aim at addressing three dif- tem [UMTS]) can enable a smooth evolution ferent deployment scenarios of next-generation 46 0163-6804/09/$25.00 © 2009 IEEE IEEE Communications Magazine • February 2009 Authorized licensed use limited to: National Chung Cheng University. Downloaded on November 29, 2009 at 14:15 from IEEE Xplore. Restrictions apply. SALKINTZIS LAYOUT 1/19/09 2:46 PM Page 47 2G/3G 3GPP core lu-cs MSC E MSC Legacy UTRAN circuit-switched lu-ps services A/lu-cs GERAN Gn SGSN GGSN Gi Gb/lu-ps S3 S4 Packet data network(s) 3GPP EPC MME S11 e.g. IMS, S1-AP Internet, etc. S1-U S5 E-UTRAN S-GW P-GW SGi S6a Gxc Gx User S2b Gxb equipment AAA/ PCRF (UE) S101 Gxa HSS ePDG S103 STa S2a SWn EPC: Evolved packet core S-GW: Serving gateway CDMA2000 P-GW: PDN gateway HRPD MME: Mobility management entity ePDG: Enhanced packet data gateway PCRF: Policy and charging rules function HSS: Home subscriber server AAA: Authentication, authorization, accounting WiMAX WLAN n Figure 1. Simplified architecture of the evolved 3GPP network. 3GPP systems. First we present handover mech- radio accesses. It is also assumed that in such anisms that enable mid-call handover of VoIP deployments there are no transport layer mecha- calls from E-UTRAN access to the legacy nisms (e.g., based on mobile IP and its deriva- UTRAN/GERAN or 1xRTT access (note that tives) that can meet the voice continuity E-UTRAN access is sometimes also referred to requirements. We also provide some background as 4G access). These mechanisms are particular- material in the next section, where we present ly important in deployment scenarios where the key aspects of the next-generation 3GPP sys- voice services are supported on E-UTRAN (as tems, and introduce the fundamental network explained later, E-UTRAN supports only IP- elements and interfaces. Finally, we wrap up our based services) but, due to limited E-UTRAN discussion by providing a number of concluding coverage, handover to UTRAN/GERAN or remarks. 1xRTT is necessary to maintain seamless voice services over wider geographical areas. Second, we focus on deployment scenarios that do not AN OVERVIEW OF NEXT- support voice services on E-UTRAN. This most- ENERATION YSTEMS ly targets initial deployments of evolved 3GPP G 3GPP S systems, in which voice services are not yet To provide some background material for our migrated to E-UTRAN (i.e., not supported yet further discussion, we present here a short over IP). For those scenarios, we present the key overview of the evolved (or next-generation) aspects of a voice call handover mechanism that 3GPP systems. Readers interested in more enables handover from E-UTRAN to UTRAN/ details on the evolved 3GPP systems are referred GERAN or 1xRTT at the beginning of a voice to the 3GPP specifications [1–5] and other arti- call. The handover is triggered by the arrival of cles in this Feature Topic. an originating or terminating voice call, which As part of Release 8 of the 3GPP specifica- can only be served on UTRAN/GERAN or tions, 3GPP has been studying and specifying an 1xRTT access and in the traditional circuit- evolved packet system (EPS) under the System switched (CS) domain [2]. We then address Architecture Evolution (SAE) work item [2]. voice call handover mechanisms enabled by the The EPS is composed of a new radio access net- IP multimedia subsystem (IMS). Such mecha- work, called E-UTRAN [1], and a new all-IP nisms mostly target deployment scenarios in core network, called evolved packet core (EPC) which voice services are supported on E- [3, 4]. The EPC can be considered an evolution UTRAN and non-3GPP-defined radio accesses, of the legacy GPRS architecture with additional such as WLAN and WiMAX, and there is also a features to improve performance, supporting need to support voice continuity across these broadband E-UTRAN access, PMIPv6 mobility, IEEE Communications Magazine • February 2009 47 Authorized licensed use limited to: National Chung Cheng University. Downloaded on November 29, 2009 at 14:15 from IEEE Xplore. Restrictions apply. SALKINTZIS LAYOUT 1/19/09 2:46 PM Page 48 and integration with non-3GPP radio technolo- SINGLE-RADIO In order to provide gies such as wireless LAN (WLAN), CDMA2000, and WiMAX. As shown in Fig. 1, VOICE CALL CONTINUITY seamless continuity the evolved 3GPP network is virtually composed of voice services in of an evolved version of the legacy 2G/3G net- Especially in the first days of E-UTRAN deploy- wide geographical work (with the well-known UTRAN/GERAN ment, coverage will be limited and available only radio accesses) and the EPC, which supports E- in scattered strategic locations, where wireless areas, it is important UTRAN access and integration with a range of broadband services are most needed. Therefore, for the next non-3GPP accesses. Note that for simplicity all in order to provide seamless continuity of voice network interfaces are not shown in Fig. 1. The services in wide geographical areas, it is impor- generation of 3GPP interfaces relevant to the voice call handover tant for the next generation of 3GPP systems to systems to seamlessly mechanisms are presented and discussed in sub- seamlessly hand over voice calls between E- handover voice calls sequent sections. UTRAN [1] and UTRAN/GERAN coverage As shown in Fig. 1, a number of diverse areas. Although this is not a new requirement between E-UTRAN access networks, such as CDMA2000, WLAN, (e.g., similar voice continuity requirements exist and UTRAN/GERAN WiMAX, GERAN, UTRAN, and E-UTRAN, between UTRAN and GERAN, and between are connected to a common core network (the CDMA2000 1xRTT and HRPD), in this case coverage areas.