LTE delivers superior voice, tooVinjett34Telephony servicesover LTE end-to-endHow to deliver telephony services for LTE-ca...
35the IMS profile for Voice and SMS (GSMApermanent reference document IR.92),          BOX c               Figure 1    The...
LTE delivers superior voice, tooVinjett36                                                                                 ...
37function (P-CSCF) in the IMS domain,         ule a large number of UEs in a limited          services for roaming users ...
LTE delivers superior voice, tooVinjett38     way, LTE users who receive or place acall temporarily (for the duration of t...
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Telephony services over LTE end-to-end


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Telephony services over LTE end-to-end

  1. 1. LTE delivers superior voice, tooVinjett34Telephony servicesover LTE end-to-endHow to deliver telephony services for LTE-capable devices is a topic of much debate.Ericsson supports two standardized solutions for delivering these services – IMSMultimedia Telephony and circuit-switched fallback for use by operators prior tomigrating to IMS/MMTel. L e n na rt Nor e ll , E r ic Pa r s on s a n d P e r S y n n e rgr e nAlthough LTE is a packet-only Introduction to telephony to full-fledged multimedia communi-technology, the standard was services over LTE BOX b cation.defined to efficiently handle The 3rd Generation Partnership Project Circuit- Fallback to second- and third-genera-voice and multimedia services, (3GPP) Long Term Evolution (LTE) stan- switched tion (2G/3G) circuit-switched telephonyproviding support for more than dard promises to deliver efficient mobile fallback is promoted as a migration step whenmobile internet access. broadband and multimedia communi- is an initial moving from circuit-switched networks This article describes how cation services. In keeping with this mechanism to to all-IP networks and MMTel. When aEricsson and telecommunica- objective, it was developed to include allow an LTE- voice call is placed or received, the CSFBtions standardization bodies the features needed to effectively capable smart- solution shifts the user equipment (UE)envision the realization of support voice-over-IP (VoIP) media and phone to move access from LTE to 2G/3G.telephony services over control. How telephony services are to to the circuit- To align the industry in support ofLTE/EPC. be realized over LTE, however, has been switched domain 3GPP standards Ericsson and sever- The authors take an end-to-end hotly debated in recent years. when handling al other vendors and network serviceperspective, outlining require- Ericsson stands behind the two solu- voice calls. providers jointly formed the One Voice tions standardized by 3GPP, namely initiative, whose main objective is toments in the LTE radio access, IP Multimedia Subsystem (IMS) Multi­ outline a baseline profile with whichEPC, IMS core, and MMTel media Telephony (MMTel) and circuit- terminal and network vendors shouldapplication server. They describe switched fallback (CSFB). Ericsson is comply to guarantee interoperability.IMS features for voice telephony, promoting MMtel as the main tele­ The work of the initiative has gainedmedia handling, radio features phony-over-LTE solution because it wider acceptance in the industry sincefor voice, QoS quality, roaming fulfills all the requirements of a tele­ its adoption by the GSMA in the begin-architecture, emergency calls, phony solution. What is more, it is ning of 2010.and solutions that coexist with future-proof, allowing an evolution The baseline profile produced by thecircuit-switched infrastructure. from today’s voice and video tele­ hony p One Voice initiative forms the basis of BOX A Terms and abbreviations 3GPP 3rd Generation Partnership Project ISIM IMS SIM ROHC robust header compression AKA authentication and key agreement ITU International Telecommunication SIM subscriber identity module AMR adaptive multi-rate Union SIP session initiation protocol CSFB circuit-switched fallback LTE Long Term Evolution SMS short message service DRX discontinuous reception MMTel IMS Multimedia Telephony SR-VCC single radio VCC eNB eNode-B or base station MSISDN mobile subscriber ISDN number TTI transmission time interval EPC evolved packet core OTDOA over-the-air time difference UE user equipment EPS 3GPP Evolved Packet System of arrival URI uniform resource identifier GBR guaranteed bit rate PCC policy and charging control USIM universal SIM GSM global system for mobile PCRF policy and charging rules function VCC voice call continuity communications P-CSCF proxy call session control function VoLTE voice-over-LTE GSMA GSM Association PDCP packet data convergence protocol VoIP voice-over-IP ICS IMS centralized services QCI QoS class identifier XCAP XML configuration access protocol IMS IP Multimedia Subsystem QoS quality of service XML Extensible Markup Language IP internet protocol RLC Radio Link ControlE r i c s s o n r e v i e w • 1 2010
  2. 2. 35the IMS profile for Voice and SMS (GSMApermanent reference document IR.92), BOX c Figure 1 The VoLTE specifications.commonly referred to as voice-over-LTE IMS(VoLTE). Centralized The requirements in the baseline Servicesprofile span every layer of the network. is a key enabler UNIAccordingly, they comprise IMS fea- for IMS/MMTel, MMTeltures, media requirements, bearer man- allowing callagement features, LTE radio require- control for aments, and common functions, such as mobile device to UNIthe IP version (Figure 1). The require- be anchored in IMSments are compliant with 3GPP Release the IMS domain8 (Rel-8) and there are some addition- through theal requirements from 3GPP Release 9 circuit-switched UNI(Rel-9) in support of packet-switched domain when notemergency calls. in LTE coverage. EPCIMS features for voiceThe VoLTE profile uses a subset of gen- UNIeral IMS features and of the MMTel LTEservice. The subset is selected to providean IP telephony service with a similaruser experience as the circuit-switchedservice used today in GSM and 3G net- Figure 2 Upwards compatibility of the VoLTE The aim of the profile is to provide aminimal set of features that can serve asa starting point. The profile must still be VoLTE VoLTEopen to functional growth in networks subset subsetand devices to reach the full 3GPP spec- terminal IMS implementationifications for IMS and MMTel, for exam-ple, using multimedia, supporting userswith multiple devices or the full set ofMMTel supplementary services. Considerable efforts have thereforebeen made to ensure that user equip-ment (UE) that supports the VoLTE Full 3GPP Full 3GPPfeature set can be used even in net- terminal network IMS implementationworks that provide all 3GPP IMS andMMTel features. Likewise, efforts havebeen made to guarantee that a networkthat only supports the VoLTE feature setcan offer this limited service to users IMS authentication and key agreement be forward compatible, however, theof UEs in which the richer 3GPP IMS/ (IMS-AKA) and IMS subscriber identity UE must be usable in a network whereMMTel service set has been implement- module (ISIM). A universal SIM (USIM) multiple devices per user are support-ed (Figure 2). may be used if the network service pro- ed in IMS. The baseline profile mandates the use vider has not deployed ISIM. Signaling compression can be usefulof different options for the IMS features The baseline profile supports both for minimizing the SIP signaling load– for example, mobile subscriber ISDN number over the air interface. Support for this S IP registration; (MSISDN) and alpha­ umeric session n option is mandated in the UE as well as a uthentication; initiation protocol-uniform resource the network but network service provid- a ddressing; identifier (SIP-URI)-based addressing. ers can turn it on or off. b asic call handling; The terminals are forward-compati- The set of supplementary services in f orking; and ble and may be used in scenarios where VoLTE was chosen to provide service s ignaling compression. network service providers employ parity with existing GSM and 3G ser-Where authentication is concerned, types of addressing. vices. The MMTel specification includesthe baseline profile recognizes that The baseline profile supports a sin- more features – for example, a morethe target device is a phone that uses gle handheld device scenario. Forking flexible Extensible Markup Languagecellular access and mandates support for is thus not required in the network. To (XML) configuration access protocol E r i c s s o n r e v i e w • 1 2010
  3. 3. LTE delivers superior voice, tooVinjett36 rate adaptation can be used to increase Figure 3 User satisfaction versus end-to-end (mouth-to-ear) delay. BOX d system capacity and coverage by lower- Single radio ing the codec rate. E model rating voice call 100 Quality of service (QoS) control continuity Users very (SRVCC) The service requirements put on bear- satisfied allows a mobile er management and charging man- 90 with an ongoing date that an LTE/evolved packet core Users satisfied voice call to (EPC) system, which provides access for transition to the MMTel, must be equipped with policy 80 Some users circuit-switch and charging control (PCC). dissatisfied domain in the PCC introduces a set of Rx and Gx event of loss of interfaces that connect EPC with the 70 Many users dissatisfied LTE coverage. IMS domain. In addition, the PCC archi- Nearly tecture introduces a policy and charg- all users dissatisfied ing rules function (PCRF) that encom- 60 passes policy decision and flow-based charging control functions. With PCC implemented, IMS can 50 0 100 200 300 400 500 600 start a network-initiated process to set Mouth to ear delay [ms] up suitable 3GPP Evolved Packet System (EPS) bearers for telephony service. The characteristics of these bearers are sig- (XCAP)-based service-management end delay is under 200ms, which is the naled via a QoS class identifier (QCI).capability that allows black- or white- typical end-to-end delay of 2G/3G cir- Two standardized classes arelist-based and time-dependent diver- cuit-switched systems. Q CI1 – a guaranteed bit-rate bearer forsions. While the VoLTE profile retains From an end-to-end latency point of VoIP media; andthis basic mechanism for upward view, the low latency of LTE access (20- Q CI5 – a high-priority non-guaranteedcompatibility, the minimum defined 30ms round-trip delay) means the min- bit-rate bearer for IMS control messagesbaseline profile uses only basic GSM-like imum end-to-end delay may be signifi- that use the session initiation protocolcontrols. cantly lower than 200ms. However, LTE (SIP) and the XCAP. radio bearers do not employ fixed delay. The LTE RAN, in other words, theMedia handling and control Instead, they use fast retransmissions to e-Node-B or base station (eNB) is respon-The objective for media handling in the repair erroneous transmissions, and LTE sible for admission control for the EPSVoLTE baseline profile was to have a set uses schedulers to control uplink and bearers. To determine if a new bearerof requirements that gives voice service downlink transmissions. Consequently, can be admitted, the eNB must ensurewith conversational quality on par with LTE transmissions introduce delay vari- that there is sufficient capacity acrossthat of legacy circuit-switched service. ations (jitter), and the endpoints that ter- the air interface, the transport network, To ensure interoperability, in line minate the voice flow must implement and on internal software or hardwarewith 3GPP, the baseline profile man- efficient de-jitter buffers. resources. Once admitted, the eNB willdates support of the adaptive multi- Notwithstanding, despite the addi- use the EPS QoS parameters to invokerate (AMR) narrowband speech codec. tional delay introduced by de-jittering, the appropriate eNB scheduling policiesHowever, seen in terms of an MMTel the end-to-end delay of commercial to meet the required end-to-end delaybusiness solution, the recommended MMTel terminals and LTE networks is and packet loss rate targets.speech codec for voice over LTE access on par with or better than that of 2G/3G PCC helps control charging for theis AMR wideband (high-definition circuit-switched telephony. service. Charging for the telephony ses-voice). This codec is mandated in 3GPP Where frame error rates are sion is managed in the IMS domain;networks and terminals that support concerned, LTE voice bearers add therefore, charging on the EPS bearer16KHz speech sampling. erroneous transmission repair (2G/3G level is set to zero. The PCRF is used to For a given codec and the set of front- circuit-switched voice bearers do not). As set the proper bearer-charging rules forend handling requirements, two vari- a result, most packets are sent success­ EPS bearers.ables influence speech quality the fully over the LTE air interface, and the An important role of PCC is tomost: end-to-end delay and frame-error residual frame error rate for voice media handle error cases when the EPS bear-rate. Figure 3 shows the relationship is low. er for voice service is lost – for example,between user satisfaction and end-to- The addition of solutions for codec when a UE travels outside radio coverage.end delay according to the International rate adaptation, which is specified in Using guaranteed bit-rate (GBR) bearersTelecommunication Union (ITU) the MMTel media specification, can LTE/EPC notifies the PCRF when the EPSE-model. User satisfaction with commu- further enhance media transport. In bearer for voice is lost. The PCRF thennication quality is greatest when end-to- high load situations, for example, codec informs the proxy call session controlE r i c s s o n r e v i e w • 1 2010
  4. 4. 37function (P-CSCF) in the IMS domain, ule a large number of UEs in a limited services for roaming users thereforewhich terminates the session and set of scheduling resources, taking into require new IMS-based roaming agree-charging. account the need to retransmit packets ments between the network service until successfully received. The pack- providers. Until such agreements are inRadio features and performance et loss rate is typically controlled in the place, the UE is required to use CSFB forfor voice link-adaptation algorithms to statisti- voice calls while roaming if LTE is usedOrdinarily, the radio network manag- cally ensure that the VoIP packets are in the visited the most constrained resource, and successfully transmitted within theas a result, it must balance a number of loss targets. The packet loss rate and Emergency callscompeting objectives, such as coverage, the packet delay budget must each be Some voice-over-LTE deployments cancapacity, quality and battery longevity accounted for in the mechanisms that rely on continuous circuit-switched(talk time). are used for cell handover. network coverage for emergency calls. To maximize coverage, which is typi- Finally, a very important aspect of But for deployments that only use VoIPcally uplink-limited, the eNB primarily VoIP support on the radio access net- technology or where the LTE coverageemploys features to reduce the packet work is the need to maximize user talk in some locations is better than the cir-size. For this purpose, it employs robust time. LTE allows UEs to turn off their cuit-switched coverage, emergency callsheader compression (ROHC) and radio transceiver periodically between VoIP must be able to use IMS. Therefore, thelink control (RLC) unacknowledged frames. A combination of appropriately VoLTE profile includes baseline specifi-mode with short packet data conver- configured active discontinuous recep- cation for IMS emergency calls.gence protocol (PDCP)/RLC sequence tion (DRX) and semi-persistent sched- 3GPP Rel-9 added the functions innumbers. By reducing the packet size, uling can potentially offer significant LTE/EPC that enable IMS-based emer-greater coding redundancy is obtained improvements over today’s solutions. gency calls over packet-switched radiofor a given uplink allocation, thereby access. From an LTE/EPC perspective,improving the reliability of data deliv- Roaming architecture 3GPP Rel-9 includes support for SIM-lessery in coverage limited scenarios. The Service control in IMS is home-network- emergency calls, user- and control-planeeNB can also employ features such as based. This allows network service pro- location services, and enhancements to t ransmission time interval (TTI) bun- viders to differentiate services, but the air interface to improve over-the- dling to allow a UE to concentrate energy might make it more complex to deliver air time difference of arrival (OTDOA) on narrow uplink allocation for a longer the service to roaming users. To enable a accuracy. These additions were made to duration; and split of service revenue and to meet reg- support more stringent international f requency hopping to reduce the effects ulatory requirements, the visited net- regulatory requirements. of interference across multiple cells. work needs to be aware of telephony ser-To maximize capacity, in other words, vices even when the service is provided Legacy coexistenceto maximize the number of UEs that from the home network. In many networks, the coverage of thecan transmit or receive VoIP packets To support this, the VoLTE profile voice-capable LTE deployments willin the cell, the eNB leverages the same uses a roaming model where the IMS be less extensive than that of circuit-tools to reduce the packet size as P-CSCF is located in the visited network switched voice. Initially, where LTE cov-mentioned above. Further enhance- and uses local breakout from the pack- erage is spotty, LTE UEs can use circuit-ments can be achieved by adding et core network (Figure 4). IMS voice switched fallback procedures. ThisExplicit Congestion Notification (ECN)to reduce the speech codec rate betweenthe two endpoints. For some UEs, Figure 4 IMS roaming architecture.bundling of VoIP packets can helpreduce packing inefficiencies, as long Visited network Home networkas the delay targets can be met. From acontrol channel perspective, the eNBmay need to employ techniques such aspredictive grant allocation to reduce theamount of signaling required to control MMTel HSS ASa large number of UEs. To maintain quality, the eNB mustensure that end-to-end delay bud-gets and packet-loss rates are met formedia packets. For packet-delay bud- IMSgets, to achieve end-to-end (mouth-to- LTE EPC P-CSCF coreear) delays of 200ms, network serviceproviders typically configure the eNBfor targets in the range of 50-80ms. Thechallenge is that the eNB must sched- E r i c s s o n r e v i e w • 1 2010
  5. 5. LTE delivers superior voice, tooVinjett38 way, LTE users who receive or place acall temporarily (for the duration of the Lennart Norell Eric Parsonscall) fall back to the circuit-switched net- joined Ellemtel in 1977 to is currently a strategicwork. CSFB is not part of the VoLTE pro- work on the development product manager for tele-file as it does not require IMS voice. of the AXE system. He phony for LTE. He has held Once VoLTE has been introduced, the moved to Ericsson in 1982 a number of senior techni-handover of ongoing calls to the circuit- and has since held various management cal leadership positions in wirelessswitched domain might still be need- and technical expert positions within product architecture and standardsed to reduce the number of dropped system and product management in the focusing on end-to-end solutions withcalls. The VoLTE specification includes telecom and datacom product areas. He over 20 years experience. He is aoptional single radio voice call continu- has been active in the design of IMS, recognized industry authority onity (SR-VCC) procedures for handing over where he has been leading the strategic CDMA-LTE interworking. He has acalls to the circuit-switched network. systems management unit. He is PhD in Computer Science from theIMS centralized services (ICS) allow the currently an expert in IMS and core University of Toronto and is currentlyIMS service domain to be used for IMS network architectures at Business Unit an Adjunct Professor with Carletonusers when the access is provided via a Networks. He has an MSc in Electrical University.circuit-switched network. These migra- Engineering from Chalmers University oftion aspects are further described in Technology, Sweden. Per SynnergrenAchieving a converged service offering forfixed and mobile telephony.2 joined Ericsson in 2001, and is currently SeniorSummary Specialist in the area ofThe companies in the VoLTE initiative Multimedia Telephony atand the GSMA have agreed on how the Business Unit Networks within Systemtelephony service is to be realized over Management. The current focus of hisLTE access. 3GPP MMTel specifications work is to ensure that Ericsson can de-make up the basis of this realization. liver a Multimedia Telephony businessThe result is a feature-rich telephony ser- solution with consistently satisfactory end-to-end performance. He has alsovice whose performance is on par with worked in Ericsson Research on radioGSM circuit-switched telephony. access functionality for IP multimedia services. He has a PhD in Experimental Mechanics and an MSc in Mechanical Engineering from Luleå University of Technology, Sweden. eferences R 1. Enström, D., Nohlgren, A., Olofsson, H., Peisa, J. 2. Granström, P., Norell, L. and Åkesson, S., 2009. and Synnergren P., 2007. Multimedia tele­ hony p Converged service for fixed and mobile tele­ for IMS – Interoperable VoIP with multimedia phony. Ericsson Review, Vol. 86(2), pp.28-32. support. Ericsson Review, Vol. 84(2), pp.44-47.E r i c s s o n r e v i e w • 1 2010