Single Radio Voice Call Continuity (SRVCC) With LTE

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White PaperSingle Radio Voice Call Continuity(SRVCC) with LTEBy: Shwetha Vittal, Lead EngineerOverviewCONTENTSLong Term Evolution (LTE) is heralded as the next big thing for mobilenetworks. It brings in promising technologies such as semi-persistentscheduling, transmission time interval (TTI) bundling, and high performancegains on Quality of end user Experience (QoE). In the end, the primary goal ofLTE is to deliver ultra-high speed mobile broadband with peak data rates over100 Mbps. However, in practical applications LTE is facing challenges to providethe same capabilities as a 2G/3G network during the initial stages of trialdeployments and operators’ metered investment in broad network build out.Why SRVCC? pg. 2One of the key issues of LTE is the delivery of voice services. Voice remains the“killer application” for operators because it still accounts for a large portion oftheir revenue. Voice will continue to remain the dominant must-have service inthe network for years, and despite the technical challenges of providing serviceover an all-Internet Protocol (IP) radio access network (RAN), voice is seen asa basic service by the consumer; in short, it is expected. However, voice servicecontinuity is not guaranteed when a Voice over IP (VoIP) subscriber roamsbetween the LTE coverage area and other wireless networks—and it is asignificant challenge to deliver voice over LTE networks.References pg. 10Single Radio Voice Call ContinuityFrom LTE pg. 3SRVCC from LTE to 3GPP2 1XCS pg. 4SRVCC from LTE to 3GPP UTRAN/GERAN pg. 7Conclusion pg. 9

Single Radio Voice Call Continuity (SRVCC) with LTE Radisys White Paper2The industry is exploring and evaluating differentpossibilities to overcome the LTE voice issues. Duringthis evaluation process two options are gainingsignificant momentum: Circuit Switched Fall Back(CSFB) and LTE VoIP-based Single Radio Voice CallContinuity (SRVCC). The latter is widely supportedin the industry and has been recommended by theLTE OneVoice Initiative, which has the support ofsome of the world’s largest operators and networkequipment providers and has been endorsed by theGSM Association (GSMA).This paper focuses on designing the LTE-IMS (IPMultimedia Subsystem) network framework tosupport SRVCC with the Circuit Switched networkby realizing the 3GPP Standard 23.216 V 8.6. 2009-12:Single Radio Voice Call Continuity (SRVCC) (Release 8).This architecture has great appeal to carriers with arobust IMS Core and both fixed and wireless componentassets in order to facilitate a converged VoIP solution.Why SRVCC?Rich multimedia services with video sharing, videoon demand, video telephony, video conferencing,VoIP, Push-To-Talk, broadband access to PersonalDigital Assistants (PDAs) and so on are currentlyoffered with the existing capabilities of the UniversalMobile Telecommunications System (UMTS) usingHigh Speed Packet Access (HSPA), Evolved HSPA(HSPA ), Code Division Multiple Access (CDMA) andIMS technologies. Increasing demand for these realtime mobile data services coupled with subscribers’expectations for always-on, high-quality services isdriving the need for expanded network capacity andthroughput. Enter LTE.Figure 1. EPS Reference Architecture for CSFB with UTRAN as Destination NetworkWith the support of LTE’s high-throughput datatransmission capacity, inter-working with 3GPP andnon-3GPP based networks, and all-IP core networkelements, the converging services listed above canbe delivered successfully. Higher bandwidth for LTEmeans that more resource blocks can be dispatchedby the LTE system, which in turn provides higherperformance gains.Recognizing this reality, CSFB is a 3GPP-definedstandard solution that requires terminals be equippedwith either dual-mode/single-standby or dual-mode/dual-standby capabilities.Figure 1 displays the reference architecture for a CSFBnetwork using an Evolved Packet System (EPS) withthe 3GPP Universal Terrestrial Radio Access Network(UTRAN).For dual-mode/single-standby mobile phones tosimultaneously use dual-network services, theInter Working Solution (IWS) node provides on-timemessage access. On the other hand, dual–mode/dualstandby mobile phones require less network changesto facilitate inter-working between two networks.However, dual-mode handsets drain the battery powerquickly and need complex terminal customization.

Single Radio Voice Call Continuity (SRVCC) with LTE Radisys White Paper3In such scenarios of converging mobile and broadbandwireless access technologies, SRVCC offers LTE-IMSbased voice service within the LTE coverage area, andCS-based voice service outside the LTE coverage area.Figure 2 displays the reference architecture for SRVCCusing EPS to 3GPP UTRAN.Whenever the VoIP subscriber moves out of LTEcoverage, SRVCC ensures smooth handoff of voicefrom the LTE to the CS network, keeping upgradesof the network to a minimum. The IMS network thatstores voice service link information during this timeguides the target CS network to establish a link,thereby replacing the original VoIP channel.Table 1 compares CSFB and SRVCC schemes.Figure 2. EPS Reference Architecture for SRVCC to UTRAN as Destination NetworkParameterThe following sections describe the SRVCC fromLTE to 3GPP2 1XCS and LTE to 3GPP UTRAN/GERANCS networks.Single Radio Voice CallContinuity From LTESRVCC service for LTE comes into the picture whena single radio User Equipment (UE) accessing IMSanchored voice call services switches from the LTEnetwork to the Circuit Switched domain—while it isable to transmit or receive on only one of these accessnetworks at a given time. This basically removesthe need for a UE to have multiple Radio AccessTechnology (RAT) capability.For single-radio terminals, measurement gaps areneeded to allow the UE to switch onto the CS networkand complete radio measurements. Measurementgaps define the time periods when no uplink ordownlink transmissions are scheduled so that theUE may perform the measurements. The EvolvedNodeB (eNodeB, i.e. LTE base station) is responsiblefor configuring the measurement gap pattern andprovides it to the UE using Radio Resource Control(RRC) dedicated signaling.SRVCCCSFBDevice/terminal capabilitySingle radio mode1Dual-mode/single-standby orDual-mode/dual-standbyTerminal customizationLess complexComplex for single standbyIMS anchoringMandatoryOptionalSwitching networks/mobility to CS networkOnly when the terminalroams out of LTEcoverage areaFor every mobile originatingand mobile terminatingvoice callCostLess expensiveExpensive due to increasednetwork signaling loadVoice call setup timeMore, as the terminal needsto establish the voice callsession with CS networkfor every access to callLess, as time is requiredonly when the terminalmoves out of LTEcoverage areaTable 1.Single radio mode terminal refers to the ability of a terminal to transmit orreceive on only one of the given radio access networks at a given time.1 The UE assists the eNodeB by informing the networkabout its gap-related capabilities, at least mentioningthat if it has a dual or single receiver. This capabilityis transferred along with the other UE capabilities.The UE accessing the SRVCC service is assumed tohave IMS Service continuity capabilities with singleradio access only.

Single Radio Voice Call Continuity (SRVCC) with LTE Radisys White PaperSRVCC from LTEto 3GPP2 1XCSIn the case of VoIP, if subscribersgeographically roam from LTE CDMAto CDMA, voice calls must be switchedfrom a VoIP to a CDMA 1x networkusing SRVCC technology. The existinginter-frequency/RAT gap patternmechanism in E-UTRAN is thereforeextended to support gap patternssuitable for 1xRTT measurements.In this approach, the eNodeB is able tointer work with the 3GPP2 1XRTT MSCusing the S1-MME interface with theEvolved Packet Core (EPC) MME. A newIWS node is required and is responsiblefor the exchange of 3GPP 1XCSsignaling messages with the MMEand for establishing a Circuit Switchedsession when the UE is in the processof switching over from the LTE networkto the 3GPP 1XCS network. Generally,Figure 3. SRVCC from LTE to 1x CS Networkthis is a case of inter system handoverfrom the LTE perspective.All the SIP and RTP packets are carried through thePacket Data Control Protocol (PDCP) payload on theA new S102 reference point or interface is definedLTE Uu interface and are tunneled on the S1-U andbetween the LTE MME node and the 3GPP 1XCS IWSthe S5 interfaces using Generic Tunneling Protocol—node. In fact, the 3GPP 1XCS signaling messages areUser (GTP-U). Mapping between these packets is donetunneled over this single link of S102 and thereafterat the eNodeB to interact with the Serving—PDNtunneled through E-UTRAN/EPS tunneling messagesGW, ultimately allowing the UE access to IMS serviceto the UE.flexibly until the handover is detected and initiatedFigure 3 displays the framework of SRVCC fromwith the UE switch over.LTE to a 3GPP2 1XCS network.The 3GPP2 1xCS IWS node enables a single radioThe S1-MME interface between the eNodeB andUE to communicate in parallel both with the sourcethe MME ensures initiation of SRVCC service andLTE network and the target 3GPP 1XRTT system.smooth transition of these signaling messagesWith this, the additional voice gap normally generatedusing the S1AP signaling protocol.by multiple RATs being jammed into one UE is reducedOnce the UE is actively attached and associatedby having transport of signaling establishmentto the EPC, it interacts with the IMS core throughwith target Circuit Switched access when the UE isthe eNodeB, Serving Gateway (SGW) and Packetconnected to the source LTE network. However, QualityData Network Gateway (PDN GW) by establishingof Service (QoS) depends on the connectivity offered bya Session Initiation Protocol (SIP) signaling session.the public IP network and is susceptible to IP delaysSIP therefore plays a key role along with Real timeand packet loss which impacts the service performance.Transport Protocol (RTP), which is used for bearerplane data transfer in the SRVCC architecture.4

Single Radio Voice Call Continuity (SRVCC) with LTE Radisys White PaperThe message flow for SRVCC for a UEfrom LTE to a 1x CS network for VoIPIMS services is shown in Figure 4.The entry criterion for the messageflow is an ongoing VoIP session tothe IMS access leg established overEvolved Packet System (EPS) access:1. 1xCS SRVCC UE sends measurementreports to the eNodeB.2. The E UTRAN makes a determinationto initiate an inter-technologyhandover to cdma2000 1xRTT.3. The E UTRAN signals the UEto perform an inter-technologyhandover by sending a Handoverfrom EUTRA Preparation Requestmessage with 3G1x OverheadParameters.4. The UE initiates signaling forestablishment of the CS accessleg by sending a UL handoverpreparation message containingthe 1xRTT Origination message.5. The E UTRAN sends an Uplink S1cdma2000 Tunneling message withFigure 4. SRVCC from LTE to 1x CS Voice SystemMEID, 1x Origination, ReferenceCell ID to the MME. The eNodeB willalso include CDMA2000 HO Requiredb. Selects the 3GPP2 1xCS IWS based on ReferenceIndication IE to Uplink S1 CDMA2000 TunnelingCell ID and encapsulates the 1x Originationmessage, which indicates to the MME that theMessage along with the MEID and RAND in ahandover preparation has started.S102 Direct Transfer message (as “1x Air InterfaceSignaling”) to the IWS, only for voice bearer.6. Upon receipt of the Uplink S1 cdma2000 Tunnelingmessage, the MME:a. Separates the voice bearer from the non-voicebearers based on the QoS Class Identifier (QCI)associated with the voice bearer (QCI 1) andCDMA2000 HO Required Indication.c. The traffic assignment is done between theIWS and RTT MSC, over the A1 interface usingthe signaling protocols to initiate the handoffto the 1XRTT system.5

Single Radio Voice Call Continuity (SRVCC) with LTE Radisys White Paper7. The traffic channel resources are established inthe 1x RTT system and 3GPP2 1xCS proceduresfor initiation of session transfer for CS accessleg are performed.8. When the 1xRTT MSC receives a positiveacknowledgment from the 1xRTT radio for trafficallocation and from the IMS for successful domaintransfer, it returns an IS-41 handoff message tothe IWS to send to the UE via the establishedsignaling tunnel.9. The 3GPP2 1xCS IWS creates a 1x message andencapsulates it in a S102 Direct Transfer message(1x, Handover indication). If the 3GPP2 accesswas able to allocate resources successfully, the1x message is a 1x Handover Direction messageand the handover indicator indicates successfulresource allocation. Otherwise, the handoverindicator indicates to the MME that handoverpreparation failed and the embedded 1x messageindicates the failure to the UE.10. The MME sends the 1x message and CDMA2000HO Status IE in a Downlink S1 cdma2000 Tunnelingmessage to the E UTRAN. The CDMA2000 HOStatus IE is set according to the handover indicatorreceived over the S102 tunnel.11. If the CDMA2000 HO Status IE indicates successfulhandover preparation, the E UTRAN forwardsthe 1x Handoff Direction message embedded inMobility from EUTRA Command message to theUE. This is perceived by the UE as a HandoverCommand message.12. The UE now tries to acquire the traffic channelwith the 1xRTT CS access as it gets awareof the traffic channel information from thecdma2000 1xRTT system.13. The UE sends a 1xRTT handoff completionmessage to the 1xRTT CS access.14. The 1xRTT CS Access sends a message to the1xRTT MSC to indicate that the handoff is done.The traffic assignment which was done duringthe session/domain transfer of the CS accessleg, between the 1x CS IWS and the 1xRTT MSC,is released now.15. An ongoing voice call over the CS access legis now established over 1xRTT access. The UEcontinues to transmit voice via the new accesssystem. The voice bearer path is no longer carriedby the EPC.16. The eNodeB now initiates the release of UEcontext on the EPS; it sends an S1 UE ContextRelease Request (Cause) message to the MME.Cause indicates that S1 release procedure isdue to handover from E-UTRAN to 1xRTT.17. The MME exchanges Suspend Request and SuspendAcknowledge messages with the Serving GW.With this the S1-U bearers are released for all EPSbearers and the Guaranteed Bit Rate (GBR) bearersare deactivated by the MME. The non-GBR bearersare preserved and are marked as suspended in theS GW. Upon receipt of downlink data the S GW shouldnot send a downlink data notification message tothe MME.18. S1 UE Context in the eNodeB and MME are nowreleased with the normal E UTRAN/EPS procedure.6

Single Radio Voice Call Continuity (SRVCC) with LTE Radisys White PaperSRVCC from LTE to3GPP UTRAN/GERANIn this scenario, the SRVCC nodeson the target CS network and IMSnetwork are enhanced with additionalcapabilities to support smoothtransition of the UE from LTE to3GPP UTRAN/GERAN based networks(e.g. 2G/3G).1. MSC Server, is enhanced with thefollowing features:a. Employed alongside the MMEin LTE network through the Svreference point.b. Mainly comprises the call control(CC) and mobility control partsof an MSC. The MSC Server isresponsible for the control ofmobile-originated and mobileterminated CC CS Domain callsfor medial channels in theCS-MGW (Media Gateway). Itterminates the user-networksignaling and translates it intothe relevant network—networksignaling, i.e., SIP Signaling in IMSand vice versa.Figure 5. SRVCC from LTE to 3GPP UTRAN/GERANc. For SRVCC, the MSC Serverprovides the following functions as needed:i. Handling the Relocation Preparation procedurerequested for the voice component from theMME via the Sv interface.ii. Invoking the session transfer procedure fromIMS to CS. This involves the access transferat the IMS level of one or more of sessionsignaling paths and associated media flowpaths of an ongoing IMS session.iii. Coordinating the CS Handover and sessiontransfer procedures.2. Session Centralization and Continuity ApplicationServer (SCC AS) is present in the IMS networkand provides the following functionality in supportof SRVCC:a. Required to enable IMS Centralized Services.The ICS User Agent (IUA) function furnishes SIPUA behavior on behalf of the UE for setup andcontrol of IMS sessions using CS bearers thatare established between the UE and the SCC AS.b. For executing and controlling the sessiontransfers needed by the UE for its accesslegs anchored in IMS.The Figure 5 displays the framework of SRVCC fora UE from LTE to a 3GPP UTRAN/GERAN CS network.7

Single Radio Voice Call Continuity (SRVCC) with LTE Radisys White PaperThe message flow is shown inFigure 6 for SRVCC from LTE to3GPP UTRAN/GERAN CS networkand is followed by a description.The entry criteria for the followingmessage flow is an ongoing VoIPsession over the IMS access legestablished over EPS access:1. The UE sends measurementreports to E-UTRAN.2. Based on UE measurement reports,the source E UTRAN decides totrigger an SRVCC handover to theCS Domain.3. Source E UTRAN sends HandoverRequired message having Target ID,generic Source to Target TransparentContainer, SRVCC HO Indication tothe source MME. The E UTRANplaces the “old BSS to new BSSinformation IE” for the CS domainin the generic Source to TargetTransparent Container. The SRVCCHO indication indicates to the MMEthat the target is only CS capable—hence is a SRVCC handover operationonly toward the CS domain.4. Bearer Splitting: 3GPP has specifiedFigure 6. Call Flow for SRVCC from E-UTRAN to GERAN without DTM Supporta QCI for VoIP services to ensurethat LTE VoIP raises the quality ofend user experience. Based on the6. The MSC Server interworks the PS-CS handoverQCI associated with the voice bearer (QCI 1) andrequest with a CS inter-MSC handover requestthe SRVCC HO indication, the source MME splitsby sending a Prepare Handover Request messagethe voice bearer from the non-voice bearers andto the target MSC.initiates the PS-CS handover procedure for the7. Target MSC performs resource allocation withvoice bearer only toward the MSC Server.the target BSS by exchanging Handover Request/5. The MME stores the STN-SR (Session TransferNumber for SRVCC), C-MSISDN of the UE from theHSS during the UE attach procedure. The MME sendsa SRVCC PS to CS Request with necessary IMSI,Target ID, STN-SR, C MSISDN and generic Source toTarget Transparent Container, MM context messageto the MSC Server.Acknowledge messages.8. The target MSC sends a Prepare HandoverResponse message to the MSC Server.9. Establishment of a circuit connection betweenthe target MSC and the MGW associated withthe MSC Server now occur.8

Single Radio Voice Call Continuity (SRVCC) with LTE Radisys White Paper10. The MSC Server initiates the Session Transfer byusing the STN-SR (e.g., by sending an ISUP IAM(STN-SR) message toward the IMS). StandardIMS Service Continuity procedures are appliedfor execution of the Session Transfer.19. The target BSS sends a Handover Completemessage to the target MSC.11. A new access leg is established by the UE towardthe SCC AS. Signaling and bearer resources areallocated in the transferring-in Access Networkand the user’s sessions are transferred from thetransferring-out Access Network. The SCC ASexecutes Access Transfer procedures.21. Completion of the establishment procedure occurswith an ISUP Answer message to the MSC Server.12. During the execution of the Session Transferprocedure the remote end is updated with the SDP ofthe CS access leg. The downlink flow of VoIP packetsis switched towards the CS access leg at this point.13. The source IMS access leg is released now.14. The MSC Server sends a SRVCC PS to CS Response(Target to Source Transparent Container) messageto the source MME.15. The source MME sends a Handover Command(Target to Source Transparent Container) messageto the source E-UTRAN. The message includesinformation about the voice component only.16. The source E-UTRAN now sends a Handoverfrom E-UTRAN Command message to the UE.17. The UE tunes to the new CS Network; HandoverDetection at the target BSS occur

Single Radio Voice Call Continuity (SRVCC) with LTE Radisys White Paper 2 The industry is exploring and evaluating different possibilities to overcome the LTE voice issues. During this evaluation process two options are gaining significant momentum: Circuit Switched Fall Back (CSFB) and LTE VoIP-based Single Radio Voice Call Continuity (SRVCC).

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