CS services in LTE- An OverviewPrepared by: Darshan PatilSr. Engineer, Alcatel Lucent Managed Solutions India
Summary• What? To support CS services in LTE• Why? Need to support CS services in LTE• When? Different Phases of evolution of Voice over LTE(VoLTE)• How? Network methodologies to implement CS servicesin LTE network
WHAT?• LTE is All- IP data only support technology usingpacket switching• This subjects to challenges for CS services in LTE asreferred to all the legacy networks which supportthem currently• Solutions to provide CS services in LTE network• Inter-networking solutions to provide CS services inLTE network
WHY?• Current legacy networks increase expectations to improvethe efficiency of the services they are providing• This includes Voice, SMS, Video Call, High data rate internetand streaming etc. Services• The evolution of LTE technology is to significantly improvethe user experience, substantially improving end-userthroughputs, increasing sector capacity, and reducing userplane latency• Though, due to lack of Circuit switching, the technology hasa need to find some way outs to provide CS servicesolutions at par with the current legacy networks• Also, till the time when LTE completely swaps the legacynetworks there is a need for internetworking with them
Comparison betweenDual Radio Solutionso use two always-on radios (andsupporting chipsets), one forpacket switched LTE data and onefor circuit switched telephony,and as a data fallback where LTEis not availableo have emerged for LTE-CDMA2000network interworking driven bytime-to-market pressureso Battery gets drained faster due totwo radio antennas and chipsetsSingle Radio Solutionso use one radio to handle bothtypes of traffic, and use networksignaling to determine when toswitch from the PS network tothe CS networko this solution is universallyaccepted for LTE-3GPP networkinterworking solutionso Power consumption of thebattery is fairly reduced due tosingle antenna – single chipset.
Phases of Single radio solutions forVoLTE Phase I: Circuit Switched Fall Back (CSFB) addresses the requirements of thefirst phase of the evolution of mobile voice services, which began oncommercial scale in 2011 primarily due to inherent cost, size and power advantages of singleradio solutions on the device side. CSFB is the solution to the reality of mixed networks today andthroughout the transition to ubiquitous all-LTE networks in the futurephases of LTE voice evolution All the voice traffic is handled by current legacy CS networks(UMTS/GSM/CDMA) All the data traffic is handled by LTE PS network wherever andwhenever available The traffic will be able to fallback to 2G/3G network in non-LTEnetwork
Phases of Single radio solutions forVoLTE Phase II: The second phase in LTE voice evolution introduces native VoIP on LTE (VoLTE)along with enhanced IP multimedia services such as video telephony, HD Voiceand Rich Communication Suite (RCS) additions like instant messaging, videoshare and enhanced/shared phonebooks This phase also uses a single radio solution with Single Radio Voice CallContinuity (SRVCC) that seamlessly maintains voice calls as mobile users movebetween LTE and non-LTE coverage areas CSFB continues to be deployed during phase 2, to provide voice services forroamers and CSFB only devices Without SRVCC, a VoLTE call on a device moving out of LTE coverage will bedropped, since no operators currently support VoIP on 3G
Phases of Single radio solutions forVoLTE Phase III:converges the native power of IP to deliverenhanced capacity, value-added services (e.g.,voice and video over IP and rich communicationservices) and interoperability across networkaccess methods and operators (LTE, 3G/ HSPA, Wi-Fi and legacy telephony domains)
HOW?• Phase I: CSFBThe architecture in Figure below shows a simplified view ofthe parallel LTE and 2G/3G networks.
Phase I: CSFB – CN procedures• The legacy network (2G/3G) and LTE network co-exists• To support CS Fallback signaling and SMS transfer forLTE devices, the MME connects to the MSC Server• MME is the mobility management entity serving userswhile in LTE access. It is responsible for maintainingSGs association towards MSC, initiating pagingprocedure towards eNodeB during CSFB• MME Support SMS procedures• MSC is responsible for maintaining SGs associationtowards MME for EPS/IMSI attached UE
SGs interface• It is the reference point between the MME and MSC server. SGsinterface is used for the mobility management and pagingprocedures between EPS and CS domain, and is based on the Gsinterface procedures.• It enables the user’s device to be both CS and PS registered whileon the LTE access network. This interface also enables the deliveryof CS pages via the LTE access, as well as SMS, without having thedevice leave LTE.• The SGs reference point is also used for the delivery of both mobileoriginating and mobile terminating SMS.• At MME - MSC Server interface a new protcol SGsAP is being addedto support CS fallback. SGsAP protocol is based on the BSSAP+.Stream Control Transmission Protocol (SCTP) is used to transportSGsAP signaling messages.
CSFB- Incoming Call• With the default LTE data network connection in operation, amobile terminating (incoming) CS voice call triggers a page viaLTE to the user’s device, as shown in below Figure
CSFB- Incoming Call continued..• This page initiates CSFB, as the device sends an extendedservice request to the network to transition to 2G/3G, asshown in below Figure• Once transitioned, the legacy call setup procedures arefollowed to setup the CS call.
CSFB- Incoming Call continued..• When the voice call ends, the device returns to LTE via idlemode or connected mode mobility procedures, as shown inbelow Figure
CSFB- Outgoing call• Mobile originating (outgoing) calls follow thesame transition from LTE (PS) to 2G/3G (CS),except for the paging step, which is notneeded.
Simultaneous PS and CS services inCSFB• Depends on the DTM feature availability with thelegacy network• In 3G networks, PS data sessions can also movefor simultaneous voice and data services.• In 2G networks, PS data sessions may besuspended until the voice call ends and thedevice returns to LTE, unless the 2G networksupports dual transfer mode (DTM), whichpermits simultaneous voice and data.
Phase I: CSFB – RAN proceduresAcquisition of the 2G/3G network and setup of the call can employ one of twoprocedures: handover or redirection.Handover• Target cell is prepared in advance• Device enters in connected modeduring IRAT• Signal strength measurementsare done prior to HO to 2G/3G(i.e in LTE access mode)• Process require more time toidentify the best cell• Under changing IRATenvironments, this methodproves to be not much efficient• Therefore it is not widelyfollowed method currentlyRedirection• Target frequency is indicated• UE may choose other IRATfrequency cell if no cell could befound out in target frequency• After UE has access to legacynetwork, call setup procedure hasto be initiated• IRAT measurements are notrequired prior to Redirection• As per statistics, a marginal loss inperformance in this method ascompared to Handover based IRATcan be accepted to compensatevariable IRAT RF environment
CSFB w/ ReselectionThree types of procedures according to System InformationBlocks sent to the UE1. Release 8 – Basic Redirection: UE reads all the SIB’s before having an access to thetarget cell2. Release 8 – with SIB skipping Mandatory SIB’s like 1,3,5,7 are only read by the UE. Afterit accesses the target cell, in connected mode it reads theneighbor info present in SIB 113. Release 9 – Enhanced (SI tunneling) where SIB information can be tunneled from the targetRadio Access Network (RAN) via the core network to thesource RAN and be included in the redirection messagesent to the device. This can avoid reading any SIBs on thetarget cell.
CSFB Performance Indices• After commercial deployments, CSFB have been par withthe 3GPP performance requirements for 3G UMTS.• These basic requirements includes Call setup time (mobileoriginated and mobile terminated), Call reliability, LTE to 3Gcell handover time, PS data interruption time.• Call reliability in case of Handover based CSFB process ispoor due to variable RF environment• Call reliability in case of Redirection based procedure isbetter than Handover based as per statistics due to nodelay in between identification and accessing the targetcell.• Rel9- SI tunneling has slightly better call reliability (about99%- 100%) than Rel8- Basic Reselection.
CSFB Outgoing Call setup penalty timeSr. No RAN procedure Transition to Sub category Call setup timepenalty1 Handover Based 3G UMTS NA 0.4S > CST of 3G2 Redirection 3G UMTS Rel8- Basic 2.5S > CST of 3GRel8- SIB skip 0.9S > CST of 3GRel9- SI tunnel 0.5S > CST of 3G3 Handover 2G GSM NA 2.6S > CST of 3G2.7S > CST of 2G4 Redirection 2G GSM Rel9- Basic 2.6S > CST of 2G2.5S > CST of 3GRel8- SIB skip NARel9- SI tunnel 0.6S > CST of 2G0.5S > CST of 3G
CSFB Incoming CallSr. No RAN procedure Transition to Sub category Call setup timepenalty1 Handover Based 3G UMTS NA 0.4S > CST of 3G2 Redirection 3G UMTS Rel8- Basic 2.5S > CST of 3GRel8- SIB skip 0.9S > CST of 3GRel9- SI tunnel 0.5S > CST of 3G3 Handover 2G GSM NA NA4 Redirection 2G GSM Rel9- BasicRel8- SIB skipRel9- SI tunnel
CSFB Data Interruption timeThe active session PS data interruption time during LTE PS to2G/3G CS and reverseSr. No RAN procedure Transition to Sub category Datainterruptiontime1 Handover Based 3G UMTS NA 0.3S2 Redirection 3G UMTS Rel8- Basic 6.8SRel8- SIB skip 5.1SRel9- SI tunnel 4.8S3 Handover 2G GSM NA NA4 Redirection 2G GSM Rel9- BasicRel8- SIB skipRel9- SI tunnel
LTE to 3G cell Handover time• Time taken to handover is never identical due to varied Rfenvironment• Delay added by LAU may increase Ho time. Further if theLte cell is overlapped with multiple LA’s, the delay canincrease up to 1-2 seconds• Fewer times, apart from updating LA’s TA’s, there would beneed to update HLR data between diff overlapping MSC’s.• More uncertainties can add to the delay further.• Solutions to the above:– IU/A Flex– MTRF– MSC pooled architecture planning
Phase II: VoLTE and SRVCC• Voice over IP can be implemented in two ways:1. using “Over the top” (OTT) VOIP applications such as skype, Ekiga,Goober etc..2. using native VOIP using additional entities such as IMS and MMTel incore network• Using OTT services is not recommended as the Qos is similar to NRTservices with no special priorities to voice. Therefore, even thevoice quality of current legacy CS services is not matched• VoLTE, in contrast, operates as a native application in the user’sdevice, enabling prioritization over other data streams to deliverquality of service levels consistent with established userexpectations• In non-LTE network, this quality is taken care by 2G/3G networkswhich can be accessed using SRVCC (Single Radio Voice CallContinuity) method of IRAT HO
MMTel and Sv• The 3GPP/NGN IP Multimedia Subsystem (IMS) multimediatelephony service (MMTel) is a global standard based on theIMS, offering converged, fixed and mobile real-timemultimedia communication using the media capabilities suchas voice, real-time video, text, file transfer and sharing ofpictures, audio and video clips. With MMTel, users have thecapability to add and drop media during a session.• Sv is defined in TS 23.216, where it is defined as the referencepoint between the MME/SGSN and MSC Server.
SRVCC Procedure1. User (UE) is in active voice call in LTE network using VoLTE with the help ofIMS as shown in below figure2. User moves from a LTE network to a non-LTE network (3G-UMTS), the activecall remains under the control of IMS throughout the handover process
SRVCC Procedure3. Two step process of SRVCC from LTE to 2G/3GIRAT HandoverIRAT handover is the traditional handover of theuser’s device from LTE radio access toWCDMA/GSM radio accessSession transferSession transfer is a new mechanism to moveaccess control and voice media anchoring fromthe LTE Evolved Packet Core (EPC) to the legacy CScore
SRVCC Procedure4. The handover process is initiated by a sessiontransfer request to the IMS/MMTel as shownbelow
SRVCC Procedure5. The IMS/MMTel responds simultaneously with two commandsi. Sending an “IRAT HO execution command” to the LTE network on whichthe voice call is in progress through MME & Lte RAN to instruct the UEto prepare to move to CS network for the voice callii. Sending a “session transfer response” to the CS n/w where the usersvoice call is being sent asking to prepare to accept the call in progress
SRVCC Procedure• Both the n/w send acknowledgementsrespectively and under the control of the IMS/MMTel, the call is switched to 2G/3G n/w
Network upgrades required for SRVCCmethodologySr. No Entity Up gradation required?1 LTE E-UTRAN Yes2 LTE EPS Yes3 IMS Core/ MMTel Yes4 2G/3G Core (MSC) Yes5 2G/3G RAN No6 UE (user equipment) Yes
PI for SRVCCo Voice interruption time:1. IRAT HO execution process requires more time than sessiontransfer procedure (of the order 0.01S). Therefore voiceinterruption time is influenced primarily by the IRAT HO executiontime.2. Further, we need to consider addition of time required to confirmthe HO execution after HO process3. The total voice interruption time is at par with the 3GPPguidelines of 0.3So Call Retention rate depends on:1. Probability of IRAT failure2. Probability of Session transfer failure3. Current stats show it as 99%
References• 3GPP specs• Qualcom (VoLTE phases)• China Mobile (CSFB)• Nokia Seimens (CSFB)• Ericsson (VoLTE phases)• Alcatel-Lucent (LTE Network Architecture)
Future Scope of VoLTE• Enhanced SRVCC – Next article• Video SRVCC – Next article• CSFB and VoLTE Roaming (Phase III)