Usługi głosowe w sieciach LTE Marcin Aronowski Cisco Systems
O czym dzisiaj postaram sie opowiadać? VoLTE - introduction Circuit Switch Fallback Architecture, Policy and Charging APN, Bearers and PDNs for VoLTE/VoHSPA Roaming Emergency Calls * I dużo TTS-ów, CzTS-ów, generalnie xts-ów ;)
LTE system architecture 3
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VoLTE 5
GSMA and Voice Standardization The Need For a Common Implementation Roaming Diverse Handsets Worldwide coverage, Billions of Subs GSMA has driven GSM standardization and is now defining VoLTE for voice evolution Single Implementation reduces complexity, promote scales and enables roaming Voice service shall look the same in any network, with any device VoLTE based on a subset of 3GPP specifications for IMS
VoLTE Timing Few VoLTE commercial networks launched in 2012 Metro PCS, SK Telecom, LG U+ VoLTE devices starting to be available in the market Samsung, LG, Sony,.. VoHSPA defined by GSMA after VoLTE Will it be deployed??? CSFB VoLTE CS Voice & SMS 2011 2012 2013 2014
VoLTE/VoHSPA Standards Map Driven by GSMA, based on 3GPP Specs for IMS, RAN & CN IR.92 (VoLTE): IMS Profile for Voice and SMS over LTE IR.58 (VoHSPA): IMS Profile for Voice over HSPA IR.94: IMS Profile for Conversational Video Service IR.64: IMS Service Centralization and Continuity IR.65: IMS Roaming IMS Call Flows and Architecture: TS 23.228, TS 24.229, etc. 3GPP MMTel Suppl. Services: TS 24.173 and many others IMS Emergency: TS 23.167 EPC and GPRS Core: TS 23.401, TS 23.060, etc. Policy anbd Charging: 23.203, 23.212, 23.214, etc. SRVCC and Service Continuity: TS 23.216, TS 23.237, etc. IR.88 LTE Roaming
The VoHSPA Business Case Today s 3G/UMTS networks use CS for voice and HSPA for data CS voice is not efficient, operators need to reduce CS bandwidth to use it for PS 2 possible evolutions to deliver voice over HSPA, both require less bandwidth than legacy CS HSPA Radio RNC CSoHS (CS over HSPA) MSC CS Network Minor changes on UE and RNC Still CS based VoHSPA HSPA Radio RNC SGSN EPC IMS/VoIP, requires IMS like VoLTE Same VoLTE/VoHSPA experience IMS Requires seamless PS-HO HSPAó LTE
Radio bearer management VoLTE VoHSPA VoLTE/VoHSPA Network Scenario Several elements, different requirements Mobility Mgnt. LTE HSPA MME SGSN S-PGW PCRF EPS Bearer & QoS Mngnt Device CS CS Iwk (SRVCC) GERAN / UTRAN (CS) MSC HSS VoLTE/VoHSPA Subscription IMS Service Control & Continuity IMS based VoIP (or video) call over LTE or HSPA radio access PCRF controlled QoS Service continuity over CS when out of LTE/HSPA coverage
IMS system and UE s IMS client (not addressed in this ppt) APN, bearers and PDN interworking So What Is Required? On the UEs and All Network Elements IMS APN, P-CSCF discovery, dedicated bearers and NRSPC, etc. Policy and Charging PCRF interaction via Gx/Rx, CDR in EPC and IMS Emergency Call Handling Specific QoS, user authentication, user localization Service continuity with CS network Mid call HO and SRVCC Roaming support LBO for voice as in CS CSFB co-existence
Circuit Switched Fallback 12
Na#ve CSFB Reference Architecture
Circuit Switched Fallback Delivery of SMS to EPC Attached Subscriber UTRAN / GERAN IuCS / A MSC 1 Incoming SMS SGs 2 SMS is delivered via SGs interface MME S6a HSS S1-MME S11 CSFB UE E-UTRAN S1-U Serving/ PDN GW During EPC attach, CSFB UE s are also attached over SGs to MSC MME maintains mapping of TA to LA to determine appropriate MSC to establish SGs association with SMS can be delivered/sent without FallBack to legacy radio (SGs interface includes SMS payload capability)
Circuit Switched Fallback Delivery of Terminating Voice Session to EPC Attached Subscriber 4 UE Responds to paging incoming call terminated via standard 2G/3G procedures UTRAN / IuCS / A CSFB UE GERAN MSC 1 Incoming Call delivered to VMSC which has SGs association for this subscriber SGs 2 UE is paged via the SGs interface 3 UE retunes to 2G/3G RAT on receipt of page MME S6a HSS S1-MME S11 CSFB UE E-UTRAN SGs interface is used to inform UE of incoming call SGs Paging triggers UE to retune to 2G or 3G legacy network Page Response sent via the legacy radio network Call terminates to UE normally S1-U Serving/ PDN GW
CSFB Deployment Challenges CSFB was initially seen as the easy interim approach As further analysis of CSFB was performed, the identified challenges mounted Some of the major challenges/difficulties facing CSFB rollout are: Legacy network dependencies Continued legacy network spend/investment Fallback latency / post-dial delay Ongoing operational complexity (TA/LA mapping)
TA to LA Mapping Implementation Challenges Most network operates deploying LTE already have both 2G and 3G networks Mapping process has to take into account When UE falling back, is it likely to go to 2G or 3G overlapping cells? Where cell boundaries between 4G and 2G (or 3G) do not perfectly align, which is provisioned? As cells breathe, boundaries become less predictable 4G networks will be growing rapidly in near-term requires constant tweaking of provisioning tables? 4G TA s 3G LA s 2G LA s
UE MME MSC HLR Dependencies for CSFB As Specified by TS 23.272 UE must support CSFB procedures as defined with TS 23.272 Must support SGs interface Must support MSC/VLR selection procedures Must support SGs interface (legacy MSC s will require addition of IP interface cards) Should support Roaming Retry procedure to minimise call drops Should support Roaming Retry procedure to minimise call drops
Architecture, Policy and Charging 19
IMS Policy and Charging Architecture * IMS functions related to PCC: P-CSCF (Rx), S-CSCF (ISC, Rf), MRFC/AS (Ro, Rf) IMS App UE SIP/SDP Rx IMS* PCRF Gx ISC Rf Gy IMS-GWF Ro OCS Bx Billing System Access IP-CAN PCEF Ga/Gz/Rf CGF Bx Voice charging controlled on the IMS side (numbering/in aware) Charging can still be enabled on EPC, e.g. for revenue assurance PCRF required for QoS control and for EPC ó IMS coordination
APN, Bearers and PDN Connectivity 21
VoLTE Bearers Based on GSMA IR.92 and IR.94 Gx PCRF IMS (SIP) Client APN IMS EPC PGW Default bearer, QCI=5 (SIP Signaling) Dedicated Bearer, QCI=1 (Voice) Dedicated Bearer, QCI=2 (Video) Dedicated IMS APN shall be used for VoLTE and the UE should not provide the IMS APN during initial attach (SIP) (RTP) Typical IMS Application Servers (AS): - TAS/MMTel Telephony AS (VoIP) - IP SM GW (SMS) - IM-SSF (IN interwork) P-CSCF Either sent by the HSS as default APN (typical case) or sent by the UE later for UE requested additional PDN Rx IMS MP/MGW AS Default bearer always active for SIP signaling, dedicated bearers established only for the duration of the voice/video call PCRF initiated Dedicated Bearers activation based on P-CSCF request initiated by the, following a request from the P-CSCF (Rx)
APN IMS, the same as VoLTE IMS (SIP) Client VoHSPA PDP Contexts Based on GSMA IR.58 and IR.94 EPC Gn-SGSN and S4-SGSN allowed PGW Primary/Secondary PDP on Gn, EPS bearers on S4 Primary PDP for SIP Signaling Primary PDP, Interactive, THP=1 NRSPC, Conversational, Source Stat.Desc=speech NRSPC, Conv./Stream./Interact PDP context with Interactive Traffic Class (TC) and Traffic Handling Priority (THP) = 1. The UE sets the Signaling Indication in the QoS IE The S4SGSN can map the GMM QoS from EPS-Subscription (TS 23.401 Ann.E) Secondary PDP (NRSPC) for media Voice PDP: Conversational TC and Source Statistic Descriptor set to Speech Video PDP: Conversational, Streaming or Interactive TC based on policies Gx (SIP) Gm (RTP) PCRF Rx P-CSCF IMS IMS-AGW
The UE does not provide the APN Attach and IMS Registration Example of High Level Call Flow (Normal Case) Subscriber profile with the IMS APN downloaded fron the HSS PCRF UE MME/ SGSN HSS SGW/ PGW CSCF Attach Req. Auth & UL Default Bearer/Primary PDP Setup for the APN IMS The PGW provides P-CSCF address(es) transferred as PCO Attach Accept Contains P-CSCF @ and the indication of IMS Voice over PS support in the network APN IMS Default Bearer, QCI=5 Optional UE Requested Additional PDN Conn. IMS (SIP) Registration New PDN connection, e.g. for APN Internet
IMS Media Session Setup Dedicated Bearers Activation UE PCEF PCRF P-CSCF Dedicated Bearer Establishment MO or MT leg for the UE RAR QCI(=1 for voice), GBR, MBR, Flow Info, Event-Trigger,.. RAA CCR-U Event-Trigger=SUCCESSFUL_... CCA-U The P-CSCF needs to know if the bearer is allocated AAR Session-Id, ICID, UE sip, Media-Component-Description, Specific-Action, AAA RAR Specific-Action=INDICATION_OF.. RAA Specific-Action = INDICATION_OF_SUCCESSFUL_RESOURCES_ALLOCATION Event-Trigger = SUCCESSFUL_RESOURCE_ALLOCATION INVITE SDP
IMS Media Session Termination Dedicated Bearers Deactivation UE PCEF PCRF P-CSCF Dedicated Bearer Deactivation RAR Charging-Rule-Remove RAA STR Session-Id, Destination-Host, Termination-Cause STA Result-Code BYE The AVP Charging-Rule-Remove causes the removal of installed rules and consequently the release of the dedicated bearer setup for the session (call) STA may happen before or after RAR
The P-CSCF must be always in the same network of the PGW P-CSCF Discovery The PGW can provide the P-CSCF address(es) to the UE during initial attach, as part of the PCO IE Multiple addresses for load balancing and resiliencies P-CSCF failure detection a) The UE detect the failure (SIP Register fails) and use another address Pro: Simple, no impact on the network Cons: After initial registration the UE will detect the failure only at registration refresh b) The PGW could check the status of the P-CSCF (TS 29.061 clause 13a.2.2) Pro: Faster failure detection Cons: Massive re-attach/re-registration when loaded P-CSFCF fails. Also additional traffic in the network for P-CSCF check (e.g. snmp polling)
IP Addressing IR92 requires the UE and the network to support both IPv4 & IPv6 The VoLTE device should request PDN Type IPv4v6 IPv6 only is normally used, the IMS APN provides access to the IMS only, isolated network Perfect case for v6-only, IPv6 pool for the Gm LAN Note: IPv6 increases the overhead on SIP/RTP (large header): ROHC is requested by IR.92 on the wireless link UE-eNB
VoLTE Roaming 29
VoLTE Roaming like CS-Voice Roaming Same Principles Are Generally Applied Routing of media for Voice over LTE (VoLTE) when call originator is Roaming should be at least as optimal as that of current Circuit-Switch (CS) Domain HPLMN CS Core Subscription management UE VPLMN CS Core Voice call Any CS Network CS Voice Roaming Voice Call routed by the VPLMN Call control via HPLMN CS (HLR/IN) VoLTE Roaming Voice RTP routed via IMS in the VPLMN Signaling/call control via IMS in HPLMN
VoLTE Roaming Normal Case IMS in VPLMN & HPLMN + VoLTE Roaming Agreement Subscriber X s profile ODB-VPLMN-APN=0 [IMS APN] VPLMN-Dyn-Addr-Allowed=1 HSS TAS S-CSCF PLMN B (H-PLMN) Subscriber X of PLMN B VoLTE UE MME Local Policy PLMN B > IMS VoPS=1 MME PGW SGW PDN Connection for IMS APN IMS APN P-CSCF IMS-AGw PLMN A (V-PLMN) The HSS in the HPLMN provides the IMS APN set as default APN and VPLMN- Dynamic-Address-Allowed = 1 The HSS shall set this only for VPLMNs with VoLTE roaming agreement! The MME can also notify the HLR/HSS of IMS VoPS support The MME in the VPLMN uses the local PGW which serves the IMS APN through the local P-CSCF for roamers and home subs The APN used for XCAP must be routed to the HPLMN
PS Roaming Without VoLTE Roaming IMS in VPLMN &HPLMN but PS Data Roaming only Subscriber X s profile ODB-VPLMN-APN=0 [IMS APN] VPLMN-Dyn-Addr-Allowed=0 Subscriber X of PLMN B VoLTE UE MME HSS SGW PGW PDN Connection for IMS APN IMS APN MME Local Policy PLMN B >> IMS VoPS=0 P-CSCF IMS-AGw PGW IMS APN PLMN B (H-PLMN) PLMN A (V-PLMN) The HSS in the HPLMN provides the IMS APN with VPLMN-Dynamic-Address- Allowed = 0 The HSS shall set this only for VPLMNs with VoLTE roaming agreement The MME in the VPLMN selects the PGW in the HPLMN for IMS APN, but set the IMS Voice over PS as not supported The UE receiving IMS VoPS not supported in the Attach Accept can use the IMS PDN connection for other IMS services (e.g. RCS) but not for voice
Emergency Calls 33
IMS Emergency Call Can be co-located GMLC Lg/SLg MME Emergency APN SGSN PGW Default Emergency bearer Dedicated Emergency bearer EPC Dedicated APN and Bearer/PDP Context for Emergency Call (SIP) Gm (RTP) LRF MI E-CSCF P-CSCF IMS SBC/MGW The APN is not part of the subscription, it is defined by the local PLMN No standard name for the APN, sos suggested by GSMA IR.88 PCRF may not be used to speed-up the call (local QoS settings in MME/SGSN) Emergency Calls can be accepted also for un-authenticated or SIM-less UE (local policy settings) User s location required to route the national emergency number
Location Service for Emergency Calls MME SGSN EPC Lg/SLg GMLC Location Retrieve INVITE 112 E- INVITE PSAP # CSCF The emergency number (e.g. 112, 911) must be translated to the number of the local Public-Safety Answering Point (PSAP) IMS LRF PSAP # retrieve PSAP # = f(112, ULI) The LRF (Location Resource Function) maps the UE s location (e.g. cell-id) to PSAP. It can be part of the GMLC, LCS Architecture via GMLC as defined in 3GPP TS 23.271 Network Induced Location Request (NI-LR): In case of emergency call, the MME sends the user location to the GMLC Alternatively, the LRF could trigger Mobile Terminates Location Req. (MT-LR) via the GMLC to retrieve the location from the MME. But this may take too long time
CSFB for Emergency Calls As CSFB is generally available before VoLTE, some operators are considering the options to use CSFB for emergency calls Avoid to implement LCS mechanisms for PSAP mapping GSMA and 3GPP include the possibility to try the emergency call in CS or PS domain and retry in the other domain if it fails GSMA IR.92 Annex A.5, 3GPP TS 23.167 clause 7.3 Using a different access domain only for emergency requires a careful setup in the network.. It may not be easy either If the IMS emergency call is rejected, the UE may retry in another RAT. But this is like deregistering from LTE/IMS and reattach on CS è long call setup If the UE is both CS and PS/VoLTE attached, a proper routing is required in the CS network to route incoming calls via IMS and the IMS shall reject the emergency call to force rerouting to CS
CS Service Continuity 37
IMS (SIP) Client CS voice VoLTE VoHSPA CS Mobility During Active IMS Call PS-HO SRVCC LTE HSPA (PS) GERAN / UTRAN (CS) MME SGSN MSC Sv SCC= Service Centralization & Continuity S/PGW IMS SCC AS LTE ó HSPA PS Handover is required to maintain the IMS/VoIP call over LTE and HSPA access (bearer update) IMS based Service Continuity over CS (SRVCC) is required for mid-call HO from LTE/ HSPA to GERAN/UTRAN CS Multiple options defined in the standards 3GPP TS 23.216, TS 29.280, TR 23.856 - GSMA IR.64
UE E-UTRAN /HSPA SRVCC High Level Call Flow 3GPP TS 23.216 MME/ GERAN SGSN MSC IMS UTRAN (SCC-AS) Active VoIP/IMS Session over LTE access Measurement HO Required (2G/3G Target,..) Initiate SRVCC For voice (STN-SR) Bearer splitting Voice ó QCI 1 Optional PS-PS HO for non-srvcc bearers CS HO Preparation Session Transfer (STN-SR) HO Cmd Coordinate SRVCC & PS-HO Responses SRVCC Response Update Remote End STN-SR= Session Transfer Number for SRVCC, defined in the subscriber profile in HSS Optional PS bearer (non-ims) Voice over CS
IMS, CSFB & Domain Selection 40
RAT Selection for Multimode UE Which RAN is selected? Core Network The LTE/EPC network may support one or more among: SMSover SGs CSFB IMS/VoLTE GERAN Multimode UE: GSM/GPRS, UMTS, LTE UTRAN E-UTRAN Voice Preference options in the UE CS Only PS Only CS Preferred, IMS secondary IMS Preferred, CS secondary The preferred shall be set in the UE and notified to the Network VoLTE UE = IMS Preferred, CS secondary
VoLTE related info between UE/RAN/CN Initial cell selection Attach/TAU Request (NAS) E-UTRAN Voice : - CS Only preference - PS Only - CS Preferred, IMS secondary - IMS Preferred, CS secondary Usage Setting: - Voice Centric - Data Centric UE Attach Accept (NAS)/Initial Context Req. (S1-AP) EPS Network - IMS Voice Over PS support Feature Support: - Emergency Call support Additional Update - SMS-Only Feature Support: - CSFB Not Preferred - No indication RRC Reconfiguration RAT priorities (SIB) Cell re-selection enb RFSP RFSP Index Index NAS S1 MME
RAT/Frequency Selection Priority Index From 3GPP TS 23.401 UE UE s voice preferences and usage settings Local policy RFSP decision HSS Camping priorities enb (modified) RFSP index MME Subscribed RFSP Index The EPC may set the RFSP Index to influence camping priorities Multiple options depending on device capabilities Example use case: enforce 2G/3G camping for voice-centric UE if the supported voice technology is not available in the network Ref. TS 23.401 (Rel-9), Clause 4.3.5.9 and 4.3.6 Standard RFSP Id values for GERAN/UTRAN/E-UTRAN priorities defined in TS 36.300 Annex I
Final UE Decision Reselection Config Voice/Data Centric Voice Pref GERAN Network Capabilities (CSFB/IMS) UE CSFB/IMS selection logic (Annex A TS 23.221) UTRAN E-UTRAN The UE selects to camp on 2G/3G or LTE considering: Network capabilities(support of CSFB and/or IMS in the CN) UE preferences (voice/data centric, IMS/CSFB preference) Decision tree for each case from TS 23.221 Annex A Try preferred voice option, then less preferred. If voice support not available, decide to stay on LTE (data centric) or 2G/3G (voice centric)
VoLTE is starting to take off Not clear if VoHSPA will ever be adopted GSMA defines a subset of IMS functions for a global and uniform VoLTE implementations Minor variants are possible for VoLTE design and should be clarified with each operator