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VoLTE flows - basics

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VoLTE Basic callflows in IMS network v2 - includes Registration, Basic VoLTE Call, SDP, Interconnect, Roaming, highlights important SIP headers for session routing and user identities.

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VoLTE flows - basics

  1. 1. realtimecommunication.wordpress.com realtimecommunication.wordpress.com VoLTE Flows Introduction v2 UASUAC SIP INVITE (SDP-O) Terminating Network Originating Network P-CSCF S-CSCF I-CSCF A-SBC O-TAS IPSMGW DNS HSS DNS HSS P-CSCF S-CSCF I-CSCF T-TAS A-SBC AGWAGW realtimecommunications.wordpress.com
  2. 2. realtimecommunication.wordpress.com VoLTE Deployments https://www.gsma.com/futurenetworks/resources/all-ip-statistics/ 3 0 12 33 56 16 0 0 1 16 35 3 8 20 12 7 3 3 0 10 20 30 40 50 60 70 80 90 100 2012 2013 2014 2015 2016 2017 ALL-IP DEPLOYMENTS VoLTE VoWifi RCS 3 0 12 33 56 19 0 0 1 16 35 4 8 20 12 7 3 4 0 10 20 30 40 50 60 70 80 90 100 2012 2013 2014 2015 2016 2017 ALL-IP DEPLOYMENTS VoLTE VoWifi RCS
  3. 3. realtimecommunication.wordpress.com What is VoLTE? LTE Tel: num1 Tel: num2 GSM Tel: num3 • VoLTE stands for Voice over LTE. LTE is a new standard for wireless communication of high-speed data for mobile phones. ViLTE means Video over LTE.
  4. 4. realtimecommunication.wordpress.com What is in the LTE? 4 LTE • A packet-switched all-IP network which provides data connectivity and mobility. • LTE access network doesn’t provide any voice, video or text services. This is to be handled by IMS. e-nodeB e-nodeB EPC MME SGW PGW E-UTRAN HSS UE • We will not discuss LTE RAN and EPC in this training.
  5. 5. realtimecommunication.wordpress.com IMS 5 LTE • VoLTE is based on the IP Multimedia Subsystem (IMS) network – a framework designed for delivering IP multimedia services. • IMS is access independent, user-centric network providing services such as authentication, session routing and management, QoS, roaming, provisioning, charging etc. LTE IMS
  6. 6. realtimecommunication.wordpress.com 4G IMS P-CSCF S-CSCF I-CSCF A-SBC TAS IPSMGW DNS HSS ALG/AGW MRF AP SGW PGW DNS HSS ePDG 2G / 3G AAA PCRF DSC MGCF MSC MGW TWAG Wifi MME Internet ePC WLAN E-CSCF BGCF LRF realtimecommunications.wordpress.com
  7. 7. realtimecommunication.wordpress.com 4G Ecosystem • The Fourth Generation of Mobile Networks is based on IP protocol. From Operator point of view it is a huge change • New Radio Network • New Enhance Packed Core • New Service Network – IMS • New Protocols, Flows, KPIs, Vendors, New Issues • 4G is user centric and supports multiple access networks • We can connect to IMS using any IP-based access network • A-SBC provides connectivity, security, NAT/NAT traversal, media handling etc. • IMS is integrated also with the legacy (Sigtran) network • VoWifi (GSMA IR.51) provides the same set of services as VoLTE and is integrated using EPC. • RCS UP is integrated with VoLTE/VoWifi.
  8. 8. realtimecommunication.wordpress.com IP-based Communication • Separates Signaling (SIP, H.323, XMPP, REST, ..) and Media (RTP, MSRP, WS, HTTP, ..) flows. • The purpose of Signaling is to • Locate the recipient • Share presence information • Exchange the parameters of the multimedia session • Binding between IP and User Identity is created during Registration. • One user can use multiple different identities and devices/sw clients. For VoLTE • SIP is used for end-to-end signaling, RTP for Voice/Video • Public User Identity (IMPU) is based on telephone number can be e.g. • sip: +<number>@volte.ims.mnc012.mcc345.3gppnetwork.org • tel: +<number> • Binding between actual IP assigned by LTE (PGW) and IMPU is stored in IMS Network (S-CSCF)
  9. 9. realtimecommunication.wordpress.com What is Signalling? LTE IP2IP1 Tel: num1 Tel: num2 IMS WLAN num1
  10. 10. realtimecommunication.wordpress.com What is ? • The standard is defined by GSMA in IR.92 and IR.94 as a minimum mandatory set of features for an interoperable, high quality IMS-based telephony service and SMS over LTE network. The scope includes: • IMS basic capabilities and supplementary services for telephony • Real-time media negotiation, transport, and codecs • LTE radio and evolved packet core capabilities • Functionality that is relevant across the protocol stack and subsystems • Additional features that need to be implemented for the UEs and networks that wish to support concurrent Circuit Switched (CS) coverage Description of basic call flows and procedures can be found in VoLTE Service Description and Implementation Guide.
  11. 11. realtimecommunication.wordpress.com IMS LTE • VoLTE is based on the IP Multimedia Subsystem (IMS) network – a framework designed for delivering IP multimedia services. • IMS is access independent, user-centric network providing services such as authentication, session routing and management, QoS, roaming, provisioning, charging etc. IMS
  12. 12. realtimecommunication.wordpress.com 3GPP IMS
  13. 13. realtimecommunication.wordpress.com IMS Simplified IMS P-CSCF S-CSCF I-CSCF A-SBC TAS IPSMGW DNS HSS ALG/AGW MRF 2G / 3G MGCF MSC MGW ePC BGCF realtimecommunications.wordpress.com
  14. 14. realtimecommunication.wordpress.com IMS Entities I Proxy - Call Session Control Function (P-CSCF) • First point of contact for SIP signaling within IMS network • Can be located in a visited as well as in a home network • Often deployed as a part of Access Session Border Controller (A-SBC) • Authorizes media plane resources, interacts with access network via PCRF • May encrypt communication using IPSec or TLS sessions Interrogating-CSCF (I-CSCF) • Contact point in the home network • The most important functionality is to provide location service - localization of an S-CSCF based on received name or capabilities from the HSS for a particular subscriber Serving-CSCF (S-CSCF) • SIP server responsible for registration and routing within IMS network • Along with P-CSCF assigned to user for the whole duration of registration • Based on service profile received from HSS triggers ASs, maintains AS associations
  15. 15. realtimecommunication.wordpress.com IMS Entities II Telephony Application Server (TAS) • Application server is responsible for telephony services which can be applied • Provides address normalization, call diverting, call forwarding, barring, triggers playing of tones/announcements, creates billing records, etc. • TAS can retrieve subscriber profile from HSS, can communicate also with CS network elements over Camel. • TAS is what makes the VoLTE enhancements on top of the pure VoIP Multimedia Resource Function (MRF) • Media Server which can play tones and announcements (early media). • Supports conferencing services where acts as a mixer. IP-SM-GW • Application server which provides Transport Level Interworking for SMS sent over IP. • SMS service still provided by SMSC in CS network.
  16. 16. realtimecommunication.wordpress.com IMS Databases DNS • Provides routing on SIP layer • Important for load-sharing and failover scenarios • Enables zero-configuration using service discovery ENUM • Used for Mobile Number Portability (MNP) • Can be used for routing towards CS network Home Subscriber Server (HSS) • Enables routing on service layer • Contains all information about subscribers and their services • The only persistent storage, hence crucial for failover/restoration scenarios
  17. 17. realtimecommunication.wordpress.com realtimecommunication.wordpress.com LTE How to Register? UE SIP REGISTER P-CSCF S-CSCF I-CSCF A-SBC TAS IPSMGW DNS HSS ALG/AGW realtimecommunications.wordpress.com
  18. 18. realtimecommunication.wordpress.com Why it is that complex?
  19. 19. realtimecommunication.wordpress.com IMS Registration Purpose • Authenticate Subscriber • Encrypt communication channel • Create a binding between IMPU and actual location (Contact address) • Update soft-state presence information • Learn capabilities of a particular device • Create a linkage between S-CSCF and P-CSCF Key Facts • All the devices belonging to a particular subscriber are handled by one S-CSCF • One S-CSCF and one P-CSCF are assigned to UE for the whole duration of registration • Registration data is stored in HSS and can be used in Restoration Scenarios • S-CSCF can provide “Implicit Registration” to other IMPUs which are listed in subscriber profile • If Registration doesn’t work, other callflows don’t work either. Therefore the Registration is the most important scenario!
  20. 20. realtimecommunication.wordpress.com Registration Call-Flow
  21. 21. realtimecommunication.wordpress.com Registration Routing UAC P-CSCF S-CSCF I-CSCF A-SBC O-TAS DNS HSS ALG/AGW DNS NAPTR/SRV/A REGISTER sip:volte.mnc123.mcc456.3gppnetwork.org SIP/2.0 Route: <sip:10.100.100.1:5060;lr> To: <sip:+123456789012@volte.mnc123.mcc456.3gppnetwork.org> REGISTER sip:volte.mnc123.mcc456.3gppnetwork.org SIP/2.0 Path: <sip:12abc@sbc01.volte.mnc123.mcc456.3gppnetwork.org:5062;lr> Cx UAR Cx MAR, SAR SIP/2.0 200 OK Service-Route: <sip:xyz@scscf01. volte.mnc123.mcc456.3gppnetwork.org:5061;lr> realtimecommunications.wordpress.com
  22. 22. realtimecommunication.wordpress.com realtimecommunication.wordpress.com Registration Identities UAC P-CSCF S-CSCF I-CSCF A-SBC O-TAS DNS HSS ALG/AGW DNS NAPTR/SRV/A REGISTER sip:volte.mnc123.mcc456.3gppnetwork.org SIP/2.0 Route: <sip:10.100.100.1:5060;lr> To: <sip:+123456789012@volte.mnc123.mcc456.3gppnetwork.org> SIP/2.0 200 OK P-Associated-URI: <+123456789@volte.mnc123.mcc456.3gppnetwork.org > Cx UAR Cx MAR, SAR realtimecommunications.wordpress.com
  23. 23. realtimecommunication.wordpress.com SIP Session IMS RTP UASUAC SIP INVITE (SDP-O) 200 OK (SDP –A) SIP ACK realtimecommunications.wordpress.com
  24. 24. realtimecommunication.wordpress.com How to Call? RTP UASUAC SIP INVITE (SDP-O) Terminating Network Originating Network P-CSCF S-CSCF I-CSCF A-SBC O-TAS IPSMGW DNS HSS DNS HSS P-CSCF S-CSCF I-CSCF T-TAS A-SBC ALG/AGWALG/AGW realtimecommunications.wordpress.com realtimecommunication.wordpress.com
  25. 25. realtimecommunication.wordpress.com IMS Session Setup Purpose • Set up a signaling path • Exchange parameters of the multimedia session • Allocate resources in access networks and on SBCs • Trigger TAS to apply originating and terminating services Key Facts • There are always two TASes and two IMS Cores on the signaling path. Although physically they can be the same servers, logically we have to distinguish the originating and terminating network elements • Routing towards ASs is driven by IFC (part of the Subscriber Profile) • Originating S-CSCF provides routing towards terminating network • IMS Network (domain) is represented by I-CSCF. The translation (domain -> IP) is done by DNS. I-CSCF implements Location Service • Both TAS and S-CSCF can modify R-URI. Both can also trigger ENUM • In case S-CSCF can’t find the routing in IMS (e.g. because of tel uri in R-URI), the message is routed to CS (via BGCF and MGCF) • Terminating S-CSCF updates R-URI with the Contact information realtimecommunications.wordpress.com RTP UASUAC P-CSCF S-CSCF I-CSCF O/T-TAS IPSMGW DNS HSS ALG/AGW
  26. 26. realtimecommunication.wordpress.com UAC P-CSCF S-CSCF I-CSCF A-SBC O-TAS DNS HSS ALG/AGW INVITE sip:+123456789012@operator.com;user=phone SIP/2.0 Route: <sip:context456@s-cscf.ims.operator.com:5061;lr> INVITE sip:+123456789012@operator.com;user=phone SIP/2.0 Route: <sip:10.0.0.1:5060;lr> Route: <sip:context123@s-cscf.ims.operator.com:5061;lr> INVITE sip:+123456789012@operator.com;user=phone SIP/2.0 Route: <sip:context123@s-cscf.ims.operator.com:5061;lr> INVITE sip:+123456789012@operator.com;user=phone SIP/2.0 Route: <sip:tas1.ims.operator.com;orig;transport=udp;lr> Route: <sip:context456@s-cscf.ims.operator.com:5061;lr> How to Route Originating Call? realtimecommunications.wordpress.com
  27. 27. realtimecommunication.wordpress.com realtimecommunication.wordpress.com How to Route Terminating Call? UAS DNS HSS P-CSCF S-CSCF I-CSCF T-TAS A-SBC ALG/AGW INVITE sip:+123456789012@operator.com; SIP/2.0 INVITE sip:+123456789012@operator.com; SIP/2.0 Route: <sip:tas2.ims.operator.com;transport=udp;lr> Route: <sip:context789@s-cscf2.ims.operator.com:5061;lr> INVITE sip:+123456789012@operator.com; SIP/2.0 Route: <sip:context789@s-cscf2.ims.operator.com:5061;lr> INVITE sip:2233445566@10.10.1.11:5060; SIP/2.0 Route: <sip:contextABC@sbc2.ims.operator.com:5070;lr;mtcall> INVITE sip:2233445566@10.10.1.11:5060; SIP/2.0 realtimecommunications.wordpress.com
  28. 28. realtimecommunication.wordpress.com Users’ Identities UASUAC P-CSCF S-CSCF I-CSCF A-SBC O-TAS IPSMGW DNS HSS DNS HSS P-CSCF S-CSCF I-CSCF T-TAS A-SBC ALG/AGWALG/AGW INVITE sip:+123456789012@operator.com;user=phone SIP/2.0 P-Preferred-Identity: <sip:99123456789@operator.com> INVITE sip:+123456789012@operator.com;user=phone SIP/2.0 P-Asserted-Identity: <sip:99123456789@operator.com> INVITE sip:+123456789012@operator.com;user=phone SIP/2.0 P-Asserted-Identity: <sip:99123456789@operator.com> P-Served-User: sip:99123456789@operator.com; sescase=orig;regstate=reg INVITE sip:+123456789012@operator.com;user=phone SIP/2.0 P-Asserted-Identity: <sip:99123456789@operator.com> P-Served-User: sip: 123456789012@operator.com; sescase=term;regstate=reg realtimecommunications.wordpress.com
  29. 29. realtimecommunication.wordpress.com Session Setup
  30. 30. realtimecommunication.wordpress.com realtimecommunication.wordpress.com SDP realtimecommunication.wordpress.com UASUAC P-CSCF S-CSCF I-CSCF A-SBC O-TAS IPSMGW DNS HSS DNS HSS P-CSCF S-CSCF I-CSCF T-TAS A-SBC ALG/AG W ALG/AG W o=BRAND-VoLTE-UE 12345 IN IP4 10.0.0.111 s=VoLTE c=IN IP4 10.0.0.111 b=AS:44 b=RS:512 b=RR:1598 m=audio 1111 RTP/AVP 116 107 118 96 111 110 o=SBC-12345 789110 IN IP4 10.114.1.21 s=IMS c=IN IP4 10.114.1.21 b=AS:80 b=RS:1000 b=RR:2000 m=audio 12034 RTP/AVP 116 107 118 96 18 8 o=SBC-12345 789110 IN IP4 10.114.1.21 s=IMS c=IN IP4 10.114.1.21 b=AS:80 b=RS:1000 b=RR:3000 m=audio 12034 RTP/AVP 116 107 118 96 18 8 realtimecommunications.wordpress.com realtimecommunications.wordpress.com
  31. 31. realtimecommunication.wordpress.com Media Routing UASUAC P-CSCF S-CSCF I-CSCF A-SBC O-TAS IPSMGW DNS HSS DNS HSS P-CSCF S-CSCF I-CSCF T-TAS A-SBC ALG/AGWALG/AGW Offer c=IN IP4 IP1 Offer c=IN IP4 IP2 Offer c=IN IP4 IP3 Answer c=IN IP4 IP5 Answer c=IN IP4 IP6 IP1 Answer c=IN IP4 IP4 IP6 IP2 IP5 IP3 IP4 realtimecommunications.wordpress.com realtimecommunication.wordpress.com
  32. 32. realtimecommunication.wordpress.com realtimecommunication.wordpress.com What is the difference betweem VoTE and OTT calling? realtimecommunications.wordpress.com realtimecommunications.wordpress.com
  33. 33. realtimecommunication.wordpress.com realtimecommunication.wordpress.com Rx, Gx interfaces realtimecommunications.wordpress.com realtimecommunications.wordpress.com Non 3GPP Access Untrusted ePDG 3GPP Access Serving Gateway S5 S2b SWn SWx A-SBC/P-CSCF ATCF/ATGW TAS IP-SM GW Rx I-CSCF/ S-CSCF AAA Presence HSS Non 3GPP Access Trusted Internet HSS WebRTC GW OAuth IMS Gx Rx/Gx Gxc SWm SWx S6b Gxb TWAN Packet Data Gateway PCRF
  34. 34. realtimecommunication.wordpress.com realtimecommunication.wordpress.com QoS? realtimecommunications.wordpress.com realtimecommunications.wordpress.com
  35. 35. realtimecommunication.wordpress.com How to Interconnect? IPX RTP UASUAC SIP INVITE (SDP-O) Operator BOperator A P-CSCF S-CSCF I-CSCF A-SBC O-TAS IPSMGW DNS HSS DNS HSS P-CSCF S-CSCF I-CSCF T-TAS A-SBC ALG/AGWALG/AGW IBCF I-SBC IBGW/TrGW IBCF IBGW/TrGW I-SBC realtimecommunications.wordpress.com realtimecommunication.wordpress.com
  36. 36. realtimecommunication.wordpress.com realtimecommunication.wordpress.com And Roaming? realtimecommunication.wordpress.com RTP UASUAC SIP INVITE (SDP-O) Operator C Operator B P-CSCF S-CSCF I-CSCF A-SBC O-TAS IPSMGW DNS HSS DNS HSS P-CSCF S-CSCF I-CSCF T-TAS A-SBC ALG/AGWALG/AGW IBCF I-SBC IBGW/TrGW IBCF I-SBC IBGW/TrGW DNS IBGW/TrGW I-SBC IBCF IBGW/TrGW I-SBC IBCF Operator A Operator D realtimecommunication.wordpress.com
  37. 37. realtimecommunication.wordpress.com CS related flows Facts • Not everywhere we have strong LTE signal • Many devices still don’t support 4G • Operators invested a lot in 3G infrastructure • Not all the services are supported or transferred in 4G network Solution MSS CS IMS P-CSCF A-SBC AGW MGW LTE S-CSCF I-CSCF T-TAS DNS HSS PGW SGW MSC MGCF IM-MGW eMSC realtimecommunications.wordpress.com realtimecommunication.wordpress.com • IMS supports interworking with legacy network via MGCF/MGW • Routing towards particular MSC is performed by BGCF • Our aim is to centralize services in IMS – that means even those subscribers who are currently in CS network will be handled by IMS. IMS Centralization (IMC) is provided by a dedicated application server – SCC AS • SCC AS does also support Voice Call Continuity. That is Access Transfer during an active call.
  38. 38. realtimecommunication.wordpress.com realtimecommunication.wordpress.com And how does it work in practice? Access Site R4 realtimecommunication.wordpress.com Core Site A P-CSCF S-CSCF I-CSCF A-SBC O-TAS IPSMGW DNS HSS S-CSCF I-CSCF T-TAS ATGW I-SBC Access site R1 Access Site R5 Access Site R2 Access Site R3 RCS MRF BGCF MGCF MSC MGW PSXDMS Core Site B E-CSCF DSC MRF BGCF E-CSCF IPSMGW ATCF
  39. 39. realtimecommunication.wordpress.com realtimecommunication.wordpress.com IMS P-CSCF S-CSCF I-CSCF A-SBC TAS IPSMGW DNS HSS + UDM ALG/AGW MRF AP PCRF + PCF E-CSCF LRF realtimecommunications.wordpress.coms E-UTRAN NG-RAN 5GS+EPC S1-MME S1-U S11 S6a N26 N2 N3 N3 N11S5-C Rx N8 N15 N4 Ut Gm N10 SGW PGW-U + UPF DNS HSS + UDM MME AMF S5-U N7 PGW-C + SMF 5G?
  40. 40. realtimecommunication.wordpress.com Take Away • VoLTE provides telecommunication services using IMS network. • IMS provides a framework for basic services such as User Identities, Authentication, Session Routing and Session Control, Service Triggering etc. • Basic IMS entities are A-SBC, P-CSCF, CSCF, DNS, HSS, Application Servers. • For Signaling IMS mandates SIP protocol. The purpose of signaling is to locate the recipient. • In order to create a binding between actual point of presence (IP) and user identity (tel/sip uri) all users have to register first. • Multimedia data goes out of band using RTP. • The GSMA/3GPP standards define the signalling Call Flows, along with the content and syntax of the protocol headers, see: VoLTE Service Description and Implementation Guide or GSMA IMS Profile for Converged IP Communications.
  41. 41. realtimecommunication.wordpress.com Where can I find more? VoLTE and CS Network IMS from 10.000 feet VoLTE in IMS SIP Illustrated: SIP by sip VoLTE flows – close encounters
  42. 42. realtimecommunication.wordpress.com Thank you!

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