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Sebastian Schumann

April 04, 2012


  1. IP Multimedia Subsystem (IMS) Components, Protocols, Signaling Evolution Sebastian Schumann

    Slovak Telekom, a.s. – Core and Control Department 4. April 2012. Žilina, Slovakia
  2. Topics §  IP Multimedia Subsystem (IMS) §  Components §  Protocols

    §  Signaling Evolution §  “Fixed agenda”: Understand IMS, its concept, protocols, and basic signaling §  Besides that: §  Ask questions (how is it done in real-world, how did Slovak Telekom do that) §  Interrupt (I don’t understand, can you provide samples, can we skip that) §  Contribute (I’ve heard/read that…, I’m interested in…) §  Discuss…
  3. The technology trends Services Data/IP Networks Mobile Networks PSTN/ISDN CATV

    Access Transport & Switching Networks Wireless Access Wireline Access IP Backbone Existing and newly emerging services Service & Network Control (QoS, Security, IP Mobility) Too costly, per-service network architecture Single/simple/cost-effective network infrastructure for e xisting & new services
  4. The long-term goal: from the current to the future all

    IP architecture NGN SERVICES and APPLICATIONS BSS TELEPHONE PC TV VIDEO ACCESS NGN TRANSPORT/ AGGREGATION MULTIMEDIA IP SERVICES INTERNET PSTN/ISDN FR/ATM LEASED LINE NGN DATA LL IP/MPLS IP SERVICES CONTROL BSS OSS VOICE IT support NGN CONTROL OSS IT support VOICE DATA Home network area TRANSPORT CPE ACCESS CPE CPE CPE CPE BSS OSS BSS OSS BSS OSS BSS OSS From circuit-switched to packet- switched (IP-based) core network From copper access infrastructure to optical as preferred access infrastructure with copper access where effective From multiple production platforms for services to shared production elements for all fix as well as nomadic and mobile services From architecture centered around voice services to network supporting broad set of applications with common service capabilities for fast product development MOBILE
  5. IMS based NGN SU TU SU IP softswitch CC IP/No7

    TU Softswitch based NGN CC AGW Broadband access No7 CS/PS TGW PS/CS Enablers IMS NASS RACF User profile App. Services Legacy PSTN SGW IP Voice only Voice oriented Multimedia enabled Switching and control within exchange Separation call control and media Distribution and decomposition of functions Telephony, IN services Enhanced Voice services Multimedia services, Shared enablers Phone world, Internet world (POTS/POIS) Separate networks, interworking possible Multiple access, Common IP core, Mobile Domain IN Telco designer view: network evolution with NGN
  6. IMS : End user perspective Communication changes. Voice/video SMS/MMS Instant

    messaging Mail Voice/Video/IM Conferencing/App sharing/MM chat/ Others content content Shared content Today services Integrated „all-IP“ services
  7. Voice TV Internet True Triple Play package Existing NGN +

    integrated (non-IMS) IPTV Integration of current IPTV into current NGN ... And complete picture at the end ... 7 Mikoczy, IMS Global Congress 2010 IMS +IMS based IPTV Context Communication Content Leveraging of IMS investment Blended services Voice TV Web 2.0 Over the Top package No NGN, No IMS, only servers with apps Leveraging of existing “Internet” infrastructure Internet
  8. IMS – subsystem within the NGN service stratum

  9. IMS: simplified concept Access & Transport Plane Core Network Session

    Control Plane Service Architecture Applications/Services Plane HSS CSCF Access Network Other Networks Web Portal Application Servers Session Control Centralized Databases Media Control & Gateways Media Server
  10. IMS service architecture IMS platform (Common Call Control & support

    functions) VoIP Carrier grade PSTN Legacy Integration with reduced feature set Messaging Presence Address book Sharing Personal & Social Networking Voice Messaging Presence Address book Collaboration Business Communication and Collaboration MS Exchange interworking Voice IPTV/mTV Manage Store Control NG-IPTV & Connected Home Media NG-IN SDP / NG-IN SDP/Service Composition Personnel Portal
  11. IMS architecture - service convergence Devices Access Transport Session Control

    Applications Managed Core IP Network Media GW Signaling GW sd Service Broker Media Server POTS phone GGSN PDSN MSC Cellular Access Point WiFi/802.11x Local Loop VoIP handset Ckt-PBX IP-PBX IP-phones ISDN-phones IP-phones DSL/Cable IP LAN Wireless Router Base Station 5ESS (IP-Centrex) Office/Hotel Hotspot Home On the Road Home/Office Dual-mode WiFi phone Soft-phones PSTN SS7 SIP Clients Ckt handset Multiple user interfaces with common look and feel Other Clients Presence Servers Telephony Servers Unified Messaging IMS Core IPTV PTT, IM, Other Active Phonebook Subscribe data
  12. IMS entities (Wiley, The IMS Concepts and Services) §  Session

    management and routing family (CSCFs) §  Databases (HSS, SLF) §  Services (e.g. AS) §  Support functions (PDF, SEG, THIG) §  Charging §  Interworking functions (BGCF, MGCF, IMS-MGW, SGW)
  13. Session management and routing family §  Proxy-CSCF – User contact

    point with the IM CN §  SIP compression, IPSec association, PDF interaction §  Interrogating-CSCF – Subscriber contact point §  Next-hop lookup from HSS, S-CSCF assignment and routing, THIG functionality §  Serving-CSCF – Service profile internal procedures §  Handling registration, challenging UE, routing decisions §  Responsible for Registration and Session Establishment, Charging Data Generation, Media content check
  14. Databases §  HSS §  data storage for all subscriber and

    service-related data §  SLF §  find HSS address for multiple HSS environment
  15. Interworking & Support functions §  Interworking between different CN § 

    for SIP message forward to another network, the S-CSCF sends a SIP session request to the BGCF, then the BGCF forwards the session to another BGCF in a selected network §  PDF §  Session and media related decisions §  THIG §  hide the configuration, capacity and topology of the network from outside an operator’s network
  16. Protocols (extract) §  SIP §  signaling protocol §  SDP § 

    embedded in SIP, describes the multimedia session, media type negotiation §  RTP §  end-to-end media delivery (audio, video) §  DNS (CN entry) §  Diameter (AAA) §  IPSec (secure communication) §  MEGACO (media gateway control)
  17. Resource and Admission Control Subsystem (RACS) RACS provides applications a

    mechanism to request and reserve the resources from access network. It provides the following main functionalities: §  Session Admission Control (various types of admission control ) §  Resource reservation (permits applications to request bearer resources in the access network) §  Service based Local Policy Control (authorizes QoS resources and defines the policies)
  18. Network Attachment Subsystem (NASS): NASS provides registration and initialization of

    CPE for access to IMS services. It provides the following main functionalities: §  IP addresses and configuration parameters §  User authentication §  Authorization of network access, based on user profile §  Access network configuration, based on user profile. §  Location management.
  19. Visited Network HSS DNS ENUM I-CSCF S-CSCF Home Network AS

    AS AS Home Subscriber Server •  Centralized DB •  HLR successor •  User profile •  Filter criteria (sent to S-CSCF) •  Which applications •  Which conditions Application Servers •  Push-to-talk •  Instant messaging •  Telephony AS •  3rd party or IMS Vendor Domain Name Server P-CSCF MGCF MGW PSTN H.248 ISUP BGCF SIP SS7 SIP SIP SIP SIP SIP SIP SIP Diameter RTP TDM Call Session Control Function •  SIP registration •  SIP session setup MS MS MRFC Media Gateway Control Function •  Interfaces to PSTN/PLMN by •  Converting SIP <-> ISUP •  Interworking RTP to circuit •  H.248 control of MGW Breakout Gateway Control Function •  Selects network (MGCF or other BGCF) in which PSTN/ PLMN breakout is to occur Media Resource Function Controller •  Pooling of Media servers (e.g. conference) Proxy CSCF •  1st contact point for UA •  QoS •  Routes to S-CSCF Interrogating CSCF •  Entry point for incoming calls •  Determines S-CSCF for Subscribers •  Hides network topology Serving CSCF •  Registrar •  Session control •  Application Interface P-CSCF SIP SIP UA/UE UA/UE
  20. Recapitulation §  IMS is an open architecture for mobile and

    fixed services §  The core and its services are independent from the access §  Layered architecture §  Transport, session control, applications §  Transparency through standard interfaces §  Session Control Layer §  End point registration §  Session establishment §  Application Layer §  Service Logic
  21. Recapitulation ctd. §  Service Control Layer §  SIP: P/I/S-CSCF § 

    DIAMETER: HSS §  Application Layer §  SIP/DIAMETER interface towards service control layer §  SIP/XCAP interface (based on HTTP) towards UE §  Call related application logic §  IMS service (e.g., Presence, PoC) §  Service Creation Environment
  22. IMS Core §  CSCF components separate logical functionality §  P-CSCF

    eq. SIP Proxy. It acts as access point for UE towards the IMS core. §  I-CSCF placed on the borders of two IMS domains. Entry point for served home users from visited networks. §  S-CSCF eq. SIP Registrar. It also acts as an anchor point for IMS service control (ISC) and service invocation (based on iFCs). §  HSS contains all subscriber and service related data
  23. User Identities §  User identities §  Private User Identity (user@realm)

    §  Authentication and Subscription identification §  Not used for routing §  Public User Identity (sip:user@domain.tld or tel:+1234567890) §  Contact to be reached by others §  SIP URI or tel URI §  Implicit set of public user identities for grouping registration §  Services and other network entities can be addressed using a SIP URI §  User identities are part of the user profile
  24. Relations between Identities IMS Subscriber Private UID 2 Private UID

    1 Public UID 1 Public UID 3 Public UID 2 Public UID n . . . } Implicit Set
  25. IMS Registration §  Required before a user can access services

    or perform calls §  Precondition: UE has IP address and knows IMS entry point §  All CSCF are used §  P-CSCF (home/visited): Entry point, determines I-CSCF §  I-CSCF (home): Determines S-CSCF §  S-CSCF (home): Authenticates the subscriber, registers IMS subscriber, interacts with service layer §  User assigned to one S-CSCF after successful registration §  Knows user profile until de-registration
  26. Registration (simplified) P-CSCF DNS UE

  27. Domain Name Service §  Link IP addresses with domain names

    §  Support in locating SIP servers §  NAPTR lookup §  SRV lookup §  A/AAAA lookup §  NAPTR resolves the preferred protocol and the DNS string to locate the service § 7200 IN NAPTR 10 50 "s“ "SIP+D2U“ §  SRV look-up for a NAPTR given address indicates the domain and port the service listens on § 7200 IN SRV 0 0 5060 §  A/AAAA to find the IP address of the domain name § 7200 IN A
  28. Registration (simplified) P-CSCF I-CSCF S-CSCF DNS UE HSS

  29. Registration procedure 1

  30. Registration procedure 2

  31. Registration procedure 3

  32. Registration procedure 4

  33. Important SIP “additions” §  P-Access-Network-Info includes port location/cell §  From/To

    eq. IMPU §  Path added by P-CSCF, inform S-CSCF about routing destination for terminating requests, values collected during registration using INVITE §  Populated to Route headers in in-registration terminating requests on S-CSCF §  Authorization contains IMPI and other values §  200 OK Service-Route to populate S-CSCF address to P-CSCF for originating requests §  Populated to Route headers in in-registration originating requests on UE
  34. Important SIP “additions” – ctd. §  P-Associated-URI informs client about

    registered IMPUs in 200 OK answer to REGISTER request §  P-Preferred-Identity (UE – P-CSCF indication), P-Asserted- Identity (P-CSCF – S-CSCF, removes P-Preferred-Identity) to choose a registered IMPU for session establishment §  Event: reg after registration to inform UE about events on S-CSCF (e.g., HSS- initiated deregistration)
  35. IMS Session Establishment §  After registration, subscriber is reachable through

    public user identity for communication §  IMS subscriber can access services now or perform calls §  P-CSCF (home or local) §  Proxy, contacts assigned S-CSCF for the calling subscriber §  S-CSCF (home) §  Service control and logic §  Contacts application or other party §  I-CSCF §  Entry point for communication from other domain
  36. Session establishment outgoing (simplified) P-CSCF I-CSCF S-CSCF DNS UE

  37. Session establishment incoming (simplified) P-CSCF I-CSCF S-CSCF HSS UE To

    simplify matters, DNS is omitted in these slides.
  38. Control Bearer Called Party Visited Network Called Party Home Network

    Calling Party Home Network Calling Party Visited Network UE P-CSCF HSS S-CSCF SIP SIP Diameter UE P-CSCF AS HSS S-CSCF SIP AS SIP Diameter I-CSCF Diameter DNS Backbone Packet Network RAN Backbone Packet Network RAN Initiate SIP Invitation 1 1 Retrieve Subscriber Profile (if needed) 2 2 3 3 Apply Service Logic Retrieve Address of CLD Party Home Network and Forward INVITE. 4 4 Identify Registrar of CLD Party and Forward INVITE. 5 5 Retrieve Subscriber Profile (if needed) 6 6 7 7 Apply Service Logic Forward INVITE to CLD Party 8 8 Ringing / Alerting 10 10 IMS Subscriber to IMS Subscriber high level call flow SDP Negotiation / Resource Reservation Control 9 9 RTP Stream Answer / Connect 11 11 NGN, SSW, and IMS architecture
  39. Application layer interaction §  User profile contains also service profile

    §  Service Profile §  Public Identification (assigned subscribers) §  Initial Filter Criteria (triggering AS interaction) §  Initial Filter Criteria (iFC) §  Trigger points with service point triggers (conditions when to interact) §  Application server (SIP URI for interaction)
  40. Service Profile

  41. Service Profile ctd.

  42. Triggering

  43. Triggering ctd.

  44. Filtering §  Only initial SIP requests §  Initial filter criteria

    (iFC) retrieved from HSS during registration §  Subsequent filter criteria (sFC) provided by application server (beyond 3GPP R8) §  Allow dynamic definition of trigger points during application runtime
  45. Application Routing §  I/S-CSCF are interaction points with the service

    layer §  I-CSCF for public service identities (PSI) §  S-CSCF for services (of served users) §  Applications have interface towards HSS §  User profile information §  Location information, service information §  Complexity of security, authorization, access interaction etc. all handled by the core
  46. Application Routing ctd. §  Application server (AS) can have different

    functions §  Terminating AS (e.g., acting as user agent) §  Originating AS (e.g., wake up service, click to dial) §  SIP Proxy server (e.g., for SIP header manipulation) §  Back-to-back user agent (e.g., for deeper modifications in SIP dialog as supplementary service enabler)
  47. Quest

  48. Quest §  Alice from IMS wants to subscribe presence

    of Bob from IMS §  Alice is currently in Chicago, Bob is at home §  Is it possible? §  Where does Bob have to publish? §  Where does Alice have to subscribe? §  Which components are involved? §  How is the call flow?
  49. Example: Presence call flow §  Service profile §  assigned to

    users that want to use presence §  IFC §  AS: Presence Server §  TP: CNF (&) §  Method and §  PUBLISH or §  SUBSCRIBE §  Event §  Header: Event §  Content: .*presence.* P-CSCF Presence Server S-CSCF SUBSCRIBE 200 OK 200 OK NOTIFY SUBSCRIBE 200 OK 200 OK NOTIFY SUBSCRIBE 200 OK 200 OK NOTIFY UE
  50. eXtensible Configuration Access Protocol §  XCAP allows clients to read,

    write and modify data stored in XML format on server §  Hard state presence information §  Watcher authorization §  Resource Lists §  XML document sub-trees and element attributes are mapped into HTTP URIs à direct access via XPath §  Various selections (e.g., one or more elements, children, attributes, content)
  51. §  Client/Server architecture like HTTP §  Application Usage for certain

    application needs §  HTTP primitives ‘Get’, ‘Put’ and ‘Delete’ are used §  Body contains XML data to be added/modified eXtensible Configuration Access Protocol
  52. Message flow §  Interface exposed by XML Document Management Server

    §  XDMS is located on application layer §  Direct communication between UE and XDMS §  Use cases §  Store resource list §  Authorize buddies XDMS UE XCAP
  53. Questions?

  54. Thank you. Sebastian Schumann Senior Designer Core and Control Slovak

    Telekom, a.s. +421 903 419 345 Thanks to Eugen Mikoczy, Ondrej Labaj, Tomas Kovacik, Juraj Matejka, and Stephan Massner for contributing to the slides.