Lecture notes about IMS services. This lecture has been presented in 2013 in Bratislava, Slovakia at the Slovak University of Technology.

1. IMS Services
Sebastian Schumann
Slovak Telekom
17. April 2013
Bratislava, Slovakia

2. About me
¡ Studied 2003 – 2007 telecommunications
and computer sciences in Leipzig,
Germany
¡ In Slovakia since 2006
¡ Working for Slovak Telekom since 2007
¡ Post-grad studies at Slovak University of
Technology since 2007
¡ Worked and implemented SIP and IMS
software as well as carrier platforms

3. Outline
¡ IMS and its Services
¡ Service Triggering
¡ Sample Services
¡ Other Aspects

4. Note!
¡ These slides only summarize the lecture. Take notes.
¡ Fixed agenda: Understand IMS services
¡ 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…

5. IMS and its Services

6. IMS: End User Perspective
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. Technology Trends
Services
Data/IPNetworks
MobileNetworks
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 archit
ecture
Single/simple/cost-effective network infr
astructure for existing & new services

8. IMS: Simplified Concept
Access & Transport
Plane
Core Network
Session
Control Plane
Service Architecture
Applications/Services
Plane
HSSCSCF
Access
Network
Other
Networks
Web Portal
Application
Servers
Session
Control
Centralized
Databases
Media
Control &
Gateways
Media
Server

9. 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, authentication
¡ Session establishment, routing, interconnect
¡ Application Layer
¡ Service Logic

10. Recapitulation ctd.
¡ Service Control Layer
¡ SIP: P/I/S-CSCF, (BGCF, I-BCF, MRFC, AS)
¡ DIAMETER: HSS, (RACS/NASS, PCRF)
¡ 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
¡ Northbound integration through service APIs

11. 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)

12. Session management & routing
¡ 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

13. Databases
¡ HSS
¡ Data storage for all subscriber and service-related data
¡ SLF
¡ Find HSS address for multiple HSS environment

14. Application Server
¡ SIP Application Server
¡ Stand-alone AS
¡ Northbound integration using various protocols possible,
e.g., HTTP REST, Parlay X
¡ Open Service Access (OSA) gateway
¡ Connect northbound to OSA Parlay based AS
¡ IM Service Switching Function (SSF)
¡ Connect northbound the AS layer to legacy services
using IN protocols (e.g. INAP, CAMEL)

15. Protocols (extract)
¡ Signaling
¡ SIP (signaling protocol)
¡ SDP (embedded in SIP, describes the session, negotiation)
¡ Media
¡ RTP (end-to-end media delivery (audio, video))
¡ MSRP (messaging, file transfer)
¡ DNS
¡ Diameter (AAA)
¡ IPSec (secure communication)
¡ MEGACO (media gateway control)

16. Service Triggering

17. Application layer interaction
¡ User profile contains 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)

18. Service Profile

19. Service Profile ctd.

20. Triggering

21. Triggering ctd.

22. 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)
¡ Allows dynamic definition of trigger points during
application runtime

23. Application Routing
¡ I/S-CSCF are interaction points with the service layer
¡ I-CSCF for public service identities (PSI) à explicit access
¡ S-CSCF for services (of served users) à implicit access
¡ Applications have interface towards HSS
¡ User profile information
¡ Location information, service information
¡ Complexity of security, authorization, access
interaction etc. all handled by the core

24. 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)

25. Sample Services

26. Services.
http://www.flickr.com/photos/soutra/
430186540/
http://gsmworld.com/images/rcs/img_gallery/eyeP-eComRCS-
Windows.3.png
http://www.flickr.com/photos/jre/3263704032/

27. IMS Services
¡ Service that are often referred to
¡ IP Messaging (page-mode, session-mode)
¡ Push-to-talk over Cellular (PoC)
¡ Conferencing
¡ Presence
¡ Commercially deployed
¡ Voice (MMTel, VoLTE)
¡ RCS-e/RCS 5 (Joyn)
¡ Voice Call Continuity (VCC)

28. Rich Communication Suite
Definition
¡ "Standard" definition
¡ End-to-end approach to define a clear set of available communication services
that are interoperable
¡ Stakeholders in RCS are all key players in the telecom market (operators, vendors)
¡ Develop concrete value propositions for different stakeholders in the ecosystem
¡ Initial focus was on enriched mobile communication services, now RCS is extending
the same services to the fixed environment
¡ Collaborative effort to facilitate the introduction of commercial IMS based rich
communication services over mobile and fixed networks
¡ Several releases available (Rel. 1-5, RCS-e aka Joyn)
¡ Focus on residential user segment
¡ Not “defined”, but PBX integration/support not defined
¡ Focus NOT on end device applications (iPhone, Android)

29. RCS Services + use cases
¡ EAB enabled services
¡ Share presence + manage presence rules
¡ Capability exchange
¡ Video call, video sharing (inside/outside of call)
¡ Image sharing, whiteboard-feature
¡ File transfer
¡ Messaging
¡ multi-party, multi-device chat
¡ x-domain
¡ Provider interworking

30. Rich Communication Suite
Releases
¡ Suite of rich communication services that can be launched from a capability
enhanced address book (EAB) à EAB enriched call and enhanced messaging
¡ Rel. 1
¡ EAB with capability exchange enables content sharing during a call and enhanced
messaging (conversational view, chat). Backup/restore in network
¡ Mobile users only, direct relation with mobile operator
¡ Rel. 2
¡ Introduction of broadband access, multiple clients, mobile phone required
¡ Network address book (NAB) allows synchronization (sharing btw. devices possible)
¡ Rel. 3
¡ Enhanced services (presence states, messaging, network value added services)
¡ Content sharing outside voice call
¡ Single broadband access possible (w/o mobile phone)
¡ Rel. 4
¡ LTE and fixed access enhancements, service enhancements
¡ Rel. 5
¡ IP voice/video call, location sharing, service improvements

31. RCS-e/Joyn
¡ RCS-e (“e” for enhanced) is a simple and interoperable
evolution to voice and text, which enables customers to
send instant messages, video chat and exchange files in
real time. All functions are built into the address book of
mobile devices and based on the IMS.
¡ Enhanced Rel. 2 for faster time-to-market
¡ Powered by the five leading European mobile operators,
incl. Deutsche Telekom
¡ Focused communication services (core services only)
¡ IM/Chat, file transfer, image/video share
¡ Social presence/profile information not mandatory
¡ Standard: “RCS-e provides a simple interoperable extension
to voice and text today“

32. RCS-e 1.2.2
¡ Dynamic capability discovery
(SIP OPTIONS)
¡ Text messaging (as in RCS R2)
¡ One-to-one chat
¡ Group chat
¡ Add. Features to Rel. 2
¡ Store &forward for chat
¡ Typing/delivery notify
¡ File Transfer (as in RCS R2)
¡ Image/Video Share during CS
phone call (as in RCS R2)

33. Real sample architecture
Mw I-SBCA-SBC
RCS-e AS
UE
P-CSCF S-CSCF
BGCF
IBCF
I-BGF
RCS-e AS
(Service)
RCS-e AS
(Config)
ENUM
A-BGF
IPX Other MNO
Mw
Mi
ISC
Gm
Mb
Mb
Ici
Izi
Mx
SIP
DNS
Media
Mw
Ma
I-CSCF
HSS
Cx
Cx
Diameter
HTTP
Sh
Mb
Mx

34. RCS-e flow diagrams
Figure 1: RCS-e alternative configuration: Initial request

35. RCS-e flow diagrams ctd.
Figure 1: RCS-e capability discovery

36. RCS-e flow diagrams ctd.
RCS-e service Tag
IM/Chat +g.3gpp.iari-ref="urn%3Aurn-7%3A3gpp-application.ims.iari.rcse.im"
File transfer +g.3gpp.iari-ref="urn%3Aurn-7%3A3gpp-application.ims.iari.rcse.ft"
Image share +g.3gpp.iari-ref="urn%3Aurn-7%3A3gpp-application.ims.iari.gsma-is"
Video share +g.3gpp.cs-voice
Social presence
information
+g.3gpp.iari-ref="urn%3Aurn-7%3A3gpp-application.ims.iari.rcse.sp"
Capability
discovery via
presence
+g.3gpp.iari-ref="urn%3Aurn-7%3A3gpp-application.ims.iari.rcse.dp"
Table 1: Complete SIP OPTIONS tag proposal for RCS-e

37. RCS-e flow diagrams ctd.
¡ SIP INVITE with ICSI/SDP for session establishment
¡ SIP MESSAGE for notifications
¡ MSRP for one-to-one/group chat and file transfer
¡ RTP for video share
¡ AS functions (samples)
¡ Group chat
¡ Aggregation
¡ Accounting, policy

38. Protocols

39. SIP
¡ SIP in the IMS has been already covered in the past
¡ Relevant main SIP headers for the service interaction
¡ P-Asserted-Identity (inserted by P-CSCF) trusted header
indicating registered user’s IMPU
¡ Contact header contains ICSI
¡ SDP used for session description & codec negotiation
¡ Sample RCS-e
¡ OPTIONS discovery
¡ MESSAGE page-mode messaging & notifications
¡ INVITE session-mode messaging & notifications,
file transfer, video share

40. Message Session Relay Protocol
(MSRP)
¡ Message content within a SIP session (similar to RTP)
¡ Rendezvous mechanism mandatory (e.g. SDP)
¡ MSRP URI’s
¡ Accepted content
¡ SDP
c=IN IP4 atlanta.example.com
m=message 7654 TCP/MSRP *
a=accept-types:text/plain
a=path:msrp://atlanta.example.com:7654/jshA7weztas;tcp

41. MSRP ctd.
¡ MSRP exchange
MSRP a786hjs2 SEND
To-Path: msrp://biloxi.example.com:12763/kjhd37s2s20w2a;tcp
From-Path: msrp://atlanta.example.com:7654/jshA7weztas;tcp
Message-ID: 87652491
Byte-Range: 1-25/25
Content-Type: text/plain
Hey Bob, are you there?
-------a786hjs2$
MSRP a786hjs2 200 OK
To-Path: msrp://atlanta.example.com:7654/jshA7weztas;tcp
From-Path: msrp://biloxi.example.com:12763/kjhd37s2s20w2a;tcp
-------a786hjs2$

42. MSRP ctd.
¡ Key concepts
¡ Framing/message chunking (+ vs $)
¡ MSRP Addressing (URIs for send/recv, lists for relays)
¡ Scheme: msrp/msrps for TLS. TCP transport.
¡ Methods (e.g. SEND) and response codes (e.g. 200 OK)
¡ MSRP relays in the path
¡ More in RFC 4975 (protocol), RFC 4976 (relays)

43. 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
¡ ngnlab.eu. 7200 IN NAPTR 10 50 "s“ "SIP+D2T“
_sip._udp.ngnlab.eu.
¡ SRV look-up for a NAPTR given address indicates the domain
and port the service listens on
¡ _sip._udp.ngnlab.eu. 7200 IN SRV 0 0 5060 icscf.ngnlab.eu.
¡ A/AAAA to find the IP address of the domain name
¡ icscf.ngnlab.eu. 7200 IN A 147.175.103.213

44. Messaging/Presence
¡ SIP MESSAGE
¡ SIP SIMPLE
¡ SIP SUBSCRIBE/NOTIFY
¡ SIP PUBLISH
¡ Many extensions
¡ Standard bodies: IETF, OMA

45. 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
200OK
200OK
NOTIFY
UE

46. eXtensible Configuration
Access Protocol (XCAP)
¡ 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)

47. ¡ 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
XCAP ctd.

48. 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

49. Other Aspects

50. Northbound Interface – API
¡ Not standardized/partially standardized integration
point with other applications/enablers or with non-
IMS clients
¡ Various protocols possible
¡ XCAP
¡ Parlay X
¡ HTTP REST
¡ Standardization approaches exist, e.g., GSMA
OneAPI, RCS-e API

51. Service Orchestration
¡ Querying multiple services
¡ Services querying other services
¡ Compilation of services by using various enablers
¡ Standardization approaches (e.g. SCIM)
¡ Approach
¡ SIP AS towards the IMS using ISC
¡ Connecting to multiple AS via ISC, optionally also to
other AS w/ different protocols

52. Q/A
Your Questions!

53. Is anything still unanswered?
¡ How do IMS services work?
¡ Why is the IMS needed for some communications services? Is it?
¡ But I have heard of service X, why don’t they use the IMS?
¡ Will we build all future services on top of IMS?
¡ Are IMS services only those inherited from the Telco past?
¡ Will Telco’s deploy multiple IMS? IMS in the cloud? Share an IMS?
¡ Will IMS bring in new revenues? Is it cheaper to deploy services
on the IMS compared to stand-alone deployments?

54. Summary
¡ Quick IMS recapitulation
¡ Focus on service layer
¡ Interaction with the IMS
¡ Sample services, incl. protocols and principles
¡ Other means of integrating IMS services
/with IMS services
¡ Hopefully covered all open questions (last chance J)

55. Sebastian Schumann
mail@s-schumann.com
@s_schumann
sschumann
Thank
you!

56. Download
¡ This lecture is available at http://bit.ly/fiit-ims-services

57. References
1. Standards
2. RCS-e related pictures from GSMA RCS-e 1.2.2 Spec
If you feel content where you hold the copyright is displayed within these
slides and you do not like it, miss a link/reference, or want me to remove
it altogether please let me know.
Thanks to Eugen Mikoczy and Stephan Massner for contributing
to the slides.