This document provides an overview of the IP Multimedia Subsystem (IMS) architecture. IMS is defined by 3GPP, 3GPP2 and TISPAN as a standardized architecture that provides integrated Internet-based multimedia and multi-session applications to mobile users while guaranteeing quality of service. The key components of IMS include the Call Session Control Function (CSCF), Home Subscriber Server (HSS), Application Servers, and Media Resource Function (MRF). IMS enables a common infrastructure for multimedia services across different access networks and devices.

1. 1
IMS – IP Multimedia
Subsystem
Architecture, Protocols and Services
Waldir R Pires Jr
Siemens Manaus
2005.06.21

2. 2
Contents
Introduction
IMS Overview
IMS Applications/Services
Conclusion
References

3. 3
Introduction
 Shift towards an all IP system
 Reasons
 Reduction of costs, creation of new revenue
 Increased competition and innovation demands
 Internet coverage
 Merge of fixed and mobile networks
 Services today are obsolete
 Multimedia services not yet available to the mobile networks – a
natural evolution
 Common Infrastructure for services

4. 4
Stove pipe
architecture horizontal
architecture

5. 5
Why IMS?
 Involved parties
 Operators, Users, Content
Providers, Legal Authorities,
…
 All-IP network
 Interactive Multimedia
 QoS guarantee
 Access transparency,
seamless handover
 AAA, Billing, Charging
 Service Aggregation
 3rd party Application Servers
 Services: No preferential
treatment of voice
 Voice (VoIP) is just one
service, positioned at same
level like any other services
 Communication security in IP
networks
 Signaling and Media
 Access and Core
 Regulatory requirements
(lawful interception, …)

6. 6
Contents
Introduction
IMS Overview
IMS Applications/Services
Conclusion
References

7. 7
IMS Definition
 The IMS is a standards-based architecture that provides:
 integrated Internet-based, multimedia, multi-session applications
to mobile users
 at the same time guaranteeing QoS across different access
network technologies and
 allowing operators to charge transparently through any medium
used.
 Defined by 3GPP, 3GPP2 and TISPAN
 OMA (service interoperability), IETF (Internet standards)

8. 8
IMS Consequences
 For users
 Person2person and
person2content
communications in voice,
text, pictures and video
 Availability anywhere
anytime through wireless
and fixed networks
 Access from different
technologies (GSM,
cdma2000, WLAN, etc)
 For operators
Definition of a
horizontal architecture
 reusability of common
functions in multiple
applications (From
stovepipe to horizontal
services)
 Interoperability and
roaming
 Bearer control, charging
and security
Key enabler for fixed-
mobile convergence
 Secure migration to an
all IP architecture

9. 9
IMS Architecture
Legacy Service Control Points
Caller ID, Local number portability and 800
services.
Application Servers
IMS Applications
OSA – GW (Open Service Access Gateway)
inter-working between SIP and APIs for communication
services
It allows applications to access presence and call state
information, application registration and resource management
in the network
Non-Telephony Services
outside the telephony call model, such as
Instant Messaging, Push To Talk and
presence enabled
TAS (Telephony Application Server)
back-to-back SIP user agent that maintains
the call state
provides the basic call processing services
including digit analysis, routing, call setup, call
waiting, call forwarding, conferencing, etc.
IM-SSF (IP Multimedia Services Switching
Function)
Provides the interworking of the SIP message
to the corresponding CAMEL, ANSI-41, INAP
or (TCAP) messages, services such as calling
name, 800 services, etc.
HSS (Home Subscriber Services)
A database that maintains the unique service profile
for each end user. This includes an end user’s current
registration information (i.e., IP address), roaming
information, telephony services (i.e., call forwarding
information), instant messaging service information
(i.e., buddies list), voice mail box options (i.e.,
greetings), etc.
CSCF (Call Session Control Function)
It provides the registration of the endpoints and
routing of the SIP signaling messages to the
appropriate application server. The CSCF interworks
with the transport and endpoint layer to guarantee
QoS across all services. In some IMS proposals, the
CSCF is divided into Serving, Proxy and Interrogating
CSCFs.
MRCF – Media Resource Control Function
It interworks the SIP signaling with the signaling used by the
media servers. It also manages the distribution of sessions
across multiple media servers. It is considered to be the central
unit for media processing (transcoding, conferencing, TTS,
etc.),
MGCF – Media Gateway Control Function
It interworks the SIP signaling with the signaling used by the
media gateway (i.e., H.248).
Media Server
Responsible for the processing and generation of multimedia
data, such as playing of announcements (audio/video),
multimedia conferencing (e.g. mixing of audio), text to speech
conversation (TTS) and speech recognition and realtime
transcoding of multimedia data (i.e. conversion between
different formats).
Media Gateway
It terminates voice data from a switched network (PSTN) and
media (RTP) streams from packet switched network (Internet).
It also acts as relays for multimedia streams i.o. end-2-end
connections. Other functions include transcoding, echo
cancellation and packet loss handling

10. 10
IMS Architecture
CSCF - Call Session Control Function
Serving CSCF
SIP server that maintains
session state for IMS
services
Proxy CSCF
the first point of contact for the user
entity and forward SIP messages to
the user’s home S-CSCF
Interrogating CSCF
the contact point for a network when
other networks need to be
contacted

11. 11
IMS Protocols
 Session Control
SIP
 Media
RTP and RTCP
 Security
Diameter (AAA)
GSM, GPRS and 3G
standards
IP
UDP TCP
SIP
RSVP RTP RTCP SAP SDP
Multimedia Applications
Audio Video Dados
Used for establishing,
modifying and terminating
multimedia sessions. It
relies on other protocols for
transport and QoS
reservation.
Real Time Protocol:
transport of real time media
using UDP
Real Time Control
Protocol: control of the real
time media being
transported, QoS statistics,
synchronization
(audio/video) of packets.

12. 12
Contents
Introduction
IMS Overview
IMS Applications/Services
Conclusion
References

13. 13
IMS Services/Application Architecture
Architecture Proposals
On the operator’s network
Horizontal
architecture
On the mobile
device
Client-server
Three-layer
From the mobile terminal down to the content
providers/application developers
Mobile Terminal
Radio Access Network
IMS
SIP Applications
3rd Party
Content Providers
Application Developers
Packet Switch Domain Circuit Switch Domain
Internet PSTN
Mobile Terminal
Radio Access Network
IMS
SIP Applications
3rd Party
Content Providers
Application Developers
Packet Switch Domain Circuit Switch Domain
Internet PSTN

14. 14
IMS Services/Application Architecture

15. 15
IMS Horizontal Architecture

16. 16
IMS Services/Application Architecture
 On the mobile device
 Client Server Architecture
 Mobile terminal and the A/S
 Three-layer application architecture
 User interface, application logic and
the protocol layer
 Provide maximum portability and
flexibility between different vendors
= Componentry
 Performance problems for multimedia
applications in Java
 Efficient Java/Native connectivity for
multimedia applications
 Various application domains
Access Independence
Push-to-talk Over Cellular
Presence
Unified Messaging
Interactive Voice
Response
Enhanced Voice Mail
Instant Messaging
Web/Audio/Video
Conferencing
Video Telephony
Multi-user Gaming

17. 17
Contents
Introduction
IMS Overview
IMS Applications/Services
Conclusion
References

18. 18
IMS Summary
 IMS provides
 AAA (authentication, authorization and accounting) of both
mobile and fixed SIP clients
 Control over sessions
 person-to-person, person-to-group and content-to-person
multimedia communication
 Charging functionality flexibility for the operator
 apply time-based, event-based or volume-based charging to
subscribers
 QoS for the IP services being used in the communications
session
 Functionality to connect roaming users to the home domain for
IP multimedia services
 Access network domain security functions

19. 19
IMS Summary
Enabler to a
true 3G IP
convergence
Horizontal
architecture to services
and applications
Common services and
roaming discovery
Interoperability, charging,
roaming control for the
operator
Combination of voice, video
and data
Access convergence GPRS,
cdma2000, WLAN, fixed
network, etc.

20. 20
Conclusion
Issues/Challenges
Costs of conversion to IMS
Competition and market divergence
Performance
Security

21. 21
Contents
Introduction
IMS Overview
IMS Applications/Services
Conclusion
References

22. 22
References
 [1] V. Koukoulidis and M. Shah, The IP Multimedia Domain in Wireless
Networks: Concepts, Architecture, Protocols and Applications, Siemens
Information and Communication Mobile, Boca Raton, Fl, Proceedings of the
IEEE Sixth International Symposium on Multimedia Software Engineering
(ISMSE’04), IEEE, 2004.
 [2] P. Polvora, C. Öström, Plan Your IP-Competence Strategically,
Ericsson Seminar, http://www.ericsson.com/learning, April 2005, Ericsson
AB 2005.
 [3] 3GPP Homepage, http://www.3gpp.org/specs/specs.htm
 [4] Open Mobile Alliance (OMA) Homepage,
http://www.openmobilealliance.org
 [5] Session Initiation Protocol Charter Homepage,
http://www.ietf.org/html.charters/sip-charter.html

23. 23
References
 [6] IP Multimedia Subsystem (IMS): Service Architecture, Lucent
Technologies, February, 2005,
 [7] IMS – IP Multimedia Subsystem: The value of using the IMS
architecture, White Paper, Ericsson, October 2004,
 [8] Motorola IP Multimedia System White Paper, February, 2004,
 [9] IP Multimedia: a New Era in Communications, Nokia White Paper,
 [10] The Protocol Dictionary Homepage,
http://www.javvin.com/protocolH245.html
 [11] P. Howard, 3GPP Security and Authentication, September 13, 2001,
http://www.ietf.org/proceedings/01aug/slides/sipping-7/index.htm

24. 24
References
 [12] Open IMS Homepage,
http://www.fokus.gmd.de/bereichsseiten/testbeds/ims_playground/playground/hss.php?lang=de
 [13] 3GPP TS 23.228: "IP Multimedia Subsystem (IMS)"
 [14] IETF RFC 3261: "SIP: Session Initiation Protocol"
 [15] 3GPP TS 23.218: "IP Multimedia (IM) session handling, IM call model"
 [16] 3GPP TS 29.228:" IP Multimedia (IM) Subsystem Cx Interface; Signalling flows and
message contents"
 [17] The Parlay Group homepage, http://www.parlay.org/
 [18] H. Montes, G Gomez, H Cuny, Nokia Networks, J. Paris, Deployment of IP Multimedia
Streaming Services in Third Generation Mobile Networks, IEEE Wireless Communications,
October 2002
 [19] Siemens IP Multimedia Subsystem (IMS): The Domain for Services, Whitepaper