The document provides an architectural overview of the IP Multimedia Subsystem (IMS), detailing various components such as the Serving CSCF, Interrogating CSCF, Proxy CSCF, and others, each with specific functions related to session control, registration, and signal processing. It also explains the roles of the Policy Decision Function, Home Subscriber Server, Application Server, and Media Gateway Control Function within the IMS framework. The IMS architecture is outlined to support multimedia services over IP networks, integrating various signaling and media resource functions.

1. 1
Architectural Overview of
IP Multimedia Subsystem -IMS
Presented by:
Masood Khosroshahy
June 2006
Project supervisor:
Prof. Elie Najm
B
E
G
I
N
N
I
N
G

2. 2
Simplified view of the layered
architecture in IMS
From [4]

3. 3
3GPP IMS Architectural Overview
IMS-MGW
IPv6 PDN
(IPv6 Network)
MGCF
PDF
I-CSCF
S-CSCF
BGCF
Application
(Ext. SIP AS, OSA AS,
CAMEL SE)
MRFC
MRFP
MRF
IPv4/IPv6
BB
CS Networks
(PSTN, CS PLMN)
CSCF
P-CSCF
SGW
OSA SCS
IM SSF
SIP AS
AS
BG
SLF
ALG
TrGW
‘IMS IPGW
IPv4 PDN
(IPv4 Network)
IP CAN ABG
TE AN
IMS
Terminal
PEF
/TP
F
HLR/AuC (‘CS/PS’)
HSS
IMS Data Handling
From [2]

4. 4
3GPP IMS Architectural Overview:
Serving CSCF (S-CSCF)
IMS-MGW
IPv6 PDN
(IPv6 Network)
MGCF
PDF
I-CSCF
S-CSCF
BGCF
Application
(Ext. SIP AS, OSA AS,
CAMEL SE)
MRFC
MRFP
MRF
IPv4/IPv6
BB
CS Networks
(PSTN, CS PLMN)
CSCF
P-CSCF
SGW
OSA SCS
IM SSF
SIP AS
AS
BG
SLF
ALG
TrGW
‘IMS IPGW
IPv4 PDN
(IPv4 Network)
IP CAN ABG
TE AN
IMS
Terminal
PEF
/TP
F
HLR/AuC (‘CS/PS’)
HSS
IMS Data Handling

5. 5
Serving CSCF (S-CSCF) Functions
• Acts like a SIP Registrar, it binds the public user ID to a location.
• The S-CSCF retrieves the subscriber profile from the HSS.
• Provides session control for the endpoint's sessions
(service logic is performed by an Application Server, not the S-
CSCF).
• Handles SIP routing for originating and terminating endpoints.
• Ensures that the media for a session, as indicated by SDP, are
within boundaries of subscriber's profile.
• Interacts with Application Server platforms for the support of
services.

6. 6
3GPP IMS Architectural Overview:
Interrogating CSCF (I-CSCF)
IMS-MGW
IPv6 PDN
(IPv6 Network)
MGCF
PDF
I-CSCF
S-CSCF
BGCF
Application
(Ext. SIP AS, OSA AS,
CAMEL SE)
MRFC
MRFP
MRF
IPv4/IPv6
BB
CS Networks
(PSTN, CS PLMN)
CSCF
P-CSCF
SGW
OSA SCS
IM SSF
SIP AS
AS
BG
SLF
ALG
TrGW
‘IMS IPGW
IPv4 PDN
(IPv4 Network)
IP CAN ABG
TE AN
IMS
Terminal
PEF
/TP
F
HLR/AuC (‘CS/PS’)
HSS
IMS Data Handling

7. 7
Interrogating CSCF (I-CSCF) Functions
• Serves as the initial point of contact from other networks.
• Performs a stateless SIP proxy function.
• Selects a S-CSCF for a user during SIP registration.
• Routes SIP requests received from another network to the
S-CSCF.
• Queries the HSS for the address of the S-CSCF.
• If no S-CSCF is currently assigned (e.g., unregistered
subscriber), then assigns an S-CSCF to handle the SIP request.
• Provide Topology Hiding Interworking Gateway (THIG) function.

8. 8
3GPP IMS Architectural Overview:
Proxy CSCF (P-CSCF)
IMS-MGW
IPv6 PDN
(IPv6 Network)
MGCF
PDF
I-CSCF
S-CSCF
BGCF
Application
(Ext. SIP AS, OSA AS,
CAMEL SE)
MRFC
MRFP
MRF
IPv4/IPv6
BB
CS Networks
(PSTN, CS PLMN)
CSCF
P-CSCF
SGW
OSA SCS
IM SSF
SIP AS
AS
BG
SLF
ALG
TrGW
‘IMS IPGW
IPv4 PDN
(IPv4 Network)
IP CAN ABG
TE AN
IMS
Terminal
PEF
/TP
F
HLR/AuC (‘CS/PS’)
HSS
IMS Data Handling

9. 9
Proxy CSCF (P-CSCF) Functions
• Serves as the initial point of contact for the user terminal to
network.
• Performs a stateful SIP proxy function.
• Sends the SIP REGISTER request received from the UE to an
I-CSCF determined using the home domain name, as provided
by the UE.
• Sends all subsequent SIP messages from the UE to the S-
CSCF, whose name the P-CSCF has received as part of
registration.
• Inserts a valid public user identity for UE initiated requests.
• Performs SIP message compression to reduce the amount of
data sent over the radio interface.

10. 10
3GPP IMS Architectural Overview:
Policy Decision Function
IMS-MGW
IPv6 PDN
(IPv6 Network)
MGCF
PDF
I-CSCF
S-CSCF
BGCF
Application
(Ext. SIP AS, OSA AS,
CAMEL SE)
MRFC
MRFP
MRF
IPv4/IPv6
BB
CS Networks
(PSTN, CS PLMN)
CSCF
P-CSCF
SGW
OSA SCS
IM SSF
SIP AS
AS
BG
SLF
ALG
TrGW
‘IMS IPGW
IPv4 PDN
(IPv4 Network)
IP CAN ABG
TE AN
IMS
Terminal
PEF
/TP
F
HLR/AuC (‘CS/PS’)
HSS
IMS Data Handling

11. 11
Policy Decision Function (PDF) Functions
• Responsible for making policy decisions based on session and
media-related information obtained from the P-CSCF
• Acts as policy decision point for Service-based Local Policy
(SBLP) control.
• Some of policy decision point functionalities:
- To store session and media-related information
- The capability to enable the usage of an authorized bearer
(e.g. PDP context)
- To inform P-CSCF when the bearer is lost or modified.
- To pass an IMS-charging identifier to the GGSN and to Pass a
GPRS-charging identifier to the P-CSCF

12. 12
3GPP IMS Architectural Overview:
Home Subscriber Server/
Subscription Locator Function
IMS-MGW
IPv6 PDN
(IPv6 Network)
MGCF
PDF
I-CSCF
S-CSCF
BGCF
Application
(Ext. SIP AS, OSA AS,
CAMEL SE)
MRFC
MRFP
MRF
IPv4/IPv6
BB
CS Networks
(PSTN, CS PLMN)
CSCF
P-CSCF
SGW
OSA SCS
IM SSF
SIP AS
AS
BG
SLF
ALG
TrGW
‘IMS IPGW
IPv4 PDN
(IPv4 Network)
IP CAN ABG
TE AN
IMS
Terminal
PEF
/TP
F
HLR/AuC (‘CS/PS’)
HSS
IMS Data Handling

13. 13
Home Subscriber Server/
Subscription Locator Function
• SLF is used as resolution mechanism to find the
address of the HSS that holds the subscriber data
• The HSS supports IMS level Authentication, and
Authorization.
• Holds IMS subscriber profile.
• Keeps track of currently assigned S-CSCF.
• Supports interactions with CSCFs and ASs.

14. 14
3GPP IMS Architectural Overview:
Application Server (AS)
IMS-MGW
IPv6 PDN
(IPv6 Network)
MGCF
PDF
I-CSCF
S-CSCF
BGCF
Application
(Ext. SIP AS, OSA AS,
CAMEL SE)
MRFC
MRFP
MRF
IPv4/IPv6
BB
CS Networks
(PSTN, CS PLMN)
CSCF
P-CSCF
SGW
OSA SCS
IM SSF
SIP AS
AS
BG
SLF
ALG
TrGW
‘IMS IPGW
IPv4 PDN
(IPv4 Network)
IP CAN ABG
TE AN
IMS
Terminal
PEF
/TP
F
HLR/AuC (‘CS/PS’)
HSS
IMS Data Handling

15. 15
Application Server (AS)
• The AS provides service control for IMS.
• May be directly connected to S-CSCF or via an OSA
Gateway for 3rd party application support with
security.
• Interacts with the HSS to obtain subscriber profile
information.
• Supports applications such as telephony services,
presence, conference control, PoC, etc.

16. 16
Application Server Types
• Not pure IMS entities, rather, functions on top of IMS
• Application Servers:
- SIP Application Server
- Open Service Architecture (OSA) Service Capability
Server
(SCS)
- CAMEL IP Multimedia Service Switching Function
(IM-SSF)
• From the perspective of the S-CSCF, all application
servers exhibit the same reference point behavior

17. 17
3GPP IMS Architectural Overview:
Breakout Gateway Control Function (BGCF)
IMS-MGW
IPv6 PDN
(IPv6 Network)
MGCF
PDF
I-CSCF
S-CSCF
BGCF
Application
(Ext. SIP AS, OSA AS,
CAMEL SE)
MRFC
MRFP
MRF
IPv4/IPv6
BB
CS Networks
(PSTN, CS PLMN)
CSCF
P-CSCF
SGW
OSA SCS
IM SSF
SIP AS
AS
BG
SLF
ALG
TrGW
‘IMS IPGW
IPv4 PDN
(IPv4 Network)
IP CAN ABG
TE AN
IMS
Terminal
PEF
/TP
F
HLR/AuC (‘CS/PS’)
HSS
IMS Data Handling

18. 18
Breakout Gateway Control Function
(BGCF) Functions
• Selects the network in which PSTN breakout is to
occur.
• Selects a local MGCF or a peer BGCF.

19. 19
3GPP IMS Architectural Overview:
Media Gateway Control Function (MGCF)
IMS-MGW
IPv6 PDN
(IPv6 Network)
MGCF
PDF
I-CSCF
S-CSCF
BGCF
Application
(Ext. SIP AS, OSA AS,
CAMEL SE)
MRFC
MRFP
MRF
IPv4/IPv6
BB
CS Networks
(PSTN, CS PLMN)
CSCF
P-CSCF
SGW
OSA SCS
IM SSF
SIP AS
AS
BG
SLF
ALG
TrGW
‘IMS IPGW
IPv4 PDN
(IPv4 Network)
IP CAN ABG
TE AN
IMS
Terminal
PEF
/TP
F
HLR/AuC (‘CS/PS’)
HSS
IMS Data Handling

20. 20
Media Gateway Control Function (MGCF)
• Controls the MGW to provide IMS connections to
PSTN trunks.
• Performs protocol conversion between ISUP and SIP.
• May process out of band information such as DTMF
signaling received in MGCF which it may forward to
the CSCF or MGW.

21. 21
3GPP IMS Architectural Overview:
Media Gateway (MGW)
IMS-MGW
IPv6 PDN
(IPv6 Network)
MGCF
PDF
I-CSCF
S-CSCF
BGCF
Application
(Ext. SIP AS, OSA AS,
CAMEL SE)
MRFC
MRFP
MRF
IPv4/IPv6
BB
CS Networks
(PSTN, CS PLMN)
CSCF
P-CSCF
SGW
OSA SCS
IM SSF
SIP AS
AS
BG
SLF
ALG
TrGW
‘IMS IPGW
IPv4 PDN
(IPv4 Network)
IP CAN ABG
TE AN
IMS
Terminal
PEF
/TP
F
HLR/AuC (‘CS/PS’)
HSS
IMS Data Handling

22. 22
Media Gateway (MGW) Functions
• Interacts with the MGCF for resource control.
• Terminates trunks from the circuit switched network and media
streams from the packet network (e.g. RTP streams).
• Supports media conversion, bearer control, and payload
processing
• May detect events (i.e. bearer loss, DTMF digits, etc.) and
notifies the MGCF.
• May perform DiffServ Code Point (DSCP) markings on the IP
packets sent towards the UE.

23. 23
3GPP IMS Architectural Overview:
Media Resource Function Controller (MRFC)
IMS-MGW
IPv6 PDN
(IPv6 Network)
MGCF
PDF
I-CSCF
S-CSCF
BGCF
Application
(Ext. SIP AS, OSA AS,
CAMEL SE)
MRFC
MRFP
MRF
IPv4/IPv6
BB
CS Networks
(PSTN, CS PLMN)
CSCF
P-CSCF
SGW
OSA SCS
IM SSF
SIP AS
AS
BG
SLF
ALG
TrGW
‘IMS IPGW
IPv4 PDN
(IPv4 Network)
IP CAN ABG
TE AN
IMS
Terminal
PEF
/TP
F
HLR/AuC (‘CS/PS’)
HSS
IMS Data Handling

24. 24
Media Resource Function Controller
(MRFC) Functions
• Controls the media stream resources in the MRFP.
• Interprets information from an AS via the S-CSCF
(using SIP) and controls the MRFP accordingly.
• May be co-located with an AS to provide capabilities
such as conference services.

25. 25
3GPP IMS Architectural Overview:
Media Resource Function Processor (MRFP)
IMS-MGW
IPv6 PDN
(IPv6 Network)
MGCF
PDF
I-CSCF
S-CSCF
BGCF
Application
(Ext. SIP AS, OSA AS,
CAMEL SE)
MRFC
MRFP
MRF
IPv4/IPv6
BB
CS Networks
(PSTN, CS PLMN)
CSCF
P-CSCF
SGW
OSA SCS
IM SSF
SIP AS
AS
BG
SLF
ALG
TrGW
‘IMS IPGW
IPv4 PDN
(IPv4 Network)
IP CAN ABG
TE AN
IMS
Terminal
PEF
/TP
F
HLR/AuC (‘CS/PS’)
HSS
IMS Data Handling

26. 26
Media Resource Function Processor
(MRFP) Functions
• Under the control of MRFC
• Mixes incoming media streams (e.g. for multiple
parties).
• Sources media streams (for multimedia
announcements).
• Processes media streams (e.g. audio transcoding).
• Provide tones and supports DTMF within the bearer
path.
• Notifies the MRFC when an event has occurred such
as DTMF digit collection.

27. 27
3GPP IMS Architectural Overview:
Signaling Gateway
IMS-MGW
IPv6 PDN
(IPv6 Network)
MGCF
PDF
I-CSCF
S-CSCF
BGCF
Application
(Ext. SIP AS, OSA AS,
CAMEL SE)
MRFC
MRFP
MRF
IPv4/IPv6
BB
CS Networks
(PSTN, CS PLMN)
CSCF
P-CSCF
SGW
OSA SCS
IM SSF
SIP AS
AS
BG
SLF
ALG
TrGW
‘IMS IPGW
IPv4 PDN
(IPv4 Network)
IP CAN ABG
TE AN
IMS
Terminal
PEF
/TP
F
HLR/AuC (‘CS/PS’)
HSS
IMS Data Handling

28. 28
Signaling Gateway
• Used to interconnect different signaling networks,
such as SCTP-IP-based signaling networks and SS7
signaling networks.
• Performs signaling conversion at the transport level.
• Does not interpret application layer messages.

29. 29
Two Procedures
• Registration
• Simple Call

30. 30
2-phase registration
UE IMS
GPRS
1. Bearer Level Registration: e.g.GPRS Attach
(2. Establishing signaling link e.g. PDP Context Activation)
3. CSCF Discovery
4. Application (IMS) Level Registration
From [2]

31. 31
SIP Registration of a Mobile Node
From [3]

32. 32
Simple SIP Call: Caller Side (1)
From [3]

33. 33
Simple SIP Call: Caller Side (2)
From [3]

34. 34
References
• [1] “The IMS”
M Poikselka et al. , 2004, John Wiley, ISBN 0-470-87113-X
• [2] “Policy and Regulatory Requirements for Future Mobile
Networks”
F. Courau & M. Olsson, June 22nd 2005,
Alcatel-Ericsson Presentation
• [3] “SIP and Mobility: IP Multimedia Subsystem in 3G Release 5”
Jorg Ott, 11 November 2002, Presentation at Bremen
• [4] “IMS –IP Multimedia Subsystem”
Oct 2004, Ericsson Whitepaper
• [5] “The IP Multimedia Subsystem”
2006, Twister Consulting whitepaper

35. 35
E
N
D
Thank you for your attention …