1. IP Multimedia Subsystem & W-CSCF
Speaker: Meng-Ta Hsu

2. Outline IP Multimedia Subsystem concepts Role of Cx interface
Application level registration procedures
Procedures for IP multimedia sessions
W-CSCF
Summary

3. IP Multimedia Subsystem
Introduced in the 3GPP Release 5
Based on the 3G GPRS Core Network
Enables IP multimedia services for the mobile users
Support interworking with PSTN

4. 3GPP Architecture PSTN CSCF HSS Internet RNC SGSN GGSN MGCF MGW MGW
Legacy mobile
signaling network
Signaling (SS7 or IP based)
Circuit
Packet (user traffic and/or signaling)
R-SGW Cx
CSCF
HSS
Internet
RNC
Iu_PS
SGSN
GGSN
Node B
MGCF MS
Iu_CS
(user traffic)
Node B
PSTN MS
MGW
MGW
Iu_CS
(control part)
MAP
MAP
MSC Server
GMSC Server
T-SGW

5. Call Session Control Function (CSCF)
A SIP Proxy
Performs session initiation, modification, deletion control
Interacting with HSS to receive user subscription information
Supports session routing and address mapping
Interacting with service platform to support advanced services

6. CSCF

7. Proxy-CSCF (P-CSCF) The first contact point within the IMS
A SIP proxy to redirect SIP messages toward MS’s home network
Its address is discovered by MS following PDP context activation procedure
P-CSCF discovery
Use DHCP/DNS procedure
Use PDP context activation message

8. Interrogating-CSCF (I-CSCF)
Assign a S-CSCF to the MS
Route a SIP message from another network towards the S-CSCF
A SIP Proxy

9. Serving-CSCF (S-CSCF)
Control the session state
Download user profile from HSS
Interact with Service Platforms
A SIP Proxy, Registrar

10. IMS Architecture for Service Provision

11. Role of Cx interface Procedure related to
Support information transfer between CSCF and HSS
Procedure related to
Location management procedures
User data handling procedures
User authentication procedures
Cx interface is implemented by DIAMETER protocol

12. Location management procedures – User registration status query
Used between the I-CSCF and the HSS during SIP registration
Diameter command: User-Authorization-Request/Answer (UAR/UAA)
To authorize the registration of the user
To obtain the user registration status
UAR I -
CSCF
HSS
UAA

13. Location management procedures – S-CSCF registration/deregistration notification
Used between the S-CSCF and the HSS during SIP registration
Diameter command: Server-Assignment-Request/Answer (SAR/SAA)
To assign and S-CSCF to a MS
To download user profile from HSS
SAR S -
CSCF
HSS
SAA

14. Location management procedures – User location query
To obtain the name of the S-CSCF where a MS is registered
Diameter command: Location-Info-Request/Answer (LIR/LIA)
LIA
LIR I -
CSCF
HSS

15. Authentication procedures
Used between the S-CSCF and the HSS to exchange authentication information
To retrieve authentication vectors from the HSS
Diameter command: Multimedia-Auth-Request/MAA
MAA
MAR S -
CSCF
HSS

16. First Time Registration
UAR: User-Authorization-Request
UAA: User-Authorization-Answer
MAR: Multimedia-Authentication-Request
MAA: Multimedia-Authentication-Answer
SAR: Server-Assignment-Request
SAA: Server-Assignment-Answer

17. Re-Registration procedures
UAR: User-Authorization-Request
UAA: User-Authorization-Answer
SAR: Server-Assignment-Request
SAA: Server-Assignment-Answer

18. Session Initiation Flow
S-CSCF
P-CSCF
GGSN
SGSN
Radio Access Network
I-CSCF
HSS
App.
Server 3 1 2
Originating Home Network
Visited/Home
Network 5 6 7 8
Terminating Home Network 4 10 9
Originating
Terminating

19. Authorize QoS Resources
iFC
iFC
Authorize QoS Resources
183Session
Progress
(SDP)
Authorize QoS Resources
PRACK
Resource
Reservation
200 OK
Resource
Reservation
UPDATE
(SDP)
200 OK(SDP)
Approval of QoS Commit
Approval of QoS Commit

20. WCSCF Architecture

21. UE Registration

22. Session Initiation

23. GPRS Authentication

24. IMS Authentication

25. W-CSCF Authentication

26. Summary
Cx interface supports information transfer between HSS and CSCF
IMS is based on SIP protocol
QoS management is important for real-time & non real-time services
W-CSCF reduces SIP signaling messages and user authentication time