IP Multimedia Subsystem architecture overview
0 likes558 views
The document provides an overview of the IMS architecture, detailing the roles and interactions between various elements such as p-CSCF, i-CSCF, and S-CSCF in managing SIP communications for mobile users. It describes processes for user registration, call handling, and the implementation of policy control functions, highlighting the significance of DNS resolution and HSS queries. Additionally, it addresses design considerations and functionalities required for efficient IMS operations, including handling of emergency calls and supplementary services.
IP Multimedia Subsystem architecture overview
- 1. IMS Architecture Overview Narasimham Settipalli
- 2. P-CSCF • First Entry point for UE when call is initiated • P-CSCF should locate I-CSCF to enter the IMS network. • How to locate I-CSCF should be specified – Using DNS domain present in SIP request, I-CSCF node should be resolved – Selection of I-CSCF should be specified • Policy control function is logic function of P- CSCF
- 3. P-CSCF Design changes • SIP Registrations are directly passed to S-CSCF from P-CSCF (if P-CSCF aware of S-CSCF domain) • Forward SIP requests received from other networks to I-CSCF. • Handling emergency calls and determining S-CSCF to handle the SIP request in visited network • If S-CSCF address not found, SIP request is forward to I-CSCF. I-CSCF uses SLF functionality to determine the S-CSCF address
- 5. P-CSCF Design changes • SIP Registration forwarding to S-CSCF • Proxy/B2BUA functionality to support UAC/UAS • Policy control functionality for charging – Rx Interfaces: 3GPP TS 23.203 and TS 29.214 – Diameter – RFCs 6733, RFC 3588 and RFC 5516 – SCTP – RFC 4960 • Rx interface needs to be developed for policy function control according to TS 23.203 and 29.214 specifications
- 6. I-CSCF Design • I-CSCF primary principle is to identify correct S-CSCF to serve the IMS subscribers. • I-CSCF queries HSS to find the S-CSCF where SIP request should be sent for roaming subscribers • SLF functionality should be implemented • I-CSCF address published in DNS and it is resolved by P- CSCF and other network elements trying to reach I-CSCF • I-CSCF implements diameter interface to HSS (RFC 3588) • I-CSCF receives SIP requests and forwards it to S-CSCF • I-CSCF located usually in home IMS Network • Multiple I-CSCF nodes installed for scalability and redundancy purpose
- 8. S-CSCF Design Interface • S-CSCF handles Users SIP Registrations • S-CSCF acts as redirect, location server • Maintains database for SIP registrations • Supports Cx and Dx interfaces to download subscriber profiles from HSS • SIP messages routing with in same PLMN or different PLMN with roaming • Services triggering towards Application Server – Conference call, adding new users in to the call • Announcements or prompts playing using MRF nodes
- 9. S-CSCF Interfaces S-CSCF I-CSCFP-CSCF HSS Cx/Dx Diameter interface MRFC/MRFP IVR/prompts IBCF MGCF MG PSTN Network SIP SIP Application Server
- 10. S-CSCF Design and implementation references • SIP Registration handling according to RFC3261. (We already handling ) – Done • New SL needed to handle communication to Application Server, handling conference calls, supplementary services using Application Server. • New Database tables needed to store SIP users according to rules specified by 3GPP IMS release in HSS. Corresponding stored procedures to handle database queries
- 11. S-CSCF Design functionalities • SIP Calls handling – Calls originated from Mobile UE, S-CSCF checks whether dialled number is part of the registered user list. Terminates the call. – If Mobile UE is not part of S-CSCF or PSTN dialled number, call is routed to IBCF/MGCF to terminate the call on PSTN Network – Any announcements or prompts playing needed, uses MRFC/MRFP to handle this scenario – Application Server is contacted whenever supplementary services needs to be handled such as: Call Transfer, Call Forward, Conference Call, Call rerouting
- 12. IMS Call Handling • When an user tries to register with IMS network, it sends REGISTER request to P-CSCF. Since the REGISTER doesn’t contain any Service-Route header (indicating the S-CSCF) P-CSCF doesn’t know which S-CSCF to forward the message (which S-CSCF is assigned to the UE ). Hence it forwards the REGISTER to the I-CSCF .If not the I-CSCF is pre-configured, it is discovered by performing the DNS query for the home domain name (identified from request-uri). • Once I-CSCF receives this REGISTER it queries the HSS ( if there are multiple HSS, then I-CSCF contacts SLF to choose the desired HSS) using User-Authorization-Request (UAR) diameter request. Now there are two possibilities at HSS, it can either specify certain pre-assigned S-CSCF by mentioning it in the Server-Name field of User-Authorization-Answer (UAA) diameter response or it specifies certain capabilities of potential S- CSCF in the Server-Capabilities filed of UAA from which the I-CSCF can choose a S-CSCF for the user. In the later case I-CSCF maintains a list of S- CSCFs available in the network along with their capabilities (through some operator specific way, not specified by 3GPP).
- 13. IMS UE Registration Visited Network Home Network UE P-CSCF I-CSCF S-CSCF HSS Register Register User Authorization Request (UAR) UAA S-CSCF is identified using HSS Register SAR SAA 200 OK 200 OK 200 OK
- 14. IMS Call Handling • Assume the case that when S-CSCF adds Service-Route Header in 200 OK registration, P-CSCF/UE should store the S-CSCF URI and use it during SIP Invite handling • If Service-Route header is not present in INVITE, P-CSCF forwards the call to I-CSCF to determine S-CSCF. – Otherwise, P-CSCF directly routes INVITE to S- CSCF
- 15. IMS Call Handling Visited Network Home Network UE1 P-CSCF I-CSCF S-CSCF HSS INVITE INVITE Location Info Request (LIR) LIAServing S- CSCF identified Invite Visited Network P-CSCF UE2 Invite Invite 200 OK200 OK200 OK 200 OK
- 16. IMS Call using I-CSCF Visited Network Home Network UE1 P-CSCF I-CSCF S-CSCF HSS INVITE INVITE Location Info Request (LIR) LIA Serving S- CSCF identified Invite Visited Network I-CSCF P-CSCF Invite Invite 200 OK200 OK 200 OK 200 OK S-CSCF Invite 200 OK
- 17. IMS Registration with SIP challenge Home Network S-CSCF HSS Registration Multimedia Authorization Request (MAR) Multimedia Authorization Answer (MAA) 401 unauthorized
- 18. IMS Registration after challenge Home Network S-CSCF HSS Registration Server Assignment Request (SAR) Server Assignment Answer (SAA) 200 OK