This presentation slides explain overview of different IMS core network elements, functionalities and interfaces.

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

4. P-CSCF Interfaces
P-CSCFUE
UE
PCRF
I-CSCF
SIP
S-CSCF
SIP
RxDiameter
SIP

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

7. I-CSCF Interface
I-CSCF S-CSCF
SIP
P-CSCF
HSS
SLF
SIP

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