IMS Core Element Illustrated

1. IMS Core Elements
IP Multimedia Subsystem Series

2. IMS Core Elements
P-CSCF
I-CSCF
S-CSCFHSS
UE
ASService
Session
Control
Access
AS :
HSS :
CSCF :
S-CSCF :
I-CSCF :
P-CSCF :
MRFC:
MRFP:
UE :
Application Server
Home Subscriber Server
Call Session Control Function
Serving – CSCF
Interrogating – CSCF
Proxy – CSCF
Media Resource Function Controller
Media Resource Function Processor
User Equipment
MRFC
MRFP

3. Proxy – Call Session Control Function
• The Proxy-CSCF (P-CSCF) is the first contact point
within the IM CN subsystem;
• The P-CSCF behaves like a Proxy (as defined in
IETF RFC 3261), i.e. it accepts requests and services
them internally or forwards them on;
• Forward the SIP request or response to the UE;
• Maintain a Security Association between itself and
each UE, as defined in TS 33.203
• Authorization of bearer resources and QoS
management. For details see TS 23.203
P-CSCF
UE

4. P-CSCF
I-CSCF
UE
Interrogating – Call Session Control Function
OtherIMSNetwork
• Interrogating-CSCF (I-CSCF) is the contact point
within an operator's network for all connections
destined to a user of that network operator, or a
roaming user currently located within that network
operator's service area;
• Route a SIP request received from another network
towards the S-CSCF;
• Assigning a S-CSCF to a user performing SIP
registration.

5. Serving – Call Session Control Function
P-CSCF
I-CSCF
UE
OtherIMSNetwork
S-CSCF
• The Serving-CSCF (S-CSCF) performs the session
control services for the UE. It maintains a session
state as needed by the network operator for support
of the services;
• Behave as a Registrar as defined in IETF RFC 3261 ,
i.e. it accepts registration requests and makes its
information available through the location server
(e.g. HSS);
• Session control for the registered endpoint's sessions;
• Handle interaction with Services Platforms for the
support of Services.

6. Home Subscriber Server
P-CSCF
I-CSCF
HSS
UE
• The HSS is the master database for a given user. It is the
entity containing the subscription-related information
to support the network entities actually handling
calls/sessions;
• The HSS is responsible for holding the following user
related information:
• User Identification, Numbering and addressing information;
• User Security information: Network access control information
for authentication and authorization;
• User Location information at inter-system level: the HSS
supports the user registration, and stores inter-system location
information, etc.;
• User profile information.
OtherIMSNetwork
S-CSCF

7. Application Server
P-CSCF
I-CSCF
HSS
UE
OtherIMSNetwork
S-CSCF
AS
• An Application Server (AS) i.e., SIP Application
Server, OSA Application Server, or CAMEL IM-SSF,
offers value added IM services and resides either in
the user's home network or in a third party location.
The third party could be a network or simply a
stand-alone AS;
• The Serving-CSCF to AS interface is used to provide
services residing in an AS;

8. Media Resource Function Controller
P-CSCF
I-CSCF
HSS
UE
S-CSCF
AS
MRFC
• Control the media stream resources in the MRFP;
• Interpret information coming from an AS and
S-CSCF (e.g. session identifier) and control MRFP
accordingly;

9. Media Resource Function Processor
P-CSCF
I-CSCF
HSS
UE
S-CSCF
AS
MRFC
MRFP
• Provide resources to be controlled by the MRFC;
• Mixing of incoming media streams (e.g. for multiple
parties);
• Media stream source (for multimedia
announcements);
• Media stream processing (e.g. audio transcoding,
media analysis).

10. thank you