IMS Core Elements
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The document summarizes the core elements of an IP Multimedia Subsystem (IMS) network. The key elements are: - The Proxy-CSCF (P-CSCF) is the first contact point for the user and handles security, authorization, and forwarding requests. - The Interrogating-CSCF (I-CSCF) routes requests between networks and assigns users a Serving-CSCF (S-CSCF). - The S-CSCF manages sessions for registered users and interacts with application servers to provide services. - The Home Subscriber Server (HSS) is the master database that stores user profiles, locations, and security information. - Application Servers
IMS Core Elements
- 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).
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Editor's Notes
- #7: A Home Network may contain one or several HSSs: it depends on the number of mobile subscribers, on the capacity of the equipment and on the organisation of the network; The HSS also generates User Security information for mutual authentication, communication integrity check and ciphering. The HSS consists of the following functionalities: IP multimedia functionality to provide support to control functions of the IM subsystem such as the CSCF; The subset of the HLR/AUC functionality required by the PS Domain (GPRS and EPC;. The subset of the HLR/AUC functionality required by the CS Domain.
- #8: The OSA Application Server does not directly interact with the IMS network entities but through the OSA Service Capability Servers (OSA SCS-s). Further information on OSA is provided in TS 22.127 [4]. A SIP Application Server specified within 3GPP that supports IMS reference points (e.g. ISC, Sh, Ut), in support of an application, is considered as part of the IM CN subsystem. Examples of such ASs are SCC AS and TAS.