The Proxy For Call Session Control Function (P-CSCF) is the first contact point within the IP Multimedia (IM) Core Network (CN) subsystem for subscribers in an IP Multimedia Subsystem (IMS) network. The
may be deployed with many functions of a P-CSCF in an IMS network. A P-CSCF is allocated to the IMS terminal during IMS registration, and does not change for the duration of the registration. From a standards perspective, the P-CSCF behaves like a SIP Proxy. In other words, it accepts requests and services them internally or forwards them onwards. The figure below depicts the
in the role of a P-CSCF.
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The P-CSCF also behaves as a User Agent in some cases. In certain conditions, it terminates and/or independently generates SIP transactions. Examples of these cases are when the P-CSCF requests the registrar for a user’s registration state updates, when performing P-CSCF initiated dialog-release etc. At a high level, the P-CSCF performs the following functions:
- Forwards the SIP REGISTER request received from the UE to an entry point determined using the home domain name, as provided by the UE
- Forwards SIP messages (other than REGISTER) received from the UE to the S-CSCF whose name the P-CSCF has received as a result of the registration procedure
- Maintains mapping between the SIP URI for the subscriber and physical UE IP address as well as a Security Association between itself and each UE, for both authentication and confidentiality
- Forwards SIP requests or responses from the IMS core to the UE
- Authorizes bearer resources and QoS management using the DIAMETER Rx interface to query an PCRF element
- Ensures SIP messages received from the UE to the S-CSCF contain the correct, or up to date, information about the access network type currently used by the UE (when the information is available from the access network)
- Performs SIP message compression/decompression
- Detects and manages an emergency session establishment request from the UE
- Generates CDRs
In IMS networks, the
acting as a P-CSCF interfaces with the following functional elements:
- S-CSCF / I-CSCF over Mw interface (SIP)
- I-CSCF over Mw interface (SIP)
- IBCF over Mx interface (SIP)
- P-CSCF/A- to MGCF over Mb interface (RTP/RTCP)
- PCRF over Rx interface (Diameter)
- DNS interface from P-CSCF to DNS server
- GM interface between UE and P-CSCF
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Sonus P-CSCF acts as a combined IMS Application Level Gateway (ALG) and IMS Access Gateway. |
For configuration details, refer to Configuring SBC as P-CSCF.
Additional topics in this section: