SBC as P-CSCF

 

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.

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-
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  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

For configuration details, refer to Configuring SBC as P-CSCF.

Additional topics in this section: