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 SBC 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 SBC Core in the role of a P-CSCF.

Figure 1: SBC as 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 SBC 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-SBC 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

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: