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.
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:
In IMS networks, the SBC acting as a P-CSCF interfaces with the following functional elements:
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: