OpenSIPS IMS - Community Edition - P-CSCF

The Proxy - Call Session Control Function (P-CSCF) serves as the initial point of contact for user equipment in a mobile network. All SIP traffic to and from the user equipment (UE) is routed through the P-CSCF.

When a UE registers with the network, it establishes an IPSec tunnel to the P-CSCF in accordance with the 3GPP TS 33.203 specifications (3G Security: Access Security for IP-based Services). All SIP communication between the UE and the network is securely transmitted through this tunnel, ensuring confidentiality and integrity.

On the core network side, the P-CSCF interacts with the home network's I-CSCF instance to handle call signaling and coordinates with the PCRF to allocate the necessary resources for the call.

Interfaces

The P-CSCF implements the following interfaces:

• Gm - (defined in 3GPP TS 24.229) describes the SIP messages that need to be exchanged in order to enables connectivity between the UE/PGW and the IMS network (P-CSCF) for registration, authentication, encryption, and session control. It contains information about the IPSec tunnel establishement and IMS AKA authentication.
• Mw - (defined in 3GPP TS 24.229) describes the SIP messages exchanged internally between the CSCF sub-components - in the P-CSCF case, the interaction between P-CSCF and I-CSCF is done over the Mw interface.
• Rx - (defined in 3GPP TS 29.211) - Diameter interface between the P-CSCF and PCRF used to allocate resources for a UE. Supported Diameter commands are:
  • AAR - Authentication and authorization request, sent by the P-CSCF to alocate resources for registration/calls
  • AAA - Answer from PCRF of the resources allocation request
  • STR - Session-Termination-Request sent to release resources allocations
  • ASR - Abort-Session-Request received from the PCRF when the UE disconnects from the network

Configuration

The P-CSCF can be configured through the environment variables defined here.