CSCF stands for Call Session Control Function, and it is a network element used in the IMS architecture to provide multimedia services over IP networks. CSCF comprises three distinct roles: the Proxy CSCF (P-CSCF), the Interrogating CSCF (I-CSCF), and the Serving CSCF (S-CSCF). The CSCF is implemented via servers using the SIP protocol to communicate with each other and application servers.
The main functions of the CSCF in the IMS network are:
- P-CSCF: The P-CSCF is the first point of contact for the user equipment (UE) in the IMS network. It acts as a proxy for the UE and forwards the SIP messages to and from the S-CSCF. It also performs security, QoS, and policy enforcement functions.
- I-CSCF: The I-CSCF is the entry point for the incoming SIP requests from other networks or domains. It acts as an interrogator and queries the Home Subscriber Server (HSS) to find out the appropriate S-CSCF for the user. It also performs load balancing and routing functions.
- S-CSCF: The S-CSCF is the main control point for the user’s session and service interactions. It registers the user with the IMS network and maintains the user’s profile and state information. It also interacts with various application servers to provide the user with the requested services, such as voice, video, messaging.