SCSCF

The Serving Call Session Control Function (S-CSCF) is a core component of the IP Multimedia Subsystem (IMS) architecture. It functions as the central SIP signaling control node for a subscriber’s session, handling registration, routing of SIP messages, and basic service logic. The S-CSCF does not execute application-level logic itself; instead, it uses a subscriber’s service profile and service policy, obtained from the Home Subscriber Server (HSS), to route signaling to the appropriate Application Servers (AS) that implement specific services.

A primary role of the S-CSCF is to determine and apply service routing using the Initial Filter Criteria (IFC) associated with the subscriber. It maintains per-session state, forwards signaling along the SIP path (between P-CSCF, I-CSCF, and AS), and ensures the correct service logic is invoked throughout the session. In roaming scenarios, a visiting S-CSCF in the visited network may serve the session, with the HSS and home network providing the necessary subscriber information to ensure proper service delivery and policy enforcement.

The S-CSCF communicates with the HSS via the Diameter-based Cx interface to fetch subscriber data, authorize usage, and obtain service profiles. It also interfaces with Application Servers using SIP to enable features such as presence, conferencing, messaging, and other multimedia services. As a central signaling control point, the S-CSCF contributes to policy enforcement and reliable session management within IMS, supporting operator-defined features while maintaining session continuity and correct routing across the signaling path.