Serving Call Session Control Function

Description

The Serving Call Session Control Function (SCSCF) is a fundamental component within the 3GPP-defined IP Multimedia Subsystem (IMS) architecture. It operates as a SIP (Session Initiation Protocol) server and acts as the central point for session control and service delivery for IMS subscribers. Upon user registration, the SCSCF authenticates the user via the HSS (Home Subscriber Server) and downloads the user's service profile, which includes initial Filter Criteria (iFC). These iFC are used to determine how incoming and outgoing SIP sessions should be routed to appropriate Application Servers (AS) for value-added services.

During a session establishment, the SCSCF performs routing decisions based on the requested service identifier and the user's profile. It interacts with other IMS core functions such as the P-CSCF (Proxy-CSCF), which is the first contact point for the UE, and the I-CSCF (Interrogating-CSCF), which is responsible for selecting an appropriate SCSCF during registration. The SCSCF also enforces the network's policy by interacting with the PCRF (Policy and Charging Rules Function) for resource authorization and quality of service (QoS) management.

Its role extends beyond basic call routing; it is responsible for maintaining the registration state, managing SIP dialogues, and generating charging data records (CDRs) for billing purposes. The SCSCF is designed to be stateless for individual transactions but stateful for dialogues, allowing for scalability and reliability. It is a mandatory element for enabling standardized multimedia services such as Voice over LTE (VoLTE), Voice over NR (VoNR), Rich Communication Services (RCS), and other real-time IP-based communication services across mobile and fixed networks.

Purpose & Motivation

The SCSCF was introduced to provide a standardized, IP-based control plane for multimedia services, separating service logic from the underlying transport network. Prior to IMS, voice and messaging services were tightly coupled with circuit-switched networks, limiting innovation and service integration. The SCSCF enables the convergence of voice, video, and data services over a common IP infrastructure, facilitating the transition from legacy circuit-switched networks to all-IP networks.

It solves the problem of fragmented service delivery by providing a central, service-agnostic session control function. This allows network operators and third-party providers to deploy and manage services independently of the access technology (e.g., LTE, 5G NR, Wi-Fi). The SCSCF's use of standardized SIP protocols ensures interoperability between different vendors' equipment and across operator boundaries, which is essential for roaming and global service delivery. Its creation was motivated by the need for a flexible, scalable architecture to support the growing demand for multimedia applications and to enable new revenue streams for operators.

Key Features

• Central SIP session control for IMS
• User authentication and registration state management
• Service triggering based on initial Filter Criteria (iFC)
• Interaction with HSS for subscriber data and Application Servers for services
• Policy enforcement via PCRF interface
• Generation of Charging Data Records (CDRs) for billing

Evolution Across Releases

Defining Specifications