Description
The Serving Call Session Control Function (S-CSCF) is the central processing node within the IP Multimedia Subsystem (IMS) architecture. It acts as a SIP registrar and SIP proxy server, handling all session control for IMS-registered users within its assigned home network. Upon user registration, the S-CSCF downloads the user's service profile from the Home Subscriber Server (HSS) and uses this information to apply service logic, trigger application servers, and manage the user's service sessions. It is responsible for routing SIP messages, interacting with other CSCFs (P-CSCF, I-CSCF), and enforcing the policies of the network operator.
Architecturally, the S-CSCF is a stateful SIP server that maintains the registration state and session state for its assigned users. It performs session establishment, modification, and termination procedures. A key component of its operation is the Initial Filter Criteria (iFC), which are part of the user's service profile. The iFC contains a set of rules that determine when and to which Application Servers (AS) SIP messages should be forwarded for value-added service execution, such as call forwarding, voicemail, or presence services.
Its role extends beyond basic SIP routing; it is integral to security, performing authentication and authorization of users in conjunction with the HSS. It also handles charging functions by generating Charging Data Records (CDRs) and interfaces with the Policy and Charging Rules Function (PCRF) for policy control related to bearer resources. The S-CSCF is a critical element for enabling rich communication services, ensuring interoperability, and providing a platform for service innovation in 4G, 5G, and fixed networks.
Purpose & Motivation
The S-CSCF was created to provide a standardized, SIP-based core for delivering multimedia services over IP networks, independent of the underlying access technology (e.g., LTE, 5G NR, Wi-Fi). Prior to IMS, telecom services were tightly coupled with circuit-switched networks, making service innovation slow and access-dependent. The S-CSCF solves this by centralizing session control, separating the service layer from the connectivity layer, and enabling the convergence of voice, video, and messaging over IP.
Its creation was motivated by the need for a flexible, scalable architecture to support the vision of 'all-IP' networks. It addresses the limitations of previous siloed service platforms by providing a common service delivery engine. This allows operators to deploy services like Voice over LTE (VoLTE), Rich Communication Services (RCS), and eventually Voice over New Radio (VoNR) efficiently, using a single core network that can serve multiple access types and facilitate interoperability between different operators' networks.
Architecture
In the Network Map
- Mobile Network → IMS Core → S-CSCF (Session Control)
Classification
Release Timeline
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (8 CRs across 3 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 16, enhancements were introduced for the S-CSCF focusing on improved reliability and selection mechanisms. Specifically, the release added procedures for S-CSCF Restoration, including restoration after registration timer expiry, to maintain service continuity. It also defined refinements for S-CSCF Selection and the handling of the S-CSCF name.
In Release 17, enhancements were made to the S-CSCF to improve network resilience and session continuity. Specifically, the release introduced procedures for S-CSCF Restoration Information Retrieval to support recovery scenarios. It also defined mechanisms for S-CSCF reselection within an eIMS (evolved IMS) architecture.
In Release 19, the enhancements for the S-CSCF focused on refining registration and deregistration procedures. Specifically, a correction was made to the S-CSCF registration procedure within the Nhss_imsUECM interface. Additionally, the standard introduced a defined mechanism for S-CSCF Deregistration Notification.
Explore further
Broader topics and technologies where S-CSCF plays a role.
Defining Specifications
3GPP specifications that define or reference S-CSCF, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 23.127 v1600 | Virtual Home Environment Stage 2 Specification | Rel-6 |
| TS 23.141 vj00 | Presence Service Stage 2 Architecture | Rel-19 |
| TS 23.179 vd50 | MCPTT Functional Architecture | Rel-13 |
| TS 23.198 v1900 | Open Service Access (OSA); Stage 2 | Rel-9 |
| TS 23.218 vj00 | IMS Call Model Specification | Rel-19 |
| TS 23.278 vj00 | CAMEL for IMS Stage 2 Specification | Rel-19 |
| TS 23.280 vk10 | Common Architecture for Mission Critical Services | Rel-20 |
| TS 23.379 vk00 | MCPTT Functional Architecture | Rel-20 |
| TS 23.417 v1700 | IMS Core Component for NGN Architecture | Rel-7 |
| TS 23.517 v1800 | IMS Core Component for NGN Architecture | Rel-8 |
| TS 23.701 vc00 | WebRTC Access to IMS Architecture Study | Rel-12 |
| TR 23.794 vh00 | Study on enhanced IMS to 5GC integration | Rel-17 |
| TS 23.849 vb00 | Study on IMS Roaming Media Optimization | Rel-11 |
| TR 23.976 vj00 | Push Service Requirements Analysis | Rel-19 |
| TS 24.141 vj00 | Presence Service Protocol Details | Rel-19 |
| TS 24.147 vj00 | IMS Conferencing Protocol Details | Rel-19 |
| TS 24.173 vj00 | Multimedia Telephony Service and Supplementary Services in IMS | Rel-19 |
| TS 24.229 vj50 | IMS call control protocol based on SIP and SDP | Rel-19 |
| TS 24.259 vj00 | Personal Network Management (PNM) Protocol Details | Rel-19 |
| TS 24.404 v1700 | Communication Diversion Services (CDIV) | Rel-7 |
| TS 24.406 v810 | Message Waiting Indication (MWI) Protocol | Rel-8 |
| TS 24.411 v1830 | ACR and CB Service Protocol Specification | Rel-8 |
| TS 24.416 v1700 | Malicious Call Identification Service | Rel-7 |
| TS 24.428 v1700 | Common Basic Communication Procedures | Rel-7 |
| TS 24.447 v800 | Advice Of Charge (AOC) Service Protocol | Rel-8 |
| TS 24.454 v840 | Closed User Group (CUG) Protocol Specification | Rel-8 |
| TS 24.504 v8m0 | Communication Diversion Services Stage 3 | Rel-8 |
| TS 24.508 v820 | TIP and TIR Service Protocol Description | Rel-8 |
| TS 24.516 v830 | MCID Protocol Specification for NGN | Rel-8 |
| TS 24.523 vj00 | NGCN-NGN Interconnection Scenarios | Rel-19 |
| TS 24.524 vj00 | Hosted Enterprise Services Architecture | Rel-19 |
| TS 24.528 v830 | Common Basic Communication Procedures for IMS Services | Rel-8 |
| TS 24.604 vj00 | Communications Diversion (CDIV) Protocol Spec | Rel-19 |
| TS 24.606 vj00 | MWI Service Protocol Description | Rel-19 |
| TS 24.608 vj00 | 3GPP TS 24608: TIP/TIR Services Protocol | Rel-19 |
| TS 24.611 vj00 | Anonymous Communication Rejection & Barring | Rel-19 |
| TS 24.616 vj00 | Malicious Call Identification (MCID) Protocol | Rel-19 |
| TS 24.628 vj00 | Common Basic Communication Procedures in IMS | Rel-19 |
| TS 24.647 vj00 | Advice of Charge (AOC) service protocol | Rel-19 |
| TS 24.802 vc10 | IMS II-NNI Traversal Scenario Determination Study | Rel-12 |
| TS 24.819 v1700 | IMS Services via Fixed Broadband Access | Rel-7 |
| TS 24.841 v1600 | Presence Service IP Multimedia Subsystem | Rel-6 |
| TS 24.880 v1820 | Media Server Control Protocol Study | Rel-8 |
| TR 24.930 vj00 | IMS Session Setup Signalling Flows | Rel-19 |
| TS 28.705 vj00 | IMS NRM IRP Information Service | Rel-19 |
| TS 29.165 vj10 | Inter-IMS Network to Network Interface (NNI) | Rel-19 |
| TS 29.228 vj20 | Cx and Dx Interface Signaling Flows | Rel-19 |
| TS 29.229 vj10 | Diameter Protocol for Cx/Dx Interfaces | Rel-19 |
| TS 29.328 vj20 | Sh and Dh Interfaces: HSS-AS Interactions | Rel-19 |
| TS 29.562 vj40 | HSS Services for IMS & GBA Interworking | Rel-19 |
| TS 29.826 vd10 | P-CSCF Restoration Enhancements for WLAN | Rel-13 |
| TS 29.866 vj00 | IMS Disaster Prevention & Restoration Enhancement | Rel-19 |
| TR 29.949 vj00 | VoLTE IMS Roaming Architecture & Procedures | Rel-19 |
| TS 32.102 vj00 | Telecom Management Physical Architecture Framework | Rel-19 |
| TS 32.409 vj00 | IMS Performance Management Measurements | Rel-19 |
| TS 32.808 v1800 | Common User Profile Storage Framework | Rel-8 |
| TS 33.107 vj00 | Lawful Interception Architecture & Functions | Rel-19 |
| TS 33.127 vj50 | Lawful Interception Architecture and Functions | Rel-19 |
| TS 33.804 vc00 | Non-UICC SSO using SIP Digest credentials | Rel-12 |
| TS 33.871 vc00 | Security for WebRTC IMS Client Access | Rel-12 |
