H-CSCF (Home Call Session Control Function) — 3GPP Glossary

The H-CSCF is a core IMS network element that serves as the home network's primary point of contact for SIP signaling. It handles session control, service triggering, and interacts with application servers. It is fundamental for establishing and managing multimedia sessions in the IMS architecture.

Description

The Home Call Session Control Function (H-CSCF) is a critical component within the IP Multimedia Subsystem (IMS) architecture defined by 3GPP. It acts as the central SIP (Session Initiation Protocol) server in the user's home network. When a user initiates or receives a call or multimedia session, the SIP signaling traverses through the H-CSCF. Its primary role is to perform session control, which includes processing SIP registrations, SIP session establishment (INVITE), modification, and termination. The H-CSCF interrogates the Home Subscriber Server (HSS) to download the user's service profile, which contains Initial Filter Criteria (iFC). These iFC are a set of triggers that determine which application servers (AS) need to be involved in the session based on the SIP request. This enables the orchestration of value-added services like voicemail, call forwarding, or video conferencing.

Architecturally, the H-CSCF resides in the control plane of the IMS core. It interfaces with several other network functions. The Cx interface connects it to the HSS for user authentication and service profile retrieval. The Sh interface may be used for communication with application servers for service data. The H-CSCF also communicates with the Serving-CSCF (S-CSCF), which is the session control entity in the visited network when the user is roaming, and the Interrogating-CSCF (I-CSCF), which acts as the first point of contact within an operator's network. The H-CSCF's decisions are driven by the user's subscription data, allowing for personalized service delivery.

In terms of operation, upon receiving a SIP REGISTER request, the H-CSCF authenticates the user via the HSS and registers the user's contact information. For a SIP INVITE, it evaluates the iFC. If a trigger condition is met—for example, the called party number matches a specific pattern—the H-CSCF will proxy the SIP request to the designated application server. The AS can then provide the service (like playing an announcement or modifying the session parameters) before the H-CSCF forwards the request towards its final destination. This model separates call control from service logic, a key principle of IMS that fosters service innovation and flexibility. The H-CSCF also plays a role in policy control, interacting with the Policy and Charging Rules Function (PCRF) via the Rx interface to ensure the session adheres to the subscriber's QoS and charging policies.

Purpose & Motivation

The H-CSCF was introduced to address the limitations of traditional circuit-switched telephony networks, which tightly coupled service logic with switching infrastructure. This made introducing new multimedia services slow and expensive. The creation of the IMS architecture, with the H-CSCF at its heart, was motivated by the need for a standardized, IP-based platform to deliver converged voice, video, and data services. It enables fixed-mobile convergence and provides a consistent service experience regardless of the access network (e.g., LTE, 5G, Wi-Fi).

Its primary purpose is to serve as the home network's anchor point for SIP-based session control. This solves the problem of how to manage subscriber identity, service profiles, and application triggering in a scalable, network-agnostic way. By centralizing this logic in the home network, operators can maintain control over service delivery even when users are roaming. The H-CSCF, in conjunction with the S-CSCF and I-CSCF, provides a robust framework for routing SIP messages, enforcing security policies, and integrating with a wide ecosystem of application servers, thereby future-proofing the network for emerging multimedia applications.

Key Features

SIP Session Control for registration, initiation, modification, and termination
Service Triggering via Initial Filter Criteria (iFC) downloaded from the HSS
Subscriber Authentication and Authorization via the Cx interface to HSS
Integration with Application Servers for value-added services
Policy Enforcement via interaction with the PCRF over the Rx interface
Anchoring point for the user's service profile in the home network

Evolution Across Releases

Rel-4 Initial

Introduced as a core component of the initial IMS architecture. Defined its basic functions: SIP registrar, session control, and service triggering based on initial filter criteria. Established interfaces like Cx to HSS and ISC to application servers.

TS 21.905

Defining Specifications

Specification	Title
TS 21.905	3GPP TS 21.905