IP Multimedia CAMEL Subscription Information

Description

IP Multimedia CAMEL Subscription Information (IM-CSI) is a critical data element stored in the Home Subscriber Server (HSS) as part of a user's IMS service profile. It contains the necessary CAMEL service logic triggers and addresses (specifically, the gsmSCF address) required to invoke CAMEL Application Part (CAP) based services for IMS sessions. When a user initiates or receives an IMS session (like a VoIP call or video session), the Serving-Call Session Control Function (S-CSCF) retrieves the user's profile from the HSS. If the IM-CSI is present, the S-CSCF uses the information to establish a dialogue with the designated CAMEL Service Environment (CSE) or Service Control Function (gsmSCF). This enables the gsmSCF to exert control over the IMS session, similar to how CAMEL controls circuit-switched calls, allowing for real-time service logic execution.

The architecture involves the HSS, S-CSCF, and the gsmSCF. The IM-CSI data includes a Service Key, gsmSCF address, and specific Detection Points (DPs) that define when the S-CSCF should contact the gsmSCF—typically at session establishment, modification, or termination events. Upon encountering a configured DP, the S-CSCF sends a CAP InitialDP or equivalent operation to the gsmSCF. The gsmSCF can then instruct the S-CSCF on how to proceed with the session, such as applying charging advice, redirecting the call, or interacting with other network elements for service blending. This mechanism allows the IMS core to be controlled by external, operator-defined service logic.

Its role is to facilitate the migration and integration of intelligent network (IN) services from the circuit-switched domain into the packet-switched IMS domain. It ensures that operators can leverage existing CAMEL investments and service creation environments to offer advanced, real-time IMS services like prepaid multimedia telephony, freephone numbers for video calls, or customized call routing based on IMS presence. The IM-CSI thus acts as a bridge, enabling CAMEL's robust service control paradigm to govern SIP-based multimedia sessions, providing a consistent service experience across network domains.

Purpose & Motivation

IM-CSI was created to address the need for intelligent network service control within the IP Multimedia Subsystem (IMS), which was introduced in 3GPP Release 5. Prior to IMS, CAMEL was extensively used in 2G and 3G circuit-switched networks to provide operator-specific, value-added services like prepaid, number translation, and call screening. As networks evolved towards all-IP architectures with IMS for multimedia services, there was a clear requirement to extend this proven service control framework to the new SIP-based session domain.

The primary problem it solves is enabling seamless service continuity and feature parity. Without IM-CSI, operators would have been forced to develop entirely new, separate service control mechanisms for IMS, duplicating effort and potentially creating service gaps. IM-CSI allows the reuse of existing CAMEL service platforms (gsmSCFs) and service logic, protecting investments and accelerating the deployment of IMS services. It specifically solves the challenge of applying real-time, event-driven control to IMS sessions, which are inherently more complex than simple voice calls, involving multiple media components and codecs.

Historically, its introduction in Release 99 was a foundational step for IMS service enablement, motivated by the desire for a smooth transition from circuit-switched intelligent networks to packet-switched multimedia services. It addressed the limitation of earlier CAMEL specifications, which were designed only for circuit-switched call control and SMS, by defining the triggers and procedures needed for SIP session control. This ensured that operators could offer familiar intelligent services (e.g., prepaid) for new IMS-based voice, video, and messaging, driving IMS adoption and commercial viability.

Key Features

• Enables CAMEL service control for IMS SIP-based sessions
• Stored as part of the user's IMS subscription profile in the HSS
• Contains the gsmSCF address and service key for service invocation
• Defines Detection Points for IMS session events (e.g., session setup)
• Allows the gsmSCF to control session routing, charging, and termination
• Supports integration of legacy IN services with next-generation IMS networks

Evolution Across Releases

Defining Specifications