Terminating IP Multimedia CAMEL Subscription Information

Description

Terminating IP Multimedia CAMEL Subscription Information (T-IM-CSI) is a critical data profile defined within the 3GPP architecture, specifically for the IP Multimedia Subsystem (IMS). It is a component of the CAMEL (Customised Applications for Mobile networks Enhanced Logic) service environment, adapted for IMS-based services. T-IM-CSI is stored in the Home Subscriber Server (HSS) as part of a subscriber's service profile. When a terminating IMS session (e.g., an incoming VoIP call or multimedia message) is initiated towards a subscriber, the Serving-Call Session Control Function (S-CSCF) retrieves the T-IM-CSI from the HSS. This retrieval occurs during the registration process or session establishment. The T-IM-CSI contains the address of the appropriate CAMEL Service Environment (CSE) or Application Server (AS) that hosts the service logic for the terminating session.

Upon receiving the T-IM-CSI, the S-CSCF uses the information to trigger the execution of specific service logic for the terminating leg of the session. This is achieved by forwarding the session signaling (e.g., SIP INVITE) to the designated Application Server identified in the T-IM-CSI. The AS then executes the custom service logic, which can involve a wide range of functionalities. The AS interacts with the S-CSCF using the IMS Service Control (ISC) interface, allowing it to influence the session routing, modify signaling, or interact with other network elements like charging systems.

The role of T-IM-CSI is fundamental for enabling operator-specific and subscriber-specific services on the terminating side in an IMS network. It decouples service logic from core session control, promoting a flexible service architecture. Key components involved include the HSS (for storage), the S-CSCF (for retrieval and triggering), and the Application Server (for logic execution). This mechanism ensures that services like terminating call screening, number translation, prepaid billing for received calls, or special alerting tones can be applied consistently based on the subscriber's profile, even in a complex, SIP-based IMS environment.

Purpose & Motivation

T-IM-CSI was created to extend the proven CAMEL-based intelligent network (IN) service control paradigm into the all-IP world of the IP Multimedia Subsystem (IMS). Prior to IMS, CAMEL was extensively used in circuit-switched (CS) and packet-switched (PS) domains for services like prepaid, freephone, and virtual private networks. As networks evolved towards IMS for delivering voice, video, and messaging over IP, there was a clear need to support the same sophisticated, operator-controlled service logic for sessions terminating at a subscriber.

Without T-IM-CSI, applying custom logic to an incoming IMS session would require hard-coding service triggers into the CSCF or using less flexible mechanisms, limiting service innovation and personalization. T-IM-CSI solves this by providing a standardized, profile-driven method to invoke external service logic. It addresses the problem of service portability and consistency, allowing operators to migrate their existing CAMEL-based terminating services (e.g., intelligent call forwarding) into the IMS domain without requiring a complete service logic rewrite. Its creation was motivated by the desire to leverage existing IN investments and expertise while transitioning to a next-generation network architecture, ensuring a smooth evolution and rich service continuity for subscribers.

Key Features

• Stored in the HSS as part of the IMS subscriber profile
• Triggered by the S-CSCF for terminating IMS session attempts
• Contains the address (e.g., SIP URI) of the applicable Application Server
• Enables CAMEL-style service control for SIP-based sessions
• Supports operator-defined logic for call handling, charging, and routing on the terminating side
• Utilizes the IMS Service Control (ISC) interface between S-CSCF and AS

Evolution Across Releases

Defining Specifications