Trunk Originated CAMEL Service Information

Description

Trunk Originated CAMEL Service Information (TO-CSI) is a specific type of CAMEL Subscription Information (CSI) stored in the Home Location Register (HLR) for a mobile subscriber. CAMEL (Customised Applications for Mobile network Enhanced Logic) is an intelligent network (IN) standard that allows operators to define custom call control services. The TO-CSI parameter is crucial for applying these intelligent services to incoming calls destined for the subscriber, specifically those calls that originate from outside the mobile network, typically from a trunk interface connected to the Public Switched Telephone Network (PSTN) or another network.

Architecturally, when a call arrives at the Gateway MSC (GMSC) from a trunk line, the GMSC queries the HLR for routing information (the Mobile Station Roaming Number - MSRN). As part of the subscriber's profile, the HLR may send the TO-CSI data to the GMSC in the response. The TO-CSI contains key information such as the address of the appropriate CAMEL Service Environment (gsmSCF) that hosts the service logic, the Service Key identifying the specific service, and default call handling parameters. Upon receiving the TO-CSI, the GMSC, which acts as the CAMEL Service Switching Function (gsmSSF), initiates a dialogue with the designated gsmSCF. It reports the incoming call event and hands over control, allowing the gsmSCF to instruct the GMSC on how to process the call based on the subscribed service logic.

How it works involves real-time interaction between network nodes. For example, a subscriber might have a prepaid service that also applies to incoming calls. When an incoming call from the PSTN arrives, the GMSC, armed with the TO-CSI, contacts the prepaid gsmSCF. The gsmSCF can then check the subscriber's account balance, apply specific routing (e.g., forward to voicemail if balance is low), or implement number translation services. The gsmSCF sends instructions back to the GMSC to continue, release, or modify the call. This mechanism ensures that intelligent network features are seamlessly applied to inbound, trunk-originated calls, providing a consistent service experience regardless of the call's origin.

Purpose & Motivation

TO-CSI was created to extend the powerful service control capabilities of the CAMEL intelligent network platform to inbound call scenarios. Early CAMEL services primarily focused on controlling calls originated by the mobile subscriber (Mobile Originated - MO). However, a significant portion of a subscriber's experience—and an operator's service offering—involves receiving calls. Without TO-CSI, intelligent services like prepaid incoming call barring, customized call forwarding for incoming calls, or incoming call screening could not be implemented in a standardized, network-based manner.

It solves the problem of applying operator-defined logic to calls coming from outside the mobile network, specifically from trunk interfaces that connect to fixed networks or other operators. Prior to its introduction, handling such calls was largely static, based on simple HLR data like Call Forwarding Unconditional (CFU). TO-CSI enabled dynamic, real-time control by a service logic server. This was particularly critical for the business model of prepaid services, where operators needed to control the cost of incoming calls (which they often bear) based on the subscriber's account status.

The motivation was driven by the commercial need for complete service parity between incoming and outgoing calls within prepaid and other value-added services. Its introduction in Release 7 was part of the broader enhancement of CAMEL to support IP Multimedia Subsystem (IMS) and more complex service interactions, ensuring that the intelligent network framework remained comprehensive and applicable to all call legs, thereby protecting operator revenue and enabling richer service features.

Key Features

• Enables CAMEL-based intelligent network services for incoming calls from trunk/PSTN origins.
• Stored as part of the subscriber's profile in the Home Location Register (HLR).
• Contains the address of the CAMEL Service Environment (gsmSCF) and a Service Key.
• Downloaded to the Gateway MSC (GMSC) during the call routing query procedure.
• Allows the gsmSCF to apply real-time service logic (e.g., prepaid check, special routing) to the inbound call.
• Essential for complete prepaid service implementation, covering both outgoing and incoming calls.

Evolution Across Releases

Defining Specifications