Visited Terminating CAMEL Subscription Information

Description

VT-CSI, or Visited Terminating CAMEL Subscription Information, is a critical data structure within the Customised Applications for Mobile networks Enhanced Logic (CAMEL) system. CAMEL is an Intelligent Network (IN) standard for GSM, UMTS, and LTE networks that allows network operators to define and deploy custom value-added services that work consistently even when the subscriber is roaming. VT-CSI specifically pertains to services triggered by mobile-terminating events, such as an incoming voice call, SMS, or packet data session addressed to the subscriber.

Architecturally, VT-CSI is part of the subscriber's profile stored in the Home Location Register (HLR). When a subscriber roams into a new network area, their profile, including any VT-CSI data, is transferred to the Visitor Location Register (VLR) or Serving GPRS Support Node (SGSN) in the visited network as part of the location update procedure. The VT-CSI contains a set of triggers and associated instructions. These triggers are detection points within the call model of the Visited Mobile Switching Center (VMSC) or visited network node. When an incoming call for the subscriber arrives at the VMSC and the call processing state machine hits a detection point configured in the VT-CSI, the VMSC's CAMEL Service Switching Function (gsmSSF) suspends call processing and initiates a dialogue with a designated CAMEL Service Control Function (gsmSCF) back in the subscriber's home network.

How it works involves a real-time signaling exchange. Upon trigger detection, the gsmSSF in the visited network sends an Initial Detection Point (IDP) message to the gsmSCF in the home network via the CAMEL Application Part (CAP) protocol. This message includes details about the call and the subscriber. The home gsmSCF, which hosts the service logic (e.g., a pre-paid balance check), then evaluates the request and sends back instructions in a Connect or Continue message. These instructions can command the visited VMSC to connect the call, apply specific charging, play an announcement, or even redirect the call based on the home operator's service logic. This allows the home operator to retain control over how services are applied to their subscribers, regardless of the visited network's capabilities.

Key components are the HLR (data storage), VLR/SGSN (temporary data storage in visited network), VMSC/gsmSSF (trigger detection and switching), and the home gsmSCF (service logic execution). Its role is to decouple service logic from basic switching functionality, enabling rapid deployment of complex services like pre-paid roaming, incoming call barring, personalized ringing tones, or fraud control for terminating calls. It ensures that a subscriber's experience and the operator's business rules are consistently enforced globally.

Purpose & Motivation

VT-CSI was created to solve a major limitation of early mobile roaming: the inability for a home network operator to apply its own intelligent services to subscribers when they were outside its network. Before CAMEL, a roaming subscriber might lose access to their home pre-paid service or custom call handling features because the visited network had no knowledge of or capability to execute those services. This created a poor user experience and restricted revenue opportunities for operators.

The development of the CAMEL standard, and VT-CSI within it, was motivated by the commercial need for consistent service delivery. It addresses the problem by allowing the home network to 'push' its service logic triggers (CSI) into the visited network. For terminating calls, this meant the home operator could still control how an incoming call was handled—for instance, checking a pre-paid balance before allowing the call to ring the phone, even though all the switching was happening in a foreign network. This enabled the successful launch of pre-paid roaming, a huge market driver.

Historically, it evolved from fixed-line IN concepts like INAP. Its creation standardized the interface (CAP) and data structures (like CSI) so that any 3GPP-compliant visited network could interact with any home network's service control point. This interoperability was essential for global service deployment. VT-CSI specifically focuses on the terminating leg, complementing other CSI types like O-CSI (for originating calls), giving operators comprehensive control over their subscribers' call experiences worldwide.

Key Features

• Stored in HLR and downloaded to VLR/SGSN during roaming registration
• Contains triggers for mobile-terminating call control events in the visited network
• Enables real-time signaling (CAP) between visited gsmSSF and home gsmSCF
• Allows home operator to control service logic for incoming calls to roaming subscribers
• Essential for implementing pre-paid charging on terminating calls while roaming
• Supports custom services like call screening, number translation, and advice of charge

Evolution Across Releases

Defining Specifications