Dialled Services CAMEL Subscription Information

Description

Dialled Services CAMEL Subscription Information (D-CSI) is a specific type of CAMEL subscription data stored in the Home Location Register (HLL) for a mobile subscriber. It is part of the broader CAMEL (Customised Applications for Mobile networks Enhanced Logic) framework, which allows network operators to deploy intelligent network (IN) services in a GSM/UMTS mobile environment. The core function of D-CSI is to trigger CAMEL service logic for mobile-originated calls based on the dialled destination number. When a subscriber initiates a call, the Visited Mobile Switching Center (VMSC) serving the subscriber queries the HLR for subscriber data. If D-CSI is present in the subscriber's profile, the HLR includes it in the response sent to the VMSC.

Architecturally, D-CSI operates within the CAMEL Application Part (CAP) protocol framework between the VMSC (acting as the gsmSSF - GSM Service Switching Function) and a dedicated external node called the gsmSCF (GSM Service Control Function). The D-CSI data itself contains critical parameters such as the gsmSCF address (the E.164 number or IP address of the service control node), the Service Key (identifying the specific CAMEL service to invoke), and, most importantly, a set of Dialled Number Triggers. These triggers are typically defined as lists of destination numbers or number ranges (e.g., premium rate numbers, international prefixes, specific service numbers). The D-CSI may also include other CAP-specific parameters like default call handling instructions and the CAMEL capability handling indicator.

When the VMSC receives a call setup request from the subscriber, it compares the dialled number against the triggers provided in the D-CSI. If a match is found, the VMSC suspends the normal call processing and initiates a CAMEL dialogue with the designated gsmSCF. The VMSC sends an InitialDP (Initial Detection Point) CAP operation to the gsmSCF, including the dialled number, calling party number, and other relevant call details. The gsmSCF, hosting the custom service logic (e.g., for prepaid credit control, call barring, or number translation), then takes control of the call. It can instruct the VMSC to continue, release, or modify the call (e.g., connect to a different destination, play an announcement) through subsequent CAP operations like RequestReportBCSMEvent, ApplyCharging, and Connect.

D-CSI's role is distinct from other CAMEL triggers like O-CSI (for originating calls) or T-CSI (for terminating calls). It provides granular, number-specific control for outgoing calls. This allows operators to apply different charging rates, barring policies, or service treatments depending on whether a user is calling a local number, an international number, or a specific service like voicemail or customer care. Its implementation is a cornerstone for enabling advanced, real-time, subscriber-specific services that are not natively supported by the basic GSM/UMTS call control protocols, effectively extending the network's service capabilities without requiring upgrades to every switching node.

Purpose & Motivation

D-CSI was created to address the need for intelligent, destination-aware call control and charging in GSM and early UMTS networks. Before CAMEL and triggers like D-CSI, mobile network services were largely static and based on subscription profiles in the HLR/VMSC. Implementing dynamic services like real-time prepaid billing, call screening based on dialled numbers, or custom call routing required proprietary, vendor-specific solutions that were difficult to standardize and interoperate across different network equipment from multiple manufacturers.

The introduction of D-CSI, as part of CAMEL Phase 2 and 3 (specified in 3GPP Release 4 and 5), provided a standardized mechanism to offload complex service logic from the core switches (MSCs) to dedicated, centralized service platforms (gsmSCFs). This solved the problem of slow service deployment and vendor lock-in. Specifically, D-CSI solved the problem of applying different policies to different types of called numbers. For example, an operator could use D-CSI to ensure that calls to premium-rate numbers are only allowed if the prepaid subscriber has sufficient credit, while calls to emergency numbers are always allowed without any credit check. It enabled the creation of sophisticated tariff plans where the cost per minute depends on the destination number dialled.

Historically, D-CSI's creation was motivated by the commercial success of prepaid mobile services and the need for more flexible service creation environments. It allowed operators to rapidly deploy and monetize new value-added services, such as dialled-number-based loyalty discounts, parental controls (barring calls to certain numbers), and intelligent call forwarding based on the dialled destination. By standardizing this trigger mechanism, 3GPP ensured that service logic developed for one network could, in principle, work with switching equipment from another vendor, fostering competition and innovation in the intelligent network services market.

Key Features

• Triggers CAMEL service invocation based on specific dialled destination numbers or number ranges
• Contains the address (gsmSCF address) of the external service control node hosting the intelligent logic
• Includes a Service Key parameter to identify the specific CAMEL application to execute
• Allows for real-time, gsmSCF-controlled call handling, charging, and interaction for mobile-originated calls
• Enables destination-specific billing policies, such as different rates for local, national, and international calls
• Supports advanced services like prepaid billing, call screening/barring, and dialled number translation

Evolution Across Releases

Defining Specifications