Terminating Basic Call State Model

Description

The Terminating Basic Call State Model (T-BCSM) is a fundamental component of the Customised Applications for Mobile network Enhanced Logic (CAMEL) protocol suite, standardized in 3GPP. It provides a formal, state-machine representation of the process for handling an incoming call destined for a subscriber within a GSM or UMTS network. The model is implemented within the Gateway Mobile Switching Centre (GMSC) or Visited MSC, which is the network node responsible for routing calls to the subscriber's current location. The T-BCSM defines a series of distinct 'Pics' (Points In Call) and detection points. Pics represent stable states in the call processing, such as 'Terminating Call Handling' where the MSC is attempting to locate and alert the subscriber. Detection points are specific events within or between these states where processing can be suspended, allowing the MSC to interact with an external service control function, like a CAMEL Service Environment (CSE).

When a call arrives at the GMSC, the call processing logic follows the T-BCSM. At predefined detection points (e.g., 'Terminating Attempt Authorized'), the GMSC can send a notification to the CSE. The CSE, which contains the subscriber's specific service logic (defined by T-CSI), can then instruct the GMSC on how to proceed. This might involve modifying call parameters, applying call forwarding rules, or implementing other value-added services. The interaction uses the CAMEL Application Part (CAP) protocol. The T-BCSM ensures that these interactions occur at consistent, well-defined moments, guaranteeing reliable service execution across different network implementations.

The model includes several key detection points like Collected Information, Analyzed Information, Terminating Call Handling, and Call Accepted. Each has specific triggers and outcomes. The architecture separates the basic switching function (MSC) from the intelligent service logic (CSE), promoting flexibility and allowing operators to introduce new services without modifying every core network switch. The T-BCSM, along with its counterpart O-BCSM (Originating BCSM), forms the core signaling framework for CAMEL-based prepaid, call screening, number translation, and other real-time services.

Purpose & Motivation

The T-BCSM was created to standardize the control of intelligent network services for terminating calls in mobile networks. Prior to CAMEL, implementing value-added services like call forwarding or incoming call screening was often proprietary and tightly integrated with switch vendor software, making deployment slow and expensive. The CAMEL initiative, inspired by fixed-line Intelligent Network (IN) concepts, aimed to decouple service logic from switching hardware. The T-BCSM provides the precise 'hook' points in the terminating call sequence where this external logic can intervene.

By defining a universal state model, 3GPP enabled interoperability between MSCs from different manufacturers and external service platforms. This allowed operators to roll out subscriber-specific services consistently across their network. It solved the problem of how to apply custom rules (e.g., 'forward all calls to voicemail after 5 rings') at the right moment during call delivery, especially when the subscriber might be roaming. The model ensures that the service control interactions are predictable and reliable, which is critical for revenue-generating services like prepaid incoming call authorization.