Originating Basic Call State Model

Description

The Originating Basic Call State Model (O-BCSM) is a core component of the CAMEL (Customised Applications for Mobile networks Enhanced Logic) service architecture, defined as a finite state machine (FSM) that models the progression of an outgoing mobile-originated call. It resides within the Service Control Function (SCF) and interacts with the Service Switching Function (SSF) in the mobile switching center (MSC) or GMSC. The model consists of a series of interconnected call states, such as 'Authorize_Origination_Attempt', 'Collect_Information', 'Analyze_Information', 'Routing_&_Alerting', and 'Active'. At key junctures between these states, known as Detection Points (DPs), the SSF can suspend call processing and request instructions from the SCF. This allows the CAMEL service logic, residing in a dedicated Service Control Point (SCP), to influence the call—for example, by modifying the dialed number, applying charging, or denying the call based on subscriber-specific logic.

The operation of the O-BCSM is event-driven. When a mobile subscriber initiates a call, the MSC/SSF triggers the O-BCSM and begins traversing its states. At each configured DP, the SSF evaluates whether it is armed for notification. If armed, it sends a message to the SCF containing call details. The SCF then executes its service logic and returns a response, which could be a 'Continue' instruction to proceed with normal call handling, a 'Request Report' to monitor for further events, or a specific command like 'Connect' or 'ReleaseCall'. This interaction is standardized through the CAMEL Application Part (CAP) protocol. The O-BCSM's precise definition ensures consistent service behavior across different network vendors and operators.

The model's architecture is designed for flexibility and control. It includes two types of Detection Points: Trigger Detection Points (TDPs), which are statically configured in the subscriber's profile (O-CSI) and armed at call setup, and Event Detection Points (EDPs), which can be dynamically armed by the SCF during the dialog. This allows for both predefined service triggers and dynamic, mid-call service interactions. The O-BCSM is strictly concerned with the call control plane; it does not handle the actual voice bearer path. Its primary role is to provide a standardized framework for the SSF to report call events and for the SCF to exert control, enabling the creation of complex, network-based intelligent services without modifying the handset.

Purpose & Motivation

The O-BCSM was created to provide a standardized, vendor-independent model for implementing intelligent network (IN) services in GSM and UMTS networks, specifically for mobile-originated calls. Prior to CAMEL and models like the O-BCSM, advanced telephony services (like freephone numbers or virtual private networks) were largely fixed within the switch software, making them difficult to create, modify, or port across different network equipment vendors. This lack of flexibility hindered rapid service deployment and innovation.

The O-BCSM, as part of the CAMEL framework, solves this by decoupling the service logic from the switching infrastructure. It allows operators to host services on centralized, scalable SCPs that can control calls in any MSC by interacting with the standardized state model. This enables the creation of subscriber-specific, real-time services such as prepaid billing, call screening, number translation, and location-based routing. The model's definition of specific detection points provides a clear contract between the switch and the service logic, ensuring reliable and predictable service execution.

Its introduction in 3GPP Release 4 was a key step in evolving mobile networks beyond basic voice telephony into programmable service platforms. It addressed the business need for operators to differentiate themselves through custom services and to implement complex charging schemes like prepaid, which was a major driver for mobile adoption in many markets. The O-BCSM provided the essential control mechanism to make these business models technically feasible on a large scale.