Basic Call State Model

Description

The Basic Call State Model (BCSM) is a core concept within the 3GPP Intelligent Network (IN) architecture, specifically defined in the CAMEL (Customised Applications for Mobile network Enhanced Logic) specifications. It provides an abstract, standardized representation of the call and session control logic from the perspective of the Service Switching Function (SSF). The BCSM is essentially a finite state machine that models the progression of a call or session through a series of distinct processing stages, known as Points In Call (PICs). Each PIC represents a significant milestone in call setup, management, or teardown, such as 'Authorize_Origination_Attempt', 'Collect_Information', 'Analyze_Information', 'Routing_and_Alerting', and 'Active'.

Between these PICs are Detection Points (DPs). DPs are specific events or conditions during call processing where the SSF can suspend its normal procedure and interact with an external Service Control Function (SCF). DPs are classified as Trigger Detection Points (TDPs), which are armed statically by subscription data, and Event Detection Points (EDPs), which are armed dynamically by the SCF during an active service logic session. When a DP is encountered and armed, the SSF sends a notification to the SCF, which can then instruct the SSF on how to proceed—for example, to continue, to connect to a different number, or to play an announcement.

Architecturally, the BCSM is implemented within the Service Switching Function (SSF), which resides in network nodes like the Mobile Switching Center (MSC) or the Call Session Control Function (CSCF) in IMS. The SSF uses the BCSM to monitor call states and manage the dialogue with the SCF. There are two primary BCSM instances: the Originating BCSM (O-BCSM), which models the call from the calling party's perspective, and the Terminating BCSM (T-BCSM), which models it from the called party's perspective. This dual-model approach allows service logic to be applied independently based on the subscriber's role in the call.

The BCSM's role is to decouple the basic call processing in the switch from the execution of complex, value-added service logic. By providing a standardized state model and well-defined interaction points (DPs), it enables network operators to deploy and manage a wide array of supplementary services—such as prepaid charging, call forwarding, freephone numbers, and VPN services—in a flexible and vendor-independent manner. The model ensures that service logic applications have a consistent and predictable view of network events, which is crucial for reliable service execution across different network equipment.

Purpose & Motivation

The BCSM was created to address the limitations of traditional telephony switches, where service logic was tightly integrated and hard-coded into the switching software. This made introducing new services a slow, expensive, and vendor-specific process, requiring software upgrades to every switch in the network. The primary motivation was to enable rapid service creation and deployment independent of the underlying switch infrastructure, a concept central to the Intelligent Network (IN) paradigm.

Its creation solved the problem of service portability and flexibility. By defining a standardized abstract model of call states, the BCSM allowed service logic to be developed once and deployed across networks from different vendors. This abstraction layer meant that service applications, running on external Service Control Points (SCPs), only needed to understand the BCSM's states and detection points, not the proprietary internals of each manufacturer's switch. This dramatically reduced time-to-market for new services.

Historically, the BCSM concept was inherited from fixed-line IN standards (ITU-T CS-1) and adapted by 3GPP for mobile networks, most notably within the CAMEL framework for GSM and UMTS. It provided the foundational mechanism for implementing operator-specific, real-time control over mobile calls and sessions, which was essential for advanced billing schemes (like prepaid), location-based services, and personalized call handling—services that were becoming critical competitive differentiators for mobile operators.