Originating IP Multimedia Basic Call State Model

Description

The O-IM-BCSM is a fundamental component within the 3GPP IP Multimedia Subsystem (IMS) architecture, specifically designed for the control plane. It is a formalized model that represents the life cycle of an originating IP multimedia session from the perspective of the network entity controlling the session, typically the S-CSCF (Serving Call Session Control Function). The model is structured as a finite state machine (FSM) consisting of a series of Points In Call (PICs) and Detection Points (DPs). PICs represent stable states in the call setup, modification, or release process, such as 'Authorize_Origination_Attempt' or 'O_Alerting'. DPs are specific events or moments within these states where the processing can be interrupted to allow for external service logic, provided by a CAMEL Service Environment (CSE), to execute. When a DP is armed, the S-CSCF suspends its standard processing and sends a notification to the CSE, which can then instruct the S-CSCF on how to proceed—for example, to continue, release the call, or modify session parameters.

The O-IM-BCSM works in conjunction with the T-IM-BCSM (Terminating IM-BCSM) to provide a complete view of a dialog. Its operation is triggered when a user equipment (UE) initiates a SIP session, such as an INVITE request. The S-CSCF, acting as the originating state machine, progresses the call through the defined PICs. The key interaction occurs at the armed DPs, which are configured based on the subscriber's O-IM-CSI (Originating IP Multimedia CAMEL Subscription Information). This data, downloaded from the HSS, tells the S-CSCF at which DPs to involve the CAMEL service. The service logic, residing in a gsmSCF, can then apply complex services like number translation, call screening, pre-paid charging, or interactive voice response interactions before the call reaches the destination.

Architecturally, the O-IM-BCSM is defined in the S-CSCF's service control logic. It interfaces with the CAMEL Application Part (CAP) over the IMS Service Control (ISC) interface to communicate with the gsmSCF. Its role is critical for migrating traditional circuit-switched intelligent network (IN) services into the packet-switched IMS domain, ensuring service continuity and enabling new, IP-based service innovations. By providing a standardized state model, it ensures interoperability between network equipment from different vendors and allows for the consistent execution of operator-defined services across the network.

Purpose & Motivation

The O-IM-BCSM was created to bridge the gap between traditional circuit-switched telephony intelligence and the new IP-based multimedia world of the IMS. Prior to IMS, advanced telephony services (like pre-paid, freephone, or virtual private networks) were delivered in GSM and UMTS circuit-switched networks using the CAMEL (Customised Applications for Mobile network Enhanced Logic) protocol and Basic Call State Models (BCSMs) for the Mobile Switching Centre (MSC). With the introduction of IMS in 3GPP Release 5, a new control paradigm based on SIP was needed, but operators required a way to port their existing, revenue-generating IN services and service creation environments to this new architecture.

The O-IM-BCSM solves this problem by providing an analogous state model for SIP-based sessions. It allows the existing CAMEL service logic and business rules, often a significant investment for operators, to be reused in the IMS domain with minimal modification. This enabled a smooth transition to all-IP networks. Furthermore, it established a standardized framework for triggering new, IMS-specific services, providing operators with the control and flexibility to create and deploy advanced multimedia services that are network-centric rather than solely handset-based. Its creation was motivated by the need for service transparency, network control, and a clear migration path for intelligent network services, which were essential for operator adoption of IMS technology.

Key Features

• Defines a finite state machine for originating SIP session control
• Integrates CAMEL service logic into IMS call processing
• Utilizes Points In Call (PICs) to represent stable call states
• Employs Detection Points (DPs) for service logic invocation
• Works in tandem with the T-IM-BCSM for complete call modeling
• Enables network-based services like pre-paid charging and call screening for multimedia sessions

Evolution Across Releases

Defining Specifications