IP Multimedia Basic Call State Model

Description

The IP Multimedia Basic Call State Model (IM-BCSM) is a conceptual model defined by 3GPP to represent the lifecycle of an IP multimedia session within the IMS architecture. It is based on a finite state machine (FSM) that outlines the possible states a call or session can be in, along with the events that trigger transitions between these states. The IM-BCSM is used primarily by the Call Session Control Function (CSCF) and Application Servers (AS) to manage session control logic and execute services consistently across the network.

The model consists of two main halves: the Originating IM-BCSM (O-IM-BCSM) and the Terminating IM-BCSM (T-IM-BCSM). The O-IM-BCSM models the session from the perspective of the originating user, covering states from session initiation to establishment or termination. The T-IM-BCSM models the session from the perspective of the terminating user, handling states from session arrival to answer or release. Each half includes points in call (PICs) which represent specific states, and detection points (DPs) where events can trigger service logic interactions. For example, states include 'Null', 'Authorize_Origination_Attempt', 'Collect_Information', 'Analyze_Information', 'Routing', and 'Alerting'. Detection points allow the invocation of service logic, such as checking for call barring or applying call forwarding rules.

IM-BCSM works by processing SIP signaling messages that correspond to the events in the state model. When a SIP INVITE is received, the CSCF maps it to the appropriate IM-BCSM state transition, moving from 'Null' to 'Authorize_Origination_Attempt'. At each detection point, the CSCF may interact with an Application Server to execute service logic, which can influence the session flow—for instance, redirecting the call or applying specific treatments. This structured approach ensures that multimedia sessions are handled uniformly, enabling interoperability between network elements and consistent service behavior. The model's role is critical for implementing standardized supplementary services and custom applications in IMS, providing a clear framework for developers and network operators to design and deploy call control features.

Purpose & Motivation

IM-BCSM was developed to address the need for a standardized call state model in IP multimedia networks, ensuring consistent and interoperable session control. Prior to its introduction, call handling in early IP multimedia systems lacked a unified model, leading to proprietary implementations and difficulties in service integration. As IMS evolved, there was a requirement for a common framework to manage the complexity of SIP-based sessions and enable advanced telephony services.

The creation of IM-BCSM was motivated by the success of the Basic Call State Model (BCSM) used in traditional Intelligent Networks (IN) for circuit-switched calls. By adapting this concept to the IP multimedia domain, 3GPP provided a familiar model for service developers and operators, facilitating the migration of IN services to IMS. IM-BCSM solves problems related to call control ambiguity, service triggering, and feature interaction, allowing for the seamless implementation of services like call waiting, hold, and multiparty conferencing. It supports the evolution towards all-IP networks by providing a robust foundation for service logic execution, essential for commercial deployments of IMS-based voice and multimedia services.

Key Features

• Finite state machine model for IP multimedia session lifecycle management
• Divided into Originating (O-IM-BCSM) and Terminating (T-IM-BCSM) halves
• Defines Points in Call (PICs) for session states and Detection Points (DPs) for service triggers
• Enables consistent call control and service logic execution in IMS networks
• Supports implementation of supplementary services like call forwarding and barring
• Based on principles from Intelligent Network BCSM, adapted for SIP-based sessions

Evolution Across Releases

Defining Specifications