Description
The Terminating IP Multimedia Basic Call State Model (T-IM-BCSM) is a fundamental concept in the 3GPP IP Multimedia Subsystem (IMS) service architecture, specifically within the Camel Application Part (CAP) and IP Multimedia Service Switching Function (IM-SSF) framework. It is a formal state machine defined in 3GPP TS 23.218 and TS 23.278 that models the lifecycle of a terminating SIP session—that is, a session where the called party is a subscriber served by the network node containing this model. The T-IM-BCSM provides a standardized abstraction of the call/ session control process, identifying specific Points In Call (PICs) and Detection Points (DPs).
Architecturally, the T-IM-BCSM is instantiated within a Call Session Control Function (CSCF), or more precisely, its service logic interaction is managed through the IM-SSF when using CAMEL for service control. The model begins with the arrival of an initial SIP request (like an INVITE) destined for a subscriber. It then progresses through states such as 'Authorize_Termination_Attempt', 'Collect_Info', 'Analyze_Info', 'Select_Route', 'Authorize_Call_Setup', and finally 'Active' and 'Disconnect'. Each state represents a specific phase of session establishment. Between these states are Detection Points (DPs)—moments where the call processing can be suspended to interact with external service logic, such as a CAMEL Service Environment (CSE) or an Application Server (AS).
How it works is integral to service triggering. For example, at the DP 'Termination_Attempt_Authorized', the IM-SSF can send a CAP message to a CAMEL service to check if the called subscriber is allowed to receive the call or if any terminating services (like call forwarding, voicemail, or incoming call screening) should be applied. The service logic can then instruct the IM-SSF/ CSCF to continue processing, release the call, or modify its parameters. This model decouples the basic call control logic in the CSCF from the complex service logic, enabling the creation of sophisticated, network-based IP multimedia services in a standardized and interoperable manner. It is the terminating counterpart to the O-IM-BCSM (Originating IM-BCSM).
Purpose & Motivation
The T-IM-BCSM was created to extend the proven Intelligent Network (IN) principles and CAMEL capabilities, widely used for circuit-switched telephony services, into the all-IP world of IMS. Before IMS, CAMEL used Basic Call State Models (BCSMs) for the mobile switching centre to trigger services like prepaid or number translation. With the move to SIP-based IMS, a new model was needed to represent SIP session control in a way that existing CAMEL service logic could understand and interact with.
Its primary purpose is to provide a standardized 'hook' or interface for external service platforms to control and influence the processing of incoming SIP sessions. This solves the problem of how to implement network-controlled services (like call forwarding unconditional, busy, or no reply) in a distributed, SIP-based architecture where the call state is not centralized in a single switch. The T-IM-BCSM defines a common language and set of events that both the session controller (CSCF/IM-SSF) and the service logic (Application Server) agree upon.
This abstraction was crucial for the smooth migration of services from circuit-switched networks to IMS and for the development of new multimedia services. It allowed operators to reuse their investments in CAMEL service creation environments and SCPs (Service Control Points) while transitioning to a next-generation network. It also ensured that service behavior was consistent and portable across different vendors' IMS core equipment, as all would implement the same state model and detection points for terminating call control.
Key Features
- Finite state machine modeling the lifecycle of a terminating IMS (SIP) session
- Defines specific Points In Call (PICs) representing stages of session establishment
- Includes Detection Points (DPs) for invoking external service logic (e.g., CAMEL)
- Enables network-controlled terminating services like call forwarding and barring
- Integrates IMS call control with the CAMEL protocol via the IM-SSF
- Standardized in 3GPP to ensure multi-vendor interoperability for service triggering
Evolution Across Releases
Initial definition of the T-IM-BCSM as part of the CAMEL Phase 3 specifications for IMS (early IMS in 3GPP R5 framework). It established the core state model, detection points (e.g., Terminating_Attempt_Authorized, Terminating_Resource_Available), and its integration with the Serving-CSCF and IM-SSF for controlling incoming SIP sessions.
Defining Specifications
| Specification | Title |
|---|---|
| TS 23.218 | 3GPP TS 23.218 |
| TS 23.278 | 3GPP TS 23.278 |