Description
The Application Server Outgoing Leg Control Model (AS-OLCM) is a fundamental architectural model within the 3GPP IP Multimedia Subsystem (IMS) for service execution. It defines the interaction between an Application Server (AS) and the Serving-Call Session Control Function (S-CSCF) during session establishment. In this model, the AS acts as a Back-to-Back User Agent (B2BUA) for the outgoing leg of a session, meaning it terminates the incoming SIP dialog from the S-CSCF and originates a new, outgoing SIP dialog towards the final destination. This allows the AS to exert full control over the session parameters, routing, and media characteristics of the outgoing leg.
The operational flow begins when the S-CSCF, executing initial Filter Criteria (iFC) from the user's service profile, forwards a SIP request (e.g., INVITE) to the designated AS. The AS, operating under the OLCM, accepts this request, terminating the incoming leg. It then processes the request based on its service logic—which may involve number translation, applying business rules, or interacting with external databases. Following this processing, the AS originates a new SIP request towards the target address, which could be the originally intended destination or a different one determined by the service logic. This new request forms the outgoing leg, and the AS remains in the signaling path for the duration of the session, enabling mid-call service control.
Key components involved are the AS itself, which hosts the service logic and acts as the B2BUA, and the S-CSCF, which performs the initial service triggering based on iFC. The model relies on the SIP protocol for all signaling interactions. The AS-OLCM is crucial for implementing a wide range of telephony and multimedia services that require the service platform to modify the session's destination or characteristics. It provides a standardized, powerful mechanism for service providers to insert value-added services into the IMS call path, ensuring interoperability and consistent behavior across different network implementations.
Purpose & Motivation
The AS-OLCM was created to provide a standardized and powerful mechanism for implementing advanced, network-hosted services within the IMS architecture. Prior to IMS and its defined service control models, intelligent network (IN) services were often tightly coupled with the circuit-switched core, making them complex to deploy and limiting innovation. The AS-OLCM, introduced in 3GPP Release 99, was part of the foundational IMS framework designed to decouple service logic from the core network transport, enabling faster service creation and deployment in a packet-switched, SIP-based environment.
It solves the problem of how to allow an external application server to actively control and modify a session after it has been initiated by the user. Without such a model, an AS could only observe or passively influence a session. The OLCM specifically addresses use cases where the service logic must change the destination or fundamental parameters of a session, such as in call forwarding unconditional, number portability resolution, or enterprise dial-plan translation. It provides the necessary architectural hooks for the AS to break the SIP dialog and re-originate it, a capability essential for many traditional telephony services translated into the IP domain.
The motivation was to ensure the IMS could support the rich service portfolio of legacy networks while enabling new multimedia services. By defining clear models like the OLCM, 3GPP ensured that service behavior was predictable and that ASs from different vendors could interoperate with any standards-compliant IMS core. This fostered a competitive ecosystem for application development and was critical for the commercial success of IMS as a service delivery platform.
Key Features
- Enables the Application Server to act as a Back-to-Back User Agent (B2BUA) for the outgoing session leg
- Allows full control over SIP signaling and session parameters for the outgoing leg
- Supports service-triggered rerouting of sessions to a different destination
- Facilitates implementation of services like call forwarding, number translation, and call screening
- Integrates with S-CSCF via initial Filter Criteria (iFC) for standardized service triggering
- Keeps the AS in the signaling path for potential mid-call service control
Evolution Across Releases
Introduced the AS-OLCM as part of the initial IMS architecture specification. Defined the core model where the AS terminates the incoming SIP dialog from the S-CSCF and originates a new outgoing dialog, establishing it as a Back-to-Back User Agent. This provided the foundational capability for implementing traditional telephony services like call forwarding within the new SIP-based IMS framework.
Defining Specifications
| Specification | Title |
|---|---|
| TS 23.218 | 3GPP TS 23.218 |