Originating Application Server

Description

The Originating Application Server (O-AS) is a core component within the 3GPP IP Multimedia Subsystem (IMS) architecture, defined in the context of the IMS Service Control (ISC) interface. It is an Application Server that is invoked by the Serving-Call Session Control Function (S-CSCF) for the originating user (the party initiating a session, such as a call or messaging service). When a user initiates an IMS session, the S-CSCF evaluates the user's initial filter criteria (iFC), which are part of the user's service profile downloaded from the Home Subscriber Server (HSS). If the iFC conditions are met (e.g., based on the called number, service type, or time of day), the S-CSCF forwards the SIP request (e.g., INVITE, MESSAGE) to the designated O-AS via the ISC interface using the SIP protocol.

Architecturally, the O-AS is a SIP application server that hosts and executes service logic. Upon receiving the SIP request, it can act in several modes: as a terminating UA (User Agent), a redirect server, a SIP proxy, or a back-to-back user agent (B2BUA). In its most common role, it acts as a B2BUA, which allows it to modify the SIP dialog, inject new headers, change the Request-URI, or interact with other network resources (like MRF for media processing or HSS for data). After applying its service logic—which could involve number translation, call screening, applying business rules, or interacting with an external service platform—the O-AS returns the SIP request to the S-CSCF for further routing, typically towards the destination network or a terminating AS.

Its role is critical for the delivery of originating services in a decoupled, service-agnostic network. It separates the core session control (handled by the CSCF) from the application logic. This allows network operators and third-party providers to deploy and modify services rapidly without impacting the underlying IMS core. The O-AS, along with its counterpart the Terminating Application Server (T-AS), enables personalized services based on the subscriber's profile. It is a fundamental entity for implementing services like Originating Call Barring, Call Forwarding Unconditional (when invoked at origination), Malicious Communication Identification, Lawful Interception for the originating leg, and various pre-call interactive voice response services.

Purpose & Motivation

The O-AS concept was created as part of the IMS architecture to solve the problem of monolithic, tightly integrated service switching in traditional telephony networks (like IN/CAMEL). In pre-IMS networks, service logic was often embedded within the call control switches (MSCs), making service introduction slow, expensive, and vendor-locked. The IMS paradigm, inspired by internet protocols, aimed to create a horizontal architecture where a common session control layer (CSCF) could interoperate with a wide variety of application servers over standardized interfaces.

The primary purpose of the O-AS is to provide a standardized, SIP-based point of integration for any service that needs to process or influence a session at its origination. This addresses the need for service innovation and multi-vendor interoperability. By defining clear trigger points (via iFC) and a standard ISC interface, 3GPP enabled network operators to mix and match application servers from different vendors to create service chains. The O-AS handles the 'originating leg' service logic, allowing for user-specific treatment of outgoing sessions before they leave the home network.

Historically, its introduction in 3GPP Release 7 alongside the full IMS specification marked a shift towards all-IP service delivery for multimedia services. It was motivated by the desire to support rich communication services (RCS), VoIP, and video telephony over packet-switched networks with the same reliability and feature richness as circuit-switched networks. The O-AS provides the mechanism to replicate and enhance classic telephony supplementary services (like call barring) and to introduce entirely new IP-based services, fulfilling the IMS promise of convergence between cellular, fixed, and internet services.

Key Features

• Invoked by the S-CSCF based on originating user's Initial Filter Criteria (iFC)
• Communicates with the S-CSCF over the standardized ISC interface using SIP
• Can operate as a Back-to-Back User Agent (B2BUA), proxy, redirect server, or UA
• Executes service logic for the calling party (e.g., call screening, number translation)
• Can interact with other IMS entities (HSS, MRF) and external application platforms
• Enables the creation of service chains by returning control to the S-CSCF for further processing

Evolution Across Releases

Defining Specifications