Application Server Incoming Leg Control Model

Description

The Application Server Incoming Leg Control Model (AS-ILCM) is a fundamental architectural component within the 3GPP CAMEL (Customised Applications for Mobile networks Enhanced Logic) framework, specifically defined for service control in circuit-switched and IP Multimedia Subsystem (IMS) networks. It defines the precise mechanisms and signaling interactions by which an external Application Server (AS) assumes control over the handling of the incoming leg of a call session. This model is invoked when a call is destined for a subscriber who has subscribed to services hosted on that AS, such as call forwarding, number translation, or pre-paid charging services that require intervention before the call is presented to the terminating user.

Architecturally, AS-ILCM operates within the broader CAMEL Application Part (CAP) or IMS Service Control (ISC) interface context. When a call setup request (e.g., an Initial Address Message in SS7 or a SIP INVITE in IMS) arrives at the network's switching node (like an MSC or an S-CSCF), the subscriber's profile indicates the invocation of CAMEL services. The switching node, acting as the gsmSSF or IM-SSF, then initiates a dialogue with the designated AS. In the AS-ILCM, the AS receives the incoming call leg information and executes its service logic. This logic can analyze parameters like the calling party number, time of day, or subscriber state, and then instruct the network on how to proceed—for example, to connect the call, redirect it to another number, play an announcement, or apply specific charging.

Key components involved in the AS-ILCM include the Service Switching Function (SSF) in the core network node, which detects the service trigger and manages the leg, and the Application Server itself, which hosts the service logic. The communication between them uses standardized protocols: CAP over SS7 for circuit-switched domains or SIP over the ISC interface for IMS. The model defines specific operations like Connect, Continue, ReleaseCall, and RequestReportBCSMEvent, which the AS uses to control the call leg. This precise control allows for complex, real-time services that are transparent to the end-user's device.

Its role in the network is critical for enabling operator-hosted value-added services without requiring modifications to the subscriber's handset. By separating service logic from basic call switching functions, AS-ILCM promotes network flexibility and rapid service deployment. It forms the basis for intelligent network (IN) services in mobile networks, ensuring that service execution is consistent, reliable, and integrated with core network functions like mobility management and charging.

Purpose & Motivation

AS-ILCM was created to address the need for standardized, network-based control of incoming call processing in mobile networks. Prior to CAMEL and models like AS-ILCM, advanced call services were often implemented using proprietary, vendor-specific intelligent network (IN) solutions that led to interoperability issues and slow service deployment across multi-vendor networks and different operators. The lack of a uniform control model made it difficult to create portable services or ensure consistent behavior for roaming subscribers.

The primary problem AS-ILCM solves is providing a clear, standardized mechanism for an external application server to intercept and control how an incoming call leg is handled before it reaches the called party. This enables a wide range of subscriber services that depend on analyzing the call attempt in real-time. For example, it allows for intelligent call forwarding (e.g., forward to voicemail if busy, or to a different number based on time), incoming call screening (accepting or rejecting calls based on black/white lists), and number translation services (like virtual private network numbers).

Historically, its introduction in 3GPP R99 as part of CAMEL Phase 3 was motivated by the growing demand for customizable, operator-controlled services beyond basic voice calls. It provided the architectural foundation for separating service logic from switching infrastructure, a key principle of intelligent networks. This separation allowed operators to develop and deploy services rapidly on dedicated application servers without needing to upgrade every core network switch, thereby reducing costs and increasing service innovation. AS-ILCM specifically addressed the incoming call scenario, complementing other models like AS-OLCM (Outgoing Leg Control) to provide comprehensive call control.