Inter-Channel Alignment

Description

Inter-Channel Alignment (ICA) is a concept within the 3GPP Charging and Management framework, specifically defined in the context of IP Multimedia Subsystem (IMS) and Packet Switched (PS) domain services. It addresses the scenario where a single user service session (e.g., a video call) utilizes multiple simultaneous IP-CAN (IP Connectivity Access Network) bearers or service data flows—such as one for voice media, one for video media, and one for signaling. ICA ensures that the charging systems (Online Charging System - OCS, Offline Charging System - OFCS) can correlate all these individual data flows to the same higher-level service instance.

The architecture involves components within the Policy and Charging Control (PCC) framework. Key elements include the Policy and Charging Rules Function (PCRF) and the charging triggers in network nodes like the P-GW/PDN-GW or SMF/UPF. When a service is initiated, the Application Function (AF), such as an IMS Call Session Control Function (CSCF), informs the PCRF about the service session and its associated media components. The PCRF then installs corresponding service data flow filters and charging rules into the gateway. Crucially, it assigns a common ICA identifier to all PCC rules belonging to the same service session. This identifier is included in all charging events (Credit Control Requests, Charging Data Records) generated for those flows.

How it works is through identifier propagation. The ICA identifier, provided by the AF, is passed via the PCRF to the gateway executing the policy. Every usage report sent to the charging system carries this identifier. The charging systems use the ICA identifier to group all partial records, apply the correct tariff, and generate a single, consolidated charge for the user's service experience. This prevents the user from receiving separate, confusing bills for voice minutes, video data, and signaling associated with one call. It also enables accurate policy enforcement, such as applying a single data quota for an entire multimedia session.

Purpose & Motivation

ICA was created to solve the billing and policy control challenges introduced by IMS and multimedia services. Traditional circuit-switched voice had a single bearer channel, making charging straightforward. With IMS, a single service like Multimedia Telephony (MMTel) can dynamically establish multiple IP flows with different QoS characteristics for audio, video, and data sharing.

The problem it addresses is the decoupling of charging records from the logical service. Without ICA, each media flow would generate independent charging records, leading to incorrect billing, inability to apply session-based tariffs, and complex customer care issues. Previous approaches lacked a standardized method to bind these flows together across the network. ICA provides the necessary correlation mechanism, motivated by the commercial need for accurate, understandable, and service-aware charging for next-generation networks. It ensures that the business logic of a service (e.g., 'per-call charging') can be correctly implemented over a packet-switched network with multiple concurrent channels.

Key Features

• Provides a unique ICA identifier to correlate multiple service data flows to a single service session
• Integrates with the PCC architecture, involving AF, PCRF, and gateways
• Supports both online and offline charging systems (OCS, OFCS)
• Enables consolidated billing for complex IMS services like video calls
• Allows for coherent policy enforcement across all media components of a session
• Defined for use with IP-CAN bearers in GPRS, EPS, and 5GS networks

Evolution Across Releases

Defining Specifications