Charging Key

Description

The charging key is a crucial parameter in 3GPP charging architecture, defined as information used by the online and offline charging system for rating purposes. It acts as an identifier that maps a service or service component to a specific tariff or rating group. When a charging event is generated, the charging key is included to inform the charging system which rating plan should be applied.

In offline charging, the charging key is used by the Charging Data Function (CDF) and the billing domain to determine the price of the service. In online charging, the Online Charging System (OCS) uses the charging key to perform real-time rating and to manage credit control. The charging key is typically derived from the service being used, the Quality of Service (QoS) parameters, or the subscriber's subscription profile.

The charging key is often associated with a rating group, which is a concept used in the Policy and Charging Control (PCC) architecture. The Policy and Charging Rules Function (PCRF) or the Policy Control Function (PCF) provides the charging key to the Policy and Charging Enforcement Function (PCEF) or the Session Management Function (SMF) as part of the PCC rules. This allows the charging system to apply the correct tariff based on the policy decisions.

The charging key concept has evolved over time. In earlier releases (Rel-6 to Rel-19), it was defined as information used by the online and offline charging system for rating purposes. In Release 20, the definition was updated to specify that it is used by the Charging Function (CHF) for rating purposes, reflecting the architectural changes in the 5G charging system where the CHF consolidates charging functions. This change aligns with the 5G Service-Based Architecture (SBA) and the convergence of charging domains.

The charging key is specified in several 3GPP specifications, including TS 23.125 (Overall high level functionality and architecture impacts of flow based charging), TS 23.203 (Policy and charging control architecture), TS 23.503 (Policy and charging control framework for the 5G System), and TS 29.512 (5G System; Session Management Policy Control Service).

Purpose & Motivation

The charging key exists to provide a standardized mechanism for identifying the rating to be applied to a service or session in both online and offline charging systems. Before its introduction, charging systems often relied on proprietary identifiers or service-specific parameters, which made it difficult to support multiple services and to ensure consistent charging across different network domains.

The charging key solves the problem of how to map a service or service component to a tariff in a flexible and scalable way. By using a charging key, the network can dynamically assign different tariffs based on policy decisions, subscriber profiles, or service characteristics. This is essential for implementing differentiated charging, such as tiered pricing, promotional offers, or usage-based billing.

Furthermore, the charging key enables the separation of charging from service logic. The network element that provides the service does not need to know the details of the tariff; it simply includes the charging key in the charging event, and the charging system applies the appropriate rating. This separation simplifies the development of new services and allows the charging system to be updated independently.

The evolution of the charging key definition in Release 20 to reference the CHF reflects the ongoing convergence of charging architectures in 5G and beyond. As networks move towards a unified charging framework, the charging key remains a fundamental building block for rating and billing.