Charge Unit

Description

In 3GPP charging architecture, a charge unit is the fundamental building block used to define how usage of a service is measured and ultimately priced. It can be expressed in either monetary terms (e.g., cents, euros, tokens of a prepaid account) or non-monetary terms (e.g., bytes, seconds, messages, events). The charge unit provides the basis for translating a measured quantity of resource consumption into a billable amount. For example, a charge unit for a voice call might be 'per minute', for data 'per megabyte', and for SMS 'per message'. The charging system collects Charging Data Records (CDRs) or performs online charging with quota management, where the consumed units are mapped to charge units according to tariff definitions.

The concept is central to both offline charging (where charging information is collected after the service usage and processed by a billing domain) and online charging (where the charging system authorises the usage in real time and reserves units from the subscriber's account). In the 3GPP charging architecture, the Charging Trigger Function (CTF) within a network element (e.g., SMF, AMF, PCF, or an application server) generates charging events. These events contain information about the used resource (e.g., volume, time, events). The charging system (e.g., CHF in 5G, OCS/CDF in pre-5G) applies tariff information that defines the cost per charge unit. The charge unit thus acts as the bridge between raw usage metrics and the monetary or non-monetary value that is deducted from a balance or included in an invoice.

Charge units are defined in the charging specifications 3GPP TS 29.458 for 5G and TS 29.658 for the 5G Charging Service Based Architecture (SBA). These specifications describe the data model, including the 'ChargingDataRequest' and 'ChargingDataResponse' messages, and the 'ChargingInformation' structure, which contains fields such as 'ratingGroup', 'serviceId', and 'chargeUnit'. The rating group identifies the tariff to be applied, while the charge unit specifies the measure of usage (e.g., time, volume, events) to which that tariff applies. Operators can define multiple charge units for a single service, allowing differentiated charging (e.g., the first 500 MB per month at one rate, and subsequent MB at another).

In practice, a charge unit is often represented in the charging system as a combination of a metric (what is counted) and a unit (the dimension, such as bytes, seconds, or occurrences). The 3GPP charging architecture supports both monetary and non-monetary units: monetary units are used directly for balance deduction in prepaid scenarios or for invoice line items, whereas non-monetary units (e.g., loyalty points, bonus bytes) can be converted later. Non-monetary units are particularly important for sponsored data, zero-rating, and service-specific allowances where the subscriber's consumption is measured against a quota that is not directly tied to currency.

The charge unit is closely related to the concept of 'rating' in charging. Rating determines the price per charge unit based on tariff, time of day, subscriber profile, and other factors. The charging system uses the reported usage (in charge units) and the rating information to compute the charge. In online charging, the OCS or CHF grants a quota (e.g., 10 minutes or 100 MB) and the network element enforces this quota. When the quota is exhausted, the network element requests a new quota, and the charging system may re-rate based on the new charge unit. This mechanism ensures that subscribers are charged accurately and that credit limits are respected in real time.

Purpose & Motivation

The charge unit concept was created to provide a standardised, technology-agnostic way to measure and price telecommunications services across diverse network types and service categories. Before its formalisation, charging systems often used proprietary or service-specific units, which made it difficult to implement convergent charging across voice, data, and messaging, and to support real-time balance management in prepaid systems. Operators needed a common vocabulary and data model to integrate charging across circuit-switched and packet-switched domains, and later across 3G, LTE, and 5G networks.

By defining charge units as either monetary or non-monetary, 3GPP enabled flexible business models such as postpaid billing, prepaid balance deduction, sponsored data, and usage-based allowances. For instance, a non-monetary charge unit can represent a number of bytes that are free of charge (zero-rated), while a monetary charge unit directly depletes a prepaid account. This flexibility is crucial for implementing tiered pricing, family plans, enterprise contracts, and roaming agreements.

The charge unit also supports interoperability between network operators and charging vendors. Standardised charge unit definitions in 3GPP TS 29.458 and TS 29.658 allow a consistent interpretation of charging data across multi-vendor networks and across generations of technology. As networks evolve towards 5G service-based architecture, the charge unit remains a foundational element that ensures accurate, timely, and transparent charging for both consumers and enterprise customers.