Chargeable Event

Description

A chargeable event in 3GPP terminology is any activity that utilises telecommunications network infrastructure and related services, which the network operator designates as subject to charging. This includes user-to-user communication (e.g., a voice call, a data session, a short message), user-to-network communication (e.g., service profile administration), inter-network communication (e.g., transferring calls, signalling, or short messages between networks), and mobility events (e.g., roaming or inter-system handover). The cost of a chargeable event may cover sending, transporting, delivery, and storage, and may also include the cost of call-related signalling. Essentially, a chargeable event is the atomic unit of usage that triggers the generation of charging information in the network.

The concept is fundamental to the 3GPP charging architecture. Network elements (e.g., MSC, SGSN, GGSN, SMF, AMF, UPF, IMS nodes) detect chargeable events and generate Charging Data Records (CDRs) or charging events that are sent to the charging system. For offline charging, the CDR is produced after the event and transferred to a Charging Data Function (CDF) or Charging Gateway Function (CGF) for processing and billing. For online charging, the network element interacts with an Online Charging System (OCS) or Charging Function (CHF) before or during the event to obtain authorisation and quota. The chargeable event definition ensures that all relevant activities are consistently identified and reported across different network domains and technologies.

Each chargeable event is characterised by parameters such as the type of event (e.g., call, SMS, data session), the involved parties (e.g., calling and called party), the time and duration, the volume of data, the location, and the QoS. These parameters are captured in the charging records and used by the billing domain to apply tariffs and produce invoices or to deduct balances. The granularity of chargeable events can vary: a single packet-switched data session may generate multiple partial CDRs due to volume or time thresholds, while a voice call typically generates one CDR at the end of the call, possibly with interim records.

The 3GPP specifications define chargeable events in a wide range of documents, including TS 22.115 (Service aspects; Charging and billing), TS 32.240 (Charging architecture and principles), and numerous technology-specific charging specifications (e.g., 32.251 for packet-switched, 32.260 for IMS, 32.270 for MMS, 32.271 for LCS, 32.272 for PoC, 32.273 for MBMS, 32.276 for GBA, 32.277 for ProSe, 32.278 for MTC, 32.293 for MMTel, 32.295 for Number Portability, 32.296 for Online Charging System, 32.849 for 5G charging, 32.850 for 5G online charging). These specifications ensure that each type of service or network function generates charging information in a consistent manner.

In 5G, the charging architecture evolves to a service-based convergence, where the Charging Function (CHF) is a network function that provides converged charging services. The chargeable event remains the core concept, but it is represented by the 'ChargingDataRequest' message, which can include multiple 'MultipleUnitUsage' and 'UsedUnitContainer' structures. The 'chargeableEvent' is identified by parameters such as 'serviceId', 'ratingGroup', and 'chargingType'. The network element triggers a charging event when a chargeable activity occurs, such as the start of a PDU session, the modification of a QoS flow, or the termination of a service. This event-driven charging model ensures real-time or near-real-time charging and supports flexible business models.

Chargeable events also play a critical role in roaming and inter-operator settlement. When a subscriber roams, the visited network generates charging records for the chargeable events and transfers them to the home network, often via TAP3 (Transferred Account Procedure) files. The home network then rates the events according to the roaming agreement and bills the subscriber. Accurate identification and reporting of chargeable events are therefore essential for revenue assurance and fraud prevention.

Purpose & Motivation

The concept of a chargeable event was introduced to provide a clear and consistent definition of what constitutes billable usage in a telecommunications network. Without such a definition, operators and vendors would have divergent interpretations of which activities should be recorded and charged, leading to revenue leakage, billing disputes, and interoperability problems. The chargeable event serves as a contractual and technical anchor: if an activity matches the definition, it must be recorded and reported to the charging system.

Historically, as networks evolved from circuit-switched voice to packet-switched data and then to all-IP multimedia services, the variety of activities that could be charged increased dramatically. Operators needed a flexible yet standardised way to identify and capture these activities across different network domains (CS, PS, IMS, EPS, 5GS) and service types (voice, SMS, data, MMS, location, presence, etc.). The chargeable event definition in TS 21.905 (Vocabulary) and TS 22.115 (Service aspects) provides this common understanding, ensuring that a voice call in a 2G network and a VoNR call in 5G are both recognized as chargeable events and generate compatible charging information.

The definition also explicitly includes mobility events (roaming, handover) and inter-network communication, which are crucial for roaming charging and interconnection settlement. By covering signalling and transfer costs, it allows operators to charge for the use of network resources even when no user data is transmitted. This broad scope ensures that emerging services and network functions can be mapped to chargeable events without redefining the entire charging architecture, making the concept future-proof and adaptable to new technologies such as network slicing, edge computing, and IoT.