Charging

Description

Charging in 3GPP is a comprehensive function that enables telecommunications operators to monetize network usage and services. It involves the collection, formatting, transfer, and evaluation of information related to chargeable events. A chargeable event is any usage of network resources or services that can be charged, such as a voice call, data session, SMS, or value-added service. Charging ensures that the Charged Party (the entity responsible for payment) is billed correctly, either through offline charging (post-paid) or online charging (pre-paid or real-time).

The charging architecture is defined in 3GPP TS 32.240, which outlines the overall principles and logical components. The key functional entities include the Charging Trigger Function (CTF), which detects chargeable events and generates charging information; the Charging Data Function (CDF), which receives and formats charging information into Charging Data Records (CDRs); the Charging Gateway Function (CGF), which collects and forwards CDRs to the Billing Domain (BD); and the Online Charging System (OCS), which performs real-time rating and credit control. The CTF resides in network elements like the SMF, AMF, or IMS nodes, and it reports to the CDF or OCS based on the charging mode.

Offline charging is used for post-paid subscribers, where charging information is collected and processed after the service is used. The CTF generates CDRs that are transferred to the CDF, which may aggregate them and forward to the CGF. The CGF then sends CDRs to the BD, which generates invoices. Offline charging does not affect the service in real-time, so it is suitable for subscribers with good credit. Online charging, on the other hand, is used for pre-paid subscribers or when real-time credit control is needed. The CTF interacts with the OCS using the Diameter Credit-Control Application (DCCA) or the Service Based Interface (SBI) in 5G. The OCS performs rating, determines the cost, and reserves units from the subscriber's account. If the account has sufficient balance, the service is allowed; otherwise, it is denied or limited.

Charging can be applied at various levels: transport, service, and content. Transport charging deals with the volume or duration of data or voice transport. Service charging is based on the service used, such as IMS telephony or messaging. Content charging is based on the content accessed, such as a video stream or application download. The 3GPP charging framework supports convergence, allowing operators to charge for combined services across domains (e.g., CS, PS, IMS) in a unified way. This is achieved through the Charging Architecture and the use of common charging identifiers and interfaces.

In 5G, the charging architecture has evolved to support new capabilities like network slicing, edge computing, and ultra-reliable low-latency communications (URLLC). The 5G Charging System (5GCS) introduces new network functions like the Charging Function (CHF) that combines CDF, CGF, and OCS functionalities. The CHF uses the Nchf service-based interface to interact with other network functions. Charging in 5G also supports exposure of charging events to third parties and integration with business support systems (BSS).

Overall, charging is a critical function for operators to generate revenue, manage credit risk, and offer flexible pricing plans. It ensures that usage is accurately measured and billed, and it supports a wide range of business models from prepaid to postpaid, sponsored data, and IoT charging.

Purpose & Motivation

Charging exists to enable telecommunications operators to monetize the usage of their networks and services. Without charging, operators would have no way to generate revenue from the services they provide, making it impossible to sustain and expand their networks. The primary purpose is to accurately measure and record usage, determine the cost, and bill the responsible party. This is essential for both post-paid and pre-paid subscribers, as well as for interconnect and roaming scenarios.

Historically, charging was relatively simple in circuit-switched networks, where call detail records (CDRs) were generated for each call. However, with the advent of packet-switched networks, IP multimedia subsystems, and now 5G, charging has become much more complex. The 3GPP charging framework was developed to provide a standardized, scalable, and flexible solution that can handle diverse services and business models. It addresses the limitations of proprietary charging systems by defining common interfaces, data formats, and procedures.

The introduction of online charging in later releases allowed operators to offer pre-paid services with real-time credit control, reducing credit risk and enabling new market segments. The evolution to 5G charging supports new use cases like network slicing, where each slice can be charged differently, and IoT, where massive numbers of devices require efficient charging. Thus, charging is not just about billing; it is a strategic enabler for operator business models and service innovation.