Accounting

Description

Accounting in 3GPP is the formal process of apportioning charges for network resource usage between the Home Environment (HPLMN), the Serving Network (VPLMN), and the Subscriber. It is a foundational concept for commercial operation of mobile networks, enabling operators to charge subscribers, settle inter-operator debts, and support roaming and wholesale agreements. The accounting process is distinct from charging, which is the act of determining the amount to be billed; accounting focuses on the apportionment and distribution of those charges among the involved parties.

The architecture for accounting is defined primarily in the 3GPP charging management specifications (32.240 series). It relies on Charging Trigger Functions (CTFs) in network elements (e.g., SMF, AMF, UPF, PCF) that generate charging events and records. These records are transferred via the Charging Gateway Function (CGF) or directly to the Charging Collection Function (CCF) and then to the Billing Domain (BD). The Billing Domain processes the records to produce bills and inter-operator settlement statements. The accounting process uses various record types, such as Call Detail Records (CDRs), Event Detail Records (EDRs), and Accounting Meter Records, each tailored to different service types (e.g., voice, data, SMS, IMS).

Key components include the Charging Trigger Function (CTF), Charging Data Function (CDF), Charging Gateway Function (CGF), and the Billing Domain. The CTF detects chargeable events and generates charging information. The CDF formats this into CDRs, which the CGF collects, validates, and routes to the BD. For online charging, the Online Charging System (OCS) interacts with the CTF in real time to authorize and control resource usage. Accounting also involves the correlation of records from multiple network elements to produce a single billable event, especially in roaming scenarios where both the HPLMN and VPLMN generate records.

In roaming scenarios, the VPLMN generates a TAP (Transferred Account Procedure) file containing accounting data, which is sent to the HPLMN via a clearing house. The HPLMN then uses this to bill its subscriber and settle with the VPLMN. The accounting process must handle currency conversion, taxation, and wholesale discounts. 3GPP defines the TAP3 format for these inter-operator exchanges. The accounting process also supports convergence, allowing a single subscriber to be billed for multiple services (e.g., mobile, fixed, broadband) through a unified charging architecture.

Accounting is critical for network slicing and 5G, where multiple tenants and service providers share infrastructure. It enables usage-based charging per slice, per QoS flow, or per application. The 5G charging architecture (Rel-15 onwards) introduces the Charging Function (CHF) as a converged charging system for both online and offline charging, simplifying the accounting process. The CHF interacts with the SMF, AMF, and other NFs via the Nchf service-based interface. This evolution supports flexible business models, such as sponsored data, network slicing as a service, and edge computing.

Purpose & Motivation

Accounting exists to solve the fundamental business problem of how to divide revenue and costs among multiple parties involved in delivering a mobile service. In early mobile networks, billing was simple because a single operator provided all services. However, with roaming, interconnection, and the introduction of new services (data, IMS, IoT), the need arose to accurately track and apportion charges among the Home Environment, Serving Network, and Subscriber. Without a standardized accounting process, operators would face disputes, revenue leakage, and inability to offer competitive roaming packages.

The 3GPP accounting framework was introduced in Rel-4 (and evolved from earlier GSM specifications) to provide a standardized, interoperable method for generating, transporting, and processing charging records. It addresses the limitations of proprietary or bilateral agreements by defining common record formats (CDRs, TAP3) and procedures. This allows any operator to exchange accounting data with any other operator, fostering global roaming and service interoperability.

Over time, accounting has evolved to support new business models such as online charging, where credit control is performed in real time, and converged charging for 5G, which unifies online and offline charging. The purpose remains to ensure fair and accurate financial settlement, enable flexible pricing, and support regulatory requirements such as lawful interception and taxation.