P-GW (enhanced by FBC) generated – Charging Data Record

Description

A PGW-CDR (P-GW generated Charging Data Record) is a standardized data structure defined by 3GPP that contains detailed accounting information for a user's Packet Data Network (PDN) connection session. It is generated by the P-GW (Packet Data Network Gateway) in the Evolved Packet Core (EPC) and is specifically associated with the gateway's Flow-Based Charging (FBC) functionality. The CDR is the primary output for offline charging systems, providing the raw data used by the operator's billing domain to calculate charges for subscriber usage.

The architecture of CDR generation involves several network functions. The P-GW, acting as the Charging Trigger Function (CTF), monitors user data sessions based on predefined service data flows and charging rules provided by the Policy and Charging Rules Function (PCRF). When specific charging events occur (e.g., session start, session stop, volume limit reached, time limit reached), the P-GW collects relevant information and assembles it into a PGW-CDR. This record is then transferred via the Gz interface to the Charging Data Function (CDF), which is part of the Offline Charging System (OFCS). The CDF may collect multiple partial CDRs for a single session before forwarding the consolidated Charging Data Record (CDR) to the Charging Gateway Function (CGF) and ultimately to the Billing Domain.

The PGW-CDR contains a comprehensive set of fields as specified in 3GPP TS 32.251. Key information includes the IMSI (International Mobile Subscriber Identity) and MSISDN (Mobile Subscriber ISDN Number) of the user, the PDN connection identifier, the Access Point Name (APN) used, the allocated IP address(es), session start and stop times, and the cause for termination. Crucially, for Flow-Based Charging, the record includes data volume counts (uplink and downlink) per service data flow or rating group. It also records QoS parameters associated with the session, such as the QoS Class Identifier (QCI) and Allocation/Retention Priority (ARP), as well as location information like the Serving Network identity and User Location Info.

The process of CDR generation is tightly integrated with the P-GW's traffic handling. As packets belonging to a service data flow pass through the gateway, they are counted against applicable volume thresholds. The P-GW applies the charging rules, which dictate when to generate interim or final CDRs. For example, a rule may trigger an interim CDR every 1 GB of data consumed or every hour. This granular, flow-based accounting enables sophisticated billing models beyond simple per-megabyte pricing, such as zero-rating for specific applications, tiered data plans, and sponsored data services. The PGW-CDR provides the auditable trail of usage required to implement these commercial policies.

Purpose & Motivation

The PGW-CDR exists to provide a standardized, detailed, and reliable mechanism for accounting subscriber usage in mobile packet-switched networks. Prior to the formalization of Flow-Based Charging (FBC) in 3GPP Release 7, charging was often based on simpler, less granular methods that could not support complex service offerings. The explosion of mobile data services and the need for differentiated pricing models (e.g., charging differently for social media, video streaming, and cloud storage) created a demand for more sophisticated charging.

The introduction of the Policy and Charging Control (PCC) architecture, which includes FBC, was the solution. It allowed operators to define charging rules based on the specific service data flow (e.g., all traffic to a video server) rather than just the PDN connection as a whole. The PGW-CDR is the output of this system. It solves the problem of how to accurately record and report this granular usage information to the billing system. Without such a detailed CDR, operators would be unable to bill for the diverse "service-aware" plans that are now commonplace.

Historically, its creation was motivated by the need to move beyond time-based or flat-rate data charging. It enabled the monetization of the mobile internet by allowing operators to create attractive, service-specific data bundles and partnerships with content providers (e.g., "video streaming not counted against your data cap"). The PGW-CDR provides the technical foundation for these business models by capturing exactly what service was used, how much data was transferred, and at what quality of service, thereby addressing the limitations of previous non-flow-based charging data records which lacked the detail required for modern service differentiation.