GGSN (PDP context) generated - Charging Data Record

Description

A GGSN-generated Charging Data Record (G-CDR) is a fundamental data structure in 3GPP charging systems, specifically for packet-switched services in GPRS, UMTS, and the evolved packet core. It is a formatted record that contains a comprehensive set of information related to a user's data session, known as a Packet Data Protocol (PDP) context. The GGSN (or, in 5G, the combined charging functions of the Session Management Function (SMF) and User Plane Function (UPF)) acts as the Charging Trigger Function (CTF). It detects charging-relevant events, such as session establishment, modification, and termination, and collects relevant information to construct the CDR.

The G-CDR includes numerous fields that capture the identity of the subscriber (e.g., IMSI, MSISDN), the network elements involved (GGSN IP address, SGSN address), session details (PDP context identifier, PDP type like IPv4), service information (Access Point Name - APN), and most critically, the usage data. This usage data comprises the total volume of data transferred in the uplink and downlink directions, the session duration (start and stop times), and potentially QoS parameters associated with the session. The record is generated at the end of a charging container closure, which can be triggered by session termination, a volume limit, a time limit, or other administrative triggers.

Once generated, the G-CDR is transferred via the Ga interface (using protocols like GTP' or Diameter) to the Charging Data Function (CDF) or directly to a billing system. The role of the G-CDR is pivotal for offline charging (post-paid billing). It provides the raw, auditable data from which a subscriber's bill is calculated. Beyond billing, G-CDRs are also used for accounting (settlements between operators), business intelligence, traffic analysis, and network planning. The specification of its fields and transfer procedures is meticulously detailed in 3GPP TS 32.298 and related specifications to ensure interoperability between network equipment and billing systems from different vendors.

Purpose & Motivation

The G-CDR exists to enable accurate, reliable, and standardized billing for packet data services in mobile networks. Before the era of GPRS and mobile data, charging was primarily based on voice call duration. The shift to packet-switched services introduced a new paradigm where usage was measured in data volume (bytes) rather than time, necessitating a new charging model and corresponding data records. The G-CDR was created to capture this volumetric usage at the network's gateway to the internet (the GGSN).

It solves the problem of correlating vast amounts of IP data traffic to individual subscribers and their service subscriptions. Without a standardized CDR format, each vendor's GGSN would produce proprietary logs, making interoperability with billing systems impossible and hindering multi-vendor network deployments. The G-CDR provides a common "language" for usage reporting. It also addresses the need for detailed records that support complex tariff plans, such as tiered data buckets, time-of-day pricing, and service-differentiated charging (e.g., charging social media data differently from video streaming).

The historical context begins with its introduction in Release 6 alongside the mature definition of the UMTS packet core. Its evolution through subsequent releases reflects the increasing complexity of services (e.g., IMS, QoS differentiation) and the transition to new core network architectures (EPC, 5GC). The G-CDR concept has been adapted but remains conceptually central, ensuring backward compatibility and a stable foundation for revenue assurance as network technology advances.