Charging Determination Point

Description

The Charging Determination Point (CDP) is a critical functional component within the 3GPP charging architecture, specifically defined within the Policy and Charging Control (PCC) framework. It operates as an intermediary between network functions that generate usage data (like the PCEF or TDF) and the backend charging systems that process this data for billing purposes. The CDP's primary function is to receive raw charging events from various network elements, apply charging rules and policies to determine the appropriate charging treatment, and then forward these standardized charging data records (CDRs) to the appropriate charging system.

Architecturally, the CDP is typically implemented as part of or in close association with the Policy and Charging Rules Function (PCRF). It receives charging information via the Gx reference point from the Policy and Charging Enforcement Function (PCEF) and via the Sd reference point from the Traffic Detection Function (TDF). The CDP processes these events according to policies received from the PCRF, which include service data flow filters, charging keys, rating groups, and other parameters that determine how specific network usage should be charged. This processing includes correlating multiple charging events, applying time-based or volume-based thresholds, and determining when to generate interim or final charging records.

The CDP's operation involves several key technical processes. First, it receives Charging Events (CEs) from network functions, which contain information about user sessions, data volumes, service usage, and other measurable parameters. The CDP then matches these events against active policy rules to determine the appropriate charging treatment. This matching process considers factors like the user's subscription profile, service characteristics, network conditions, and operator-defined business rules. Once matched, the CDP applies charging parameters such as rating groups (which map to specific tariff rates) and charging keys (which identify the service or content being charged).

After processing, the CDP generates standardized Charging Data Records (CDRs) or Charging Events Records (CERs) that contain all necessary information for billing. These records include timestamps, user identifiers, service identifiers, usage quantities, charging parameters, and other metadata required for accurate billing. The CDP then forwards these records to the appropriate charging system: to the Charging Data Function (CDF) via the Ga reference point for offline charging (post-paid billing), or to the Online Charging System (OCS) via the Gy reference point for online charging (pre-paid or real-time credit control). This separation ensures that charging logic is centralized and consistent regardless of the billing model.

The CDP plays several important roles in the network beyond basic charging determination. It enables service-aware charging by distinguishing between different types of traffic and applying appropriate charging rules. It supports converged charging across multiple access technologies (2G, 3G, 4G, 5G) by providing a unified charging interface. It facilitates policy-driven charging where billing rules can be dynamically adjusted based on network conditions, user behavior, or promotional offers. Additionally, the CDP provides charging correlation capabilities, allowing operators to combine charging events from multiple sources (voice, data, messaging, value-added services) into coherent billing records for each user session.

Purpose & Motivation

The Charging Determination Point was created to address the growing complexity of charging in modern mobile networks. As 3GPP networks evolved from simple voice services to rich multimedia and data services, the traditional charging approaches became inadequate. Earlier charging systems typically received raw usage data directly from network elements without intermediate processing, leading to inconsistencies, complex integration requirements, and limited flexibility in charging rules application. Operators needed a more sophisticated mechanism to apply complex charging policies consistently across different network functions and services.

The CDP was introduced specifically to separate charging determination logic from both the enforcement functions (which handle traffic) and the billing functions (which process records). This architectural separation allows operators to implement complex charging rules without modifying every network element. It enables service-aware charging where different types of traffic (streaming video, web browsing, IoT data) can be charged differently based on content, quality, or business agreements. The CDP also addresses the need for real-time charging capabilities required for pre-paid services and spending limit controls, providing a standardized interface between network usage and credit management systems.

Furthermore, the CDP solves the problem of charging correlation in multi-service environments. Modern users simultaneously use voice, data, messaging, and various value-added services, often within the same session. The CDP provides the intelligence to correlate these diverse charging events into coherent billing records. It also enables policy-driven charging where billing can be dynamically adjusted based on network conditions (like congestion), user behavior (like roaming status), or business rules (like promotional offers). This flexibility became essential as operators moved from simple per-minute or per-megabyte billing to complex tiered plans, bundled services, and quality-based charging models.

Key Features

• Centralized charging event processing from multiple network functions
• Application of dynamic charging rules and policies received from PCRF
• Generation of standardized Charging Data Records (CDRs) for billing systems
• Support for both offline (post-paid) and online (real-time) charging interfaces
• Service-aware charging differentiation based on traffic type and content
• Charging correlation across multiple services within user sessions

Evolution Across Releases

Defining Specifications