Description
QoS flow Based Charging (QBC) is a charging architecture defined within the 3GPP 5G Core network, specified primarily in the 32-series (Charging) specifications. It represents a paradigm shift from bearer-based charging in 4G EPS to a more granular, flow-based model aligned with the 5G Core's service-based architecture and QoS model. In 5G, user plane traffic is organized into QoS Flows, each with a unique QoS Flow Identifier (QFI) and a specific QoS profile defining parameters like 5G QoS Identifier (5QI), Guaranteed Flow Bit Rate (GFBR), and Maximum Flow Bit Rate (MFBR). QBC operates by generating Charging Data Records (CDRs) or Charging Events that are correlated to these individual QoS Flows.
The architecture involves several key network functions. The Session Management Function (SMF) is central, as it establishes, modifies, and releases QoS Flows within a PDU Session. The SMF interacts with the Policy Control Function (PCF) to receive policy and charging control (PCC) rules. These PCC rules include charging instructions that dictate how a specific service data flow, mapped to a QoS Flow, should be charged. When charging is triggered, the SMF, acting as a Charging Trigger Function (CTF), collects relevant charging information such as QoS Flow identifiers, data volumes, duration, and associated network slice information. It then forwards this information to the Charging Data Function (CDF) or Online Charging System (OCS) via the Nchf service-based interface.
How QBC works is intrinsically linked to the 5G QoS model. A single PDU Session can contain multiple QoS Flows—for example, one for high-priority voice traffic, one for best-effort internet browsing, and another for a low-latency gaming service. QBC allows the operator to apply distinct charging rates, quotas, or billing models to each of these flows independently. The charging data records can capture not just volume and time, but also the specific 5QI value, the network slice instance (S-NSSAI) serving the flow, and the data network name (DNN). This enables highly detailed billing reports and facilitates service-differentiated pricing, such as charging a premium for guaranteed low-latency flows or offering zero-rating for specific application flows.
Purpose & Motivation
QBC was created to address the limitations of the EPS bearer-based charging model, which was not granular enough for the diverse service landscape envisioned for 5G. In 4G, charging was typically associated with an EPS bearer, which aggregated traffic with similar QoS requirements. This made it difficult to implement fine-grained, service-specific charging policies, especially as networks evolved to support network slicing and a wide variety of vertical industry services with distinct QoS profiles.
The primary problem QBC solves is enabling monetization models that match the technical capabilities of 5G. With network slicing, IoT services, ultra-reliable low-latency communication (URLLC), and enhanced mobile broadband (eMBB) all coexisting on the same infrastructure, a one-size-fits-all charging approach is inadequate. QBC provides the mechanism to charge based on the actual value or cost of delivering a specific QoS level. This allows operators to create innovative tariff plans, offer service level agreement (SLA)-based billing to enterprise customers, and implement fair usage policies that consider the quality of service consumed, not just the raw data volume. Its introduction was motivated by the business need to monetize 5G's advanced capabilities beyond simple data buckets.
Key Features
- Charging granularity at the level of individual 5G QoS Flows, identified by QFI
- Integration with the 5G Policy and Charging Control (PCC) framework via the PCF
- Supports both offline (CDR-based) and online (credit management) charging scenarios
- Charging data can be correlated with Network Slice Instance (S-NSSAI) and DNN
- Enables service-differentiated pricing models (e.g., premium rates for guaranteed bit rate/low latency flows)
- Utilizes the Nchf service-based interface between CTF (e.g., SMF) and the charging systems
Evolution Across Releases
Introduced as part of the foundational 5G System architecture. Defined the core principles of QoS flow based charging, specifying how the SMF acts as a Charging Trigger Function, the information collected per QoS flow, and the interaction with the new Charging Service (Nchf). Established the framework for correlating charging data with 5QI, QFI, and network slice identifiers.
Enhanced QBC to support new 5G capabilities. This included charging enhancements for network slicing, integration with access traffic steering, switch and splitting (ATSSS) for multi-access PDU sessions, and support for edge computing scenarios, ensuring charging records could reflect the use of local breakout and edge application servers.
Further refinements for vertical and IoT services. Introduced charging aspects for non-public networks (NPNs), support for QoS monitoring and reporting which could influence charging, and enhancements for service specific charging in relation to 5G LAN-type services.
Ongoing evolution to support advanced 5G-Advanced features. Studies and specifications likely focus on charging for AI/ML model training and inference services, enhanced support for extended reality (XR) traffic with multiple concurrent flows, and further automation in charging policy derivation.
Defining Specifications
| Specification | Title |
|---|---|
| TS 32.255 | 3GPP TR 32.255 |
| TS 32.291 | 3GPP TR 32.291 |
| TS 32.298 | 3GPP TR 32.298 |