Guaranteed Flow Bit Rate

Description

Guaranteed Flow Bit Rate (GFBR) is a key QoS parameter introduced in 5G System (5GS) specifications, particularly in TS 23.501 (System Architecture for the 5G System) and TS 24.501 (Non-Access-Stratum protocol for 5GS). GFBR defines the bit rate that is guaranteed to be provided to a QoS flow by the network. It is applicable to Guaranteed Bit Rate (GBR) QoS flows, which are used for services that require dedicated bandwidth, such as Voice over NR (VoNR), real-time gaming, and mission-critical communications. GFBR is negotiated between the UE and the network during PDU session establishment or modification, and it represents the minimum guaranteed data rate for the flow.

In the 5G QoS framework, each QoS flow is associated with a QoS profile that includes parameters like 5QI (5G QoS Identifier), Allocation and Retention Priority (ARP), and for GBR flows, GFBR and Maximum Flow Bit Rate (MFBR). GFBR is enforced by the network through admission control, resource reservation, and scheduling. The Session Management Function (SMF) is responsible for authorizing the requested GFBR based on subscription data and policy control (PCF). The User Plane Function (UPF) and the Radio Access Network (RAN) ensure that the flow receives at least the GFBR by allocating sufficient radio resources and prioritizing the flow's packets.

GFBR can be set per QoS flow and can be different for uplink and downlink. It is typically expressed in bits per second. The network uses GFBR to determine whether a new GBR flow can be admitted without compromising existing guarantees. If resources are insufficient, the network may reject the establishment of the flow or modify its QoS parameters. GFBR is also used in handover procedures to ensure that the target cell can support the required bit rate before admitting the UE.

The 3GPP specifications define GFBR in various contexts, including network slicing, where each slice may have its own GFBR guarantees, and in interworking with 4G LTE, where GFBR maps to the Guaranteed Bit Rate (GBR) of EPS bearers. In TS 38.300 (NR overall description) and TS 38.835 (Study on enhancement for data collection for NR and EN-DC), GFBR is discussed in the context of RAN QoS management and performance measurements.

Overall, GFBR is a fundamental QoS parameter that enables deterministic service quality in 5G networks, supporting a wide range of applications from enhanced mobile broadband to ultra-reliable low-latency communications.

Purpose & Motivation

GFBR exists to provide a guaranteed minimum bit rate for specific QoS flows in 5G networks, addressing the need for deterministic quality of service for real-time and mission-critical applications. In earlier generations like 4G LTE, the concept of Guaranteed Bit Rate (GBR) was introduced for EPS bearers, but 5G introduced a more granular flow-based QoS model where GFBR applies per QoS flow. This allows operators to offer differentiated services with strict performance guarantees, such as voice calls, video conferencing, and industrial automation.

The motivation for GFBR stems from the limitations of best-effort and non-GBR services, which cannot assure consistent data rates under network congestion. By defining GFBR, 3GPP enables admission control and resource reservation mechanisms that ensure the required bandwidth is available throughout the lifetime of the flow, even during handovers or network load variations. This is essential for services that are sensitive to packet loss and delay, where falling below a certain bit rate would degrade the user experience.

Furthermore, GFBR supports network slicing and multi-operator scenarios by allowing each slice or tenant to specify their guaranteed bit rate requirements. It also facilitates interworking with 4G, where GFBR maps to GBR, ensuring continuity of QoS guarantees during mobility between generations. As 5G evolves towards Rel-20 and beyond, GFBR remains a cornerstone for QoS enforcement in diverse deployment scenarios.