Fixed Network User Rate

Description

Fixed Network User Rate (FNUR) is a QoS class identifier standardized within 3GPP specifications, specifically detailed in TS 22.034 and the vocabulary document TS 21.905. It belongs to a family of QoS parameters designed to characterize the traffic handling capabilities required for different services across the network. FNUR specifically defines a traffic class where the user data transfer rate is guaranteed to be constant and fixed by the network. This is in contrast to variable or best-effort rates, providing a deterministic bandwidth allocation.

Architecturally, FNUR is implemented within the core network's policy and charging control (PCC) framework. When a service data flow is established—for example, for a high-quality video call—the application function (AF) or the user equipment (UE) itself can request a specific QoS, including an FNUR. This request is processed by the Policy and Charging Rules Function (PCRF), which translates it into specific policy and charging control (PCC) rules. These rules are then enforced at the gateway nodes, such as the Packet Data Network Gateway (P-GW) in 4G or the User Plane Function (UPF) in 5G, ensuring the data flow receives the pre-allocated, constant bit rate throughout its session.

The key operational principle involves resource reservation and admission control. The network must check if sufficient resources (e.g., bandwidth on fixed-line backhaul links) are available to honor the requested fixed rate before admitting the session. Once admitted, traffic policing and shaping functions at the enforcement points ensure the user's traffic conforms to the contracted FNUR, potentially marking or dropping excess packets. This mechanism is crucial for maintaining service level agreements (SLAs) and providing a predictable user experience for real-time, constant-bit-rate applications, forming a foundational element of managed QoS in converged fixed-mobile networks.

Purpose & Motivation

FNUR was introduced to address the need for guaranteed, predictable data throughput for specific applications in telecommunications networks. Early data services primarily offered best-effort delivery, which was insufficient for real-time applications like voice over IP (VoIP) or video conferencing that suffer from jitter and packet loss under variable bandwidth. The creation of standardized QoS parameters like FNUR allowed network operators to offer tiered services with performance guarantees.

The historical context lies in the evolution of 3GPP standards to support multimedia services. As networks progressed from basic voice and SMS to rich multimedia, the requirement for differentiated traffic handling became paramount. FNUR, as part of the broader QoS framework defined in Release 4, provided a tool to model and support constant bit rate (CBR) traffic sources. It solved the problem of how to formally request and network-manage a service that needs a steady, unvarying pipe of bandwidth, enabling the reliable support of legacy CBR codecs and certain streaming protocols within a packet-switched paradigm.

It addressed limitations of previous 'one-size-fits-all' IP networks by enabling explicit resource reservation. Without such a parameter, supporting high-quality, real-time services would require massive over-provisioning or would result in poor quality. FNUR allowed for efficient network utilization while meeting strict performance criteria, facilitating the commercial rollout of premium, guaranteed-bit-rate services.

Key Features

• Defines a constant, guaranteed bit rate for user data flows
• Integrates with the 3GPP Policy and Charging Control (PCC) architecture for dynamic policy enforcement
• Enables admission control to prevent network over-subscription for guaranteed services
• Supports traffic policing and shaping at network gateways to enforce the contracted rate
• Essential for supporting real-time applications with strict delay and jitter requirements
• Provides a basis for Service Level Agreements (SLAs) between operators and customers

Evolution Across Releases

Defining Specifications