Guaranteed Bit Rate

Description

Guaranteed Bit Rate (GBR) is a fundamental Quality of Service (QoS) attribute defined in 3GPP standards for EPS (Evolved Packet System) and 5GS (5G System). It applies to dedicated bearers (in EPS) or QoS Flows (in 5GS) that require a minimum level of bandwidth assurance from the network. When a bearer/flow is designated as GBR, the network performs Admission Control during its establishment: it checks if sufficient radio and transport resources are available to honor the requested GBR value. If resources are available, the network commits them, reserving capacity to ensure the bit rate is maintained for the duration of the session, barring exceptional conditions like radio link failure.

The GBR value is coupled with other QoS parameters such as the Maximum Bit Rate (MBR), which caps the peak rate, and the QoS Class Identifier (QCI or 5QI). The network, particularly the eNodeB/gNodeB and the core network's policy control (PCRF/PCF), continuously monitors the resource usage of GBR bearers. In congestion scenarios, resources allocated to GBR bearers are protected, while non-GBR traffic may be throttled. The enforcement of GBR happens primarily at the radio scheduler in the base station, which allocates time-frequency resources to meet the guaranteed rate for each active GBR bearer.

GBR bearers are typically used for real-time conversational services (e.g., Voice over LTE/5G, video calls) and real-time streaming services. The guarantee ensures low latency, jitter, and packet loss, providing a consistent user experience. In contrast, Non-GBR bearers are used for elastic traffic like web browsing or email, where the available bit rate can vary based on network load. The distinction between GBR and Non-GBR is a cornerstone of the 3GPP QoS model, enabling the network to efficiently multiplex traffic with diverse requirements while meeting strict service level agreements.

Purpose & Motivation

GBR was introduced to support real-time, delay-sensitive applications over packet-switched mobile networks. Early mobile data services (e.g., basic GPRS) offered "best-effort" service, which was insufficient for voice, video telephony, or live streaming, as these applications suffer severely from variable throughput and latency.

The creation of GBR as a QoS parameter solved this by enabling the network to provide resource assurance. It allows operators to offer Service Level Agreements (SLAs) for specific applications or user tiers. From a technical perspective, GBR motivates the necessary resource reservation and admission control mechanisms within the RAN and core network. It addresses the limitation of stateless, best-effort IP routing by introducing a stateful, connection-oriented QoS model within the packet core, ensuring that real-time services can coexist with background traffic without degradation. This was crucial for the convergence of all services onto a single IP-based access network, a key goal of EPS and 5GS.