Maximum Flow Bit Rate

Description

The Maximum Flow Bit Rate (MFBR) is a Quality of Service (QoS) parameter defined in the 5G QoS model. It applies to individual QoS Flows within a Protocol Data Unit (PDU) Session. A QoS Flow is the finest granularity for QoS differentiation in the 5G system, and each flow is associated with a QoS profile containing parameters like the 5G QoS Identifier (5QI), Guaranteed Flow Bit Rate (GFBR), and the MFBR. The MFBR specifies the peak bit rate that the network should not exceed for that particular QoS Flow. It represents an upper limit on the sustained data rate.

In operation, the MFBR is used by the network's User Plane functions, particularly the UPF (User Plane Function), to perform traffic policing. When user data packets for a specific QoS Flow arrive, the UPF measures the flow's bit rate. If the flow's rate attempts to exceed the configured MFBR, the UPF will typically drop or mark (e.g., using Explicit Congestion Notification) the excess packets to enforce the limit. This prevents a single QoS Flow from consuming disproportionate network resources, which is crucial for maintaining service quality for other flows and users. The MFBR is enforced on a per-flow basis, allowing for granular control.

The MFBR is signaled during PDU Session establishment or modification procedures. The SMF (Session Management Function) is responsible for deriving the QoS parameters, including MFBR, based on subscriber policies from the PCF (Policy Control Function) and the requested service. The SMF then provides these parameters to the UPF for enforcement and to the RAN (via the AMF) for potential radio resource consideration. While the RAN primarily uses the Aggregate Maximum Bit Rate (AMBR) for non-GBR flows, the per-flow MFBR is essential in the core network for precise traffic management. Its value is typically expressed in kilobits per second (kbps) or megabits per second (Mbps) and is a critical component in realizing network slicing, as different slices can have vastly different MFBR limits tailored to their service requirements (e.g., enhanced Mobile Broadband vs. IoT).

Purpose & Motivation

The MFBR exists to provide a mechanism for precise traffic policing and resource management at the granularity of a QoS Flow. In the absence of such a parameter, data flows could potentially consume unlimited bandwidth, leading to network congestion, unfair resource allocation, and degradation of service for other users. It is a fundamental tool for implementing service-level agreements (SLAs) and differentiated charging models, where users or services are granted specific maximum bandwidth entitlements.

Historically, similar concepts existed in earlier systems (like Maximum Bit Rate in EPS), but the 5G QoS model refines it within a more flexible, flow-based architecture. The motivation for MFBR in 5G is heightened by the diverse range of services, from ultra-high-definition video (requiring high MFBR) to massive IoT (requiring very low MFBR). It allows network operators to efficiently package and sell tiered data services, enforce fair use policies, and ensure that critical services (with guaranteed bit rates) are not starved by best-effort traffic attempting to use excessive bandwidth. It is a key enabler for network efficiency and monetization in 5G.