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.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (83 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the MFBR (Maximum Flow Bit Rate) function was introduced for GBR QoS Flows, with enforcement specified for both the uplink and downlink directions. Specifically, the MFBR is enforced in the UPF for the downlink and in the (R)AN for both downlink and uplink, while for non-3GPP access, the UE is responsible for uplink MFBR enforcement. The release also included clarifications on supporting an MFBR value that is greater than the GFBR (Guaranteed Flow Bit Rate).
- How to determine the maximum number of established PDU sessions TS 24.501CR0077
- MFBR Enforcement for GBR QoS Flows TS 23.501CR0226
- Handling of maximum supported data rate per UE for integrity protection TS 23.501CR0334
- Clarification on support of MFBR greater than GFBR TS 23.501CR0417
- Correction for maximum data rate per UE for integrity protection for DRBs TS 24.501CR0016
- Correction for maximum data rate per UE for integrity protection for DRBs for PDU sessions in non-3GPP access which are transferable to 3GPP access TS 24.501CR0371
+ 9 more changes
In Release 16, the MFBR (Maximum Flow Bit Rate) was made a mandatory parameter for GBR (Guaranteed Bit Rate) QoS Flows. The specification clarified enforcement, stating the MFBR is enforced in the UPF in the downlink and in the (R)AN for both downlink and uplink directions. Furthermore, a syntactical error was declared for QoS profiles where both the uplink and downlink MFBR values are set to zero.
- Clarification of PCF behaviour to honor UE provided maximum packet filter support TS 29.512CR0491
- NIDD Description Update for Maximum Packet Size TS 23.501CR1364
- PLMN+CAG information - minimum, maximum storage and survival of power cycle TS 23.501CR1520
- Fix terminology on maximum number of CAGs per cell instead of per NG-RAN node TS 23.501CR2217
- Modification of the maximum number of supported packet filters TS 24.501CR1114
- Handling of maximum number of allowed active DRBs TS 24.501CR1563
+ 4 more changes
In Release 17, the specifications introduced clarifications for indicating the subscribed Maximum Flow Bit Rate (MFBR) for Guaranteed Bit Rate (GBR) QoS Flows. The release also provided corrections and clarifications on handling the maximum number of established PDU sessions, including specific scenarios for Multi-Access (MA) PDU sessions and NB-N1 mode. Furthermore, it defined that the MFBR is enforced in the User Plane Function (UPF) in the downlink and in the (R)AN for both downlink and uplink directions.
- Introduction of support of GSMA NG.116 attributes Maximum DL/UL throughput per slice/UE TS 23.501CR2822
- S-NSSAI rejected due to maximum number of UEs reached and BO timer value TS 24.501CR3123
- Maximum number of established PDU sessions already reached for a NW slice TS 24.501CR3213
- Clarifications on indicating subscribed MFBR/GFBR uplink/downlink TS 24.501CR2675
- Correction for NB-N1 mode and maximum number of PDU sessions with active user plane resources TS 24.501CR2976
- Clarification on handling maximum number of established PDU sessions for MA PDU session TS 24.501CR3181
+ 13 more changes
In Release 18, the specification introduced new control and rejection procedures for the Maximum Flow Bit Rate (MFBR) function, specifically for PDU session establishment when group data rate limits are exceeded. This enhancement is part of the broader "Maximum Group Data Rate control" feature, where the network can now reject a session establishment request to enforce these aggregate rate limits. The MFBR itself continues to be enforced in the UPF for the downlink and in the (R)AN for both downlink and uplink for GBR QoS Flows, as defined in previous releases.
- Equivalent SNPNs: NSSAIs, network-assigned UE radio capability ID, maximum number of established PDU sessions and 5GMM parameters in annex C stored per selected entry TS 24.501CR5027
- New Maximum signalling waiting time due to discontinuous coverage TS 24.501CR5240
- Updating the name for maximum time offset for unavailability period TS 24.501CR5810
- PDU session establishment rejection for Maximum Group Data Rate control TS 24.501CR5877
- Maximum Group Data Rate control TS 29.512CR1187
- Introduction of maximum time duration to initiate CG-SDT in Stage-2 [CG-SDT-Enh] TS 38.300CR0743
+ 23 more changes
In Release 19, the primary changes related to the MFBR function were not directly about its enforcement but involved significant clarifications and corrections to the associated "Maximum Time Offset" procedures for managing discontinuous coverage. These updates specifically refined the UE's behavior upon the expiry of the discontinuous coverage maximum time offset timer and standardized the related terminology and information elements. The modifications ensured consistent handling of the timer that governs when a UE initiates a Mobility Registration Update before losing coverage or after returning to coverage, which supports the maintenance of QoS flows and their associated MFBR.
- Clarification on the maximum time offset TS 24.501CR6364
- Correction regarding the discontinuous coverage maximum time offset timer TS 24.501CR6388
- Correction on maximum time offset TS 24.501CR6375
- UE behavior at expiry of discontinuous coverage maximum time offset timer TS 24.501CR6449
- Stop discontinuous coverage maximum time offset timer TS 24.501CR6501
- The correction on discontinuous coverage maximum time offset TS 24.501CR6459
+ 4 more changes
Explore further
Broader topics and technologies where MFBR plays a role.
Defining Specifications
3GPP specifications that define or reference MFBR, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 24.554 vj40 | 5G Proximity Services (ProSe) Protocols | Rel-19 |
| TS 24.890 vg00 | 5G NAS Protocol for 5GS Stage 3 | Rel-16 |
| TS 26.510 vj10 | Media Delivery APIs for 5GMS and RTC Systems | Rel-19 |
| TS 26.512 vj10 | 5G Media Streaming Protocols & APIs | Rel-19 |
| TR 26.926 vj00 | Traffic Models & Quality Evaluation for Media/XR in 5G | Rel-19 |
| TR 26.928 vj00 | Study on eXtended Reality (XR) in 5G | Rel-19 |
| TS 29.061 vj00 | Packet Domain Interworking for PLMN | Rel-19 |
| TS 29.512 vj40 | 5G Session Management Policy Control Service | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |