Description
The Aggregated Maximum Bit Rate (AMBR) is a fundamental Quality of Service (QoS) parameter within the 3GPP Evolved Packet System (EPS) and 5G System (5GS). It operates at two distinct levels: the UE-AMBR (per User Equipment) and the APN-AMBR (per Access Point Name, known as Session-AMBR in 5GS). The UE-AMBR is enforced by the base station (eNodeB in 4G, gNB in 5G) on the radio interface and limits the aggregate bit rate across all non-Guaranteed Bit Rate (non-GBR) bearers or QoS Flows associated with a single UE. The APN-AMBR (or Session-AMBR) is enforced by the core network's User Plane Function (PGW-U/UPF) and applies to the aggregate of all non-GBR bearers/QoS Flows within a specific PDN connection or PDU Session. This hierarchical enforcement ensures control both at the air interface for radio resource fairness and at the Gi/SGi/N6 interface for backhaul and core network resource management.
Architecturally, AMBR is a subscription parameter stored in the Home Subscriber Server (HSS) or Unified Data Management (UDM). During session establishment or modification, this parameter is retrieved and provided to the Policy and Charging Rules Function (PCRF) or Policy Control Function (PCF), which then communicates the authorized AMBR values to the enforcement points via the Packet Data Network Gateway (PGW) or Session Management Function (SMF). The enforcement nodes, such as the eNodeB/gNB and PGW-U/UPF, utilize traffic policing mechanisms like token bucket algorithms to monitor and shape the aggregate traffic, ensuring it does not exceed the configured AMBR limit. Non-compliant packets may be delayed or dropped.
AMBR applies exclusively to non-GBR bearers or QoS Flows, which are used for best-effort services like web browsing, email, and video streaming. This is in contrast to GBR bearers, which have dedicated, guaranteed bandwidth for services like voice or real-time gaming. The separation allows the network to provide strict guarantees for latency-sensitive traffic while efficiently sharing remaining capacity among elastic, non-real-time applications. In 5GS, the concept is enhanced with the Reflective QoS feature, where the UE can derive QoS rules for uplink traffic based on downlink packet markings, but the Session-AMBR remains a network-enforced ceiling.
Its role is pivotal for network efficiency and user experience. By capping the total best-effort bandwidth per user or per session, AMBR prevents network congestion caused by a few heavy users, promotes equitable resource distribution among all connected UEs, and allows operators to implement tiered service plans. It is a key tool for traffic management and monetization, forming the basis for differentiated data packages without requiring per-flow policing for every application.
Purpose & Motivation
AMBR was introduced in 3GPP Release 8 with the EPS to address the limitations of earlier 3G systems in managing the explosive growth of mobile data traffic. Pre-4G networks primarily relied on per-bearer QoS parameters but lacked an efficient mechanism to control the aggregate consumption of a user's multiple simultaneous data sessions (e.g., web, email, app updates). Without AMBR, a user could establish multiple non-GBR bearers, each requesting high bandwidth, and potentially exhaust radio and core network resources, degrading service for other users. AMBR solves this by providing a simple, enforceable ceiling on a user's total best-effort traffic.
The creation of AMBR was motivated by the need for scalable and practical QoS management in all-IP networks. It allows operators to offer 'unlimited' data plans with fair usage policies by technically limiting the maximum achievable throughput, thus protecting network integrity. It also enables service differentiation; for example, a premium subscriber can have a higher UE-AMBR than a basic subscriber, even if they are using the same applications. This addressed a key business requirement for tiered pricing models.
Furthermore, AMBR provides a crucial separation between guaranteed and best-effort service handling. By only applying to non-GBR traffic, it ensures that the network can always reserve necessary resources for GBR services (like VoLTE) to meet their strict latency and reliability requirements, while dynamically sharing the remaining capacity. This architectural principle of aggregate policing for elastic traffic and dedicated resources for inelastic traffic has proven essential for supporting the diverse mix of services on modern mobile networks.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (96 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, the AMBR function was enhanced to support higher bitrate values through the introduction of an Extended APN-AMBR information element. The release also specifically defined the PDU Session AMBR as a new parameter and clarified the handling of the session-AMBR during procedures like EPS bearer context modification. Furthermore, corrections and clarifications were made to the encoding of the Extended APN-AMBR IE, addressing the use of its most and least significant bits.
- How to determine the maximum number of established PDU sessions TS 24.501CR0077
- Extending maximum bitrate in EPS QoS IE TS 24.301CR2891
- Handling of extended EPS quality of service IE and extended APN-AMBR IE TS 24.301CR2919
- Correction of APN-AMBR TS 24.301CR3031
- MSB & LSB in the coding of Extended QoS and Extended APN AMBR IE TS 24.301CR3037
- Reset of PLMN's maximum number of EPS bearer contexts TS 24.301CR3122
+ 15 more changes
In Release 16, the enhancements to the AMBR function included new Serving PLMN control over UE AMBR and corrections to the placement of UE-AMBR parameters. The release also introduced clarifications for the PCF to honour UE-provided maximum packet filter support and modifications to handle the maximum number of supported packet filters and active DRBs. Furthermore, it defined the application of Session-AMBR within the context of a Multi-Access PDU session.
- Serving PLMN UE AMBR control TS 29.507CR0062
- Correct the wrong placement of UE-AMBR TS 23.503CR0443
- Clarification of PCF behaviour to honour UE provided maximum packet filter support TS 23.503CR0466
- Correct UE behavior when maximum number of active EPS bearer contexts is reached and the upper layers request more DRBs TS 24.301CR3317
- Modification of the maximum number of supported packet filters TS 24.501CR1114
- Handling of maximum number of allowed active DRBs TS 24.501CR1563
+ 2 more changes
In Release 17, the enhancements to AMBR-related functions primarily involved clarifications and corrections for session-AMBR handling in Multi-Access (MA) PDU sessions and during PDU session establishment. The release also introduced the new concept of a Slice Maximum Bit Rate (Slice-MBR) to complement existing per-session and per-UE bit rate controls. Furthermore, numerous refinements were made to the network's handling of session and slice rejection causes, particularly for scenarios like reaching the maximum number of UEs per network slice or the maximum number of PDU sessions.
- KI#3 - Slice Maximum Bit Rate (Slice-MBR) TS 23.503CR0544
- 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
- Fix typo in the minimum range of APN-AMBR for downlink or uplink (extended-2) TS 24.301CR3525
- 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
+ 16 more changes
In Release 18, the enhancements to AMBR functions primarily focused on new rate control mechanisms and specific handling for satellite and discontinuous coverage scenarios. Key introductions included the "Maximum Group Data Rate" control for PDU session establishment rejection and the "Maximum Slice Data Rate" capability. Furthermore, corrections and considerations were made for session-AMBR handling and for estimated maximum wait times related to discontinuous coverage, particularly for satellite access.
- Introduce Maximum time offset IE in the TAU ACCEPT message TS 24.301CR3970
- 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
- Introduction of maximum time duration to initiate CG-SDT in Stage-2 [CG-SDT-Enh] TS 38.300CR0743
+ 27 more changes
In Release 19, the main update related to AMBR was the refinement of procedures and terminology for the "discontinuous coverage maximum time offset." The changes focused on correcting and clarifying UE behavior and network instructions for stopping or handling the expiry of the associated timer. This included updates to the relevant Information Element (IE) and its value to ensure consistent implementation.
- UE behavior at expiry of discontinuous coverage maximum time offset timer TS 24.301CR4113
- Stop discontinuous coverage maximum time offset timer TS 24.301CR4131
- The correction on discontinuous coverage maximum time offset in EPS TS 24.301CR4174
- 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
+ 6 more changes
Explore further
Broader topics and technologies where AMBR plays a role.
Defining Specifications
3GPP specifications that define or reference AMBR, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.203 vj20 | Policy and charging control architecture | Rel-19 |
| TS 23.401 vj50 | Evolved Packet System (EPS) Stage 2 Description | Rel-19 |
| TS 23.503 vk00 | 5G Policy and Charging Control Framework | Rel-20 |
| TS 23.700 vk00 | XR Services Application Enablement Layer | Rel-20 |
| TS 24.301 vj60 | NAS protocol for Evolved Packet System | Rel-19 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 24.801 v810 | CT1 SAE NAS Aspects for EPC | Rel-8 |
| TS 29.061 vj00 | Packet Domain Interworking for PLMN | Rel-19 |
| TS 29.212 vj00 | Gx/Gxx/Sd/St Diameter Protocol | Rel-19 |
| TS 29.274 vj50 | GTPv2-C Control Plane Protocol Specification | Rel-19 |
| TS 29.507 vj40 | 5G Access & Mobility Policy Control Service | Rel-19 |
| TS 29.508 vj40 | 5G Session Management Event Exposure Service | Rel-19 |
| TS 29.513 vj40 | 5G PCC Signalling Flows & QoS Mapping | Rel-19 |
| TS 29.890 vg00 | CT3 5G System Technical Report | Rel-16 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.875 vd10 | Dual Connectivity Extension Requirements | Rel-13 |
| TR 37.985 vj00 | Overview of V2X features in LTE and NR | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |