Access Network Bit rate Recommendation

Description

The Access Network Bit rate Recommendation (ANBR) is a Quality of Service (QoS) signaling mechanism defined within the 3GPP framework, specifically for IP Multimedia Subsystem (IMS) and Packet Switched Streaming (PSS) services. It operates by allowing the core network, specifically the Policy and Charging Rules Function (PCRF) or the Access Network Discovery and Selection Function (ANDSF) in certain architectures, to communicate a recommended maximum bit rate to the application function (AF) or directly to the User Equipment (UE). This recommendation pertains to the bit rate for media flows (e.g., voice, video) as they traverse the radio access network (RAN). The ANBR value is not a guaranteed or enforced bit rate but an advisory, typically derived from network policy, subscription profiles, and real-time or predicted radio conditions. The primary architectural interaction involves the Rx interface between the AF (e.g., a P-CSCF in IMS) and the PCRF, where the ANBR is carried within the Authentication and Authorization Request (AAR) and Answer (AAA) Diameter messages. For UE-assisted scenarios, the information can be conveyed via SIP signaling or within media descriptions.

Upon receiving the ANBR, the application layer—such as a media encoder in the UE or a media resource function in the network—uses this value to adapt its media transmission characteristics. For example, a video streaming client may select a lower bit rate video codec profile or adjust its encoding parameters to ensure the generated media stream does not exceed the recommended rate. This adaptation is dynamic; the ANBR can be updated during an ongoing session in response to changes in network conditions, such as cell load changes or user mobility events triggering a handover to a cell with different capacity. The mechanism is integral to the QoS architecture for Evolved Packet System (EPS) and 5G System (5GS), working in conjunction with other QoS parameters like the Guaranteed Bit Rate (GBR) and Maximum Bit Rate (MBR) to manage radio and transport resources efficiently.

ANBR's role is crucial for optimizing the utilization of the often-constrained radio interface while maintaining satisfactory Quality of Experience (QoE). By preventing the application from sending media at a rate higher than the access network can sustainably support, ANBR helps avoid packet loss, jitter, and excessive delays that degrade real-time communications. It enables a more intelligent, network-assisted adaptation compared to purely end-to-end, application-layer adaptation protocols like DASH or WebRTC, as it incorporates network policy and radio-level awareness. The mechanism supports both uplink and downlink directions, with separate ANBR values possible for each, allowing for asymmetric network conditions. Its implementation is specified across multiple 3GPP technical specifications covering IMS, multimedia telephony, and streaming services, ensuring interoperability between network elements and client devices.

Purpose & Motivation

ANBR was introduced to address the challenge of efficiently delivering high-quality multimedia services over cellular networks with variable and sometimes limited radio resources. Prior to its standardization, media adaptation was primarily handled by end-to-end protocols or based on passive observations of packet loss and delay, which could be reactive and inefficient. This often led to suboptimal user experiences, such as video freezing or voice call dropouts during network congestion, because the application was unaware of the specific capacity constraints of the radio access link. The motivation for ANBR stemmed from the need for a network-assisted mechanism that could proactively guide application behavior, aligning media bit rates with the available access network bandwidth as determined by network policy and conditions.

The creation of ANBR was driven by the evolution towards all-IP networks and the proliferation of IMS-based services like Voice over LTE (VoLTE) and Video over LTE (ViLTE). These real-time services have strict latency and jitter requirements, making efficient radio resource management critical. Traditional QoS mechanisms in 3GPP, such as dedicated bearers with GBR, provide resource reservation but are relatively static and resource-intensive. ANBR complements these by offering a more flexible, policy-driven recommendation that does not require the establishment of a dedicated bearer, reducing signaling overhead and enabling dynamic adaptation for a large number of concurrent sessions. It solves the problem of network congestion caused by uncoordinated high-bit-rate media streams, thereby improving overall network capacity and stability.

Furthermore, ANBR enables service differentiation and personalized QoS. Operators can use it to enforce subscription-based policies, recommending different bit rates for premium versus standard users even under the same radio conditions. It also facilitates seamless mobility; as a user moves, the network can update the ANBR to reflect the capabilities of the new serving cell, allowing the media session to adapt gracefully without service interruption. This addresses limitations of previous approaches that lacked direct network input, providing a standardized interface for cross-layer optimization between the application and the radio access network.

Key Features

• Provides network-generated bit rate recommendations for media flows
• Enables dynamic, in-session adaptation of media encoding bit rates
• Works in both uplink and downlink directions independently
• Operates over standardized interfaces like Rx between AF and PCRF
• Complements existing QoS mechanisms (GBR, MBR) without requiring dedicated bearer setup
• Supports policy-driven service differentiation based on subscription and network conditions

Evolution Across Releases

Defining Specifications