Guaranteed Bit Rate Bearer

Description

A Guaranteed Bit Rate (GBR) bearer is a type of EPS bearer in 3GPP networks that provides a guaranteed minimum bit rate for the user's traffic. It is characterized by dedicated network resources that are permanently allocated at the time of bearer establishment or modification, ensuring that the specified GBR is met under normal network conditions. GBR bearers are used for services that require a consistent and predictable quality of service, such as Voice over IP (VoIP), video conferencing, real-time gaming, and streaming media. The GBR value represents the minimum bit rate that the network guarantees to provide, while the Maximum Bit Rate (MBR) defines the upper limit.

In the EPS architecture, bearers are classified into two categories: GBR bearers and Non-GBR bearers. GBR bearers are typically established for services that have strict delay and loss requirements, and they are associated with QoS Class Identifiers (QCIs) that define their characteristics. For example, QCI 1 is for conversational voice, QCI 2 for conversational video, QCI 3 for real-time gaming, and QCI 4 for non-conversational video. Each QCI has a corresponding GBR and MBR, along with other parameters like packet delay budget and packet error loss rate. The network allocates dedicated resources for GBR bearers, which may include radio resources, transport resources, and core network resources. These resources are reserved for the lifetime of the bearer, ensuring that the GBR is maintained even during congestion.

The establishment of a GBR bearer involves signaling between the UE, eNodeB, MME, S-GW, P-GW, and PCRF (in non-5G networks). The PCRF provides the QoS parameters (QCI, GBR, MBR) based on the service requirements and subscription data. The MME then requests the eNodeB to set up the bearer with the appropriate QoS. The eNodeB performs admission control to ensure that it has sufficient resources to meet the GBR; if resources are insufficient, the bearer establishment may be rejected or the GBR may be downgraded. Once established, the bearer is maintained with the allocated resources, and the network monitors the traffic to ensure compliance. If the UE exceeds the MBR, the network may drop or delay packets. If the UE's traffic falls below the GBR, the resources remain allocated, which can be inefficient but ensures guaranteed service.

GBR bearers are particularly important in LTE and 5G networks for supporting real-time services. In 5G, the concept of GBR bearers has evolved into QoS flows, which are similar but more flexible. A 5G QoS flow with a GBR resource type also has a guaranteed flow bit rate (GFBR) and maximum flow bit rate (MFBR). The 5G QoS model allows for more granular QoS handling and supports reflective QoS. However, the fundamental principle of guaranteeing a bit rate remains the same. GBR bearers are also used in IMS for voice calls, where the dedicated bearer is established for the duration of the call to ensure voice quality.

The management of GBR bearers involves several 3GPP specifications, including TS 23.203 (Policy and charging control architecture), TS 24.301 (NAS protocol for EPS), TS 24.801 (NAS protocol for 5G), and TS 29.804 (Policy and charging control for 5G). These specifications define the procedures for bearer establishment, modification, and release, as well as the interaction with the policy and charging functions. The GBR bearer concept has been stable since its introduction in Release 7 (as IP-CAN bearer) and was adapted for EPS in Release 8. It continues to be a cornerstone of QoS management in mobile networks.

Purpose & Motivation

The GBR bearer was created to provide a reliable and predictable quality of service for applications that require a minimum bit rate to function correctly. Before GBR, all traffic was treated as best effort, which meant that real-time services like voice and video could suffer from degradation during network congestion. GBR bearers solve this by reserving dedicated resources for the duration of the service, ensuring that the guaranteed bit rate is always available.

The motivation came from the need to support services like VoIP and video telephony over IP-based networks, where packet loss and delay can severely impact user experience. By guaranteeing a bit rate, the network can ensure that these services meet their stringent QoS requirements. This is particularly important in mobile networks, where radio resources are scarce and varying channel conditions can affect throughput.

GBR bearers also enable operators to offer differentiated services and charge accordingly. They provide a mechanism for policy control and charging, allowing operators to enforce QoS policies and generate revenue from premium services. The introduction of GBR bearers in 3GPP Release 7 (for IP-CAN) and Release 8 (for EPS) marked a significant step towards a fully QoS-enabled all-IP network. The concept has been carried forward to 5G with QoS flows, demonstrating its enduring importance.