Bearer

Description

In 3GPP networks, a bearer is a logical channel that provides the capability for information transfer between network elements, such as between a user equipment (UE) and the core network. It is characterized by a set of parameters that define its quality of service (QoS), including capacity, delay, bit error rate, and traffic class. Bearers are essential for establishing, modifying, and releasing data connections in response to service requests, and they form the basis for QoS management in both circuit-switched and packet-switched domains.

The concept of a bearer was first introduced in Release 99 (R99) as part of the Universal Mobile Telecommunications System (UMTS) specifications. In UMTS, bearers are classified into two main categories: real-time (RT) and non-real-time (NRT). RT bearers are designed for services that require strict delay guarantees, such as voice and video calls, while NRT bearers support applications that are more tolerant to delay, such as web browsing and file downloads. Each bearer class is associated with a set of QoS parameters, such as guaranteed bit rate (GBR), maximum bit rate (MBR), and allocation and retention priority (ARP), which are used by the network to allocate resources and enforce policies.

With the evolution to Long Term Evolution (LTE) and the System Architecture Evolution (SAE), the bearer concept was refined to support an all-IP network. In LTE, bearers are established between the UE and the Packet Data Network Gateway (P-GW) and are known as EPS bearers. An EPS bearer is composed of a radio bearer between the UE and the eNodeB, an S1 bearer between the eNodeB and the Serving Gateway (S-GW), and an S5/S8 bearer between the S-GW and the P-GW. This hierarchical structure allows for flexible QoS management across different network segments. The concept of a default bearer and dedicated bearers was introduced, where the default bearer provides basic connectivity and dedicated bearers are used for specific services with distinct QoS requirements.

In 5G, the bearer concept has evolved further with the introduction of QoS flows. A QoS flow is the finest granularity of QoS treatment in the 5G System (5GS), and it is mapped to data radio bearers (DRBs) over the air interface. Unlike EPS bearers, which are established per UE, QoS flows are established per PDU session and can be dynamically added or modified to meet changing service requirements. The 5G QoS model supports both GBR and non-GBR flows, as well as reflective QoS, which allows the UE to derive QoS rules from downlink packets. This evolution reflects the need for more granular and dynamic QoS management in 5G to support diverse use cases like URLLC and mMTC.

Despite the evolution, the fundamental purpose of a bearer remains the same: to provide a defined quality of service for information transfer. Bearers are used for both user plane data and control plane signaling, and they are managed by the core network's policy and charging control (PCC) framework. The establishment, modification, and release of bearers are triggered by events such as session initiation, handover, or changes in network conditions, and they are governed by QoS rules and policies configured by the operator. Understanding bearers is essential for anyone working with mobile networks, as they are the building blocks for delivering services with guaranteed performance.

Purpose & Motivation

The bearer concept exists to provide a standardized mechanism for establishing and managing information transmission paths with specific quality of service characteristics in mobile networks. It solves the problem of how to differentiate and prioritize different types of traffic, ensuring that real-time services like voice receive the necessary resources while non-real-time services can tolerate lower quality. Before bearers, networks lacked a unified way to define and enforce QoS, leading to inconsistent user experiences and inefficient resource utilization.

The motivation for creating bearers in R99 was to support the diverse QoS requirements of UMTS services, which ranged from circuit-switched voice to packet-switched data. By defining bearers with parameters like bit rate, delay, and error rate, operators could offer service level agreements (SLAs) and manage network resources effectively. In LTE, the bearer concept was adapted to an all-IP architecture, introducing EPS bearers and the concept of default and dedicated bearers to simplify connectivity and enable differentiated services. This allowed for more scalable and flexible QoS management, which was crucial for the success of mobile broadband.

In 5G, the bearer concept evolved into QoS flows to meet the demands of new use cases that require ultra-low latency, high reliability, and massive connectivity. QoS flows provide finer granularity and dynamic control, enabling the network to adapt to changing conditions and service requirements in real-time. This evolution ensures that the bearer concept remains relevant and capable of supporting the advanced services envisioned for 5G and beyond.