Linked Bearer Identity

Description

The Linked Bearer Identity (LBI) is a fundamental identifier within the 3GPP Evolved Packet Core (EPC) and 5G Core (5GC) bearer and QoS flow management architecture. It is a numerical value that uniquely identifies the default bearer within a Packet Data Network (PDN) connection to which a dedicated bearer is linked. Every PDN connection established by a User Equipment (UE) must have one, and only one, default bearer. This default bearer provides the IP connectivity and serves as the permanent control plane anchor for that PDN connection. Any additional bearers created within the same PDN connection for specific services (like VoIP or video streaming) are dedicated bearers, and each must contain the LBI pointing to its parent default bearer.

Architecturally, the LBI is assigned by the network during default bearer establishment and is known to key core network functions: the Mobility Management Entity (MME) in EPC, the Serving Gateway (SGW), the PDN Gateway (PGW), and later the Session Management Function (SMF) and User Plane Function (UPF) in 5GC. When a dedicated bearer is established—typically triggered by the Policy and Charging Rules Function (PCRF) or the Policy Control Function (PCF) based on detected service data flows—the creation request (e.g., Bearer Resource Command) includes the LBI. This tells the network within which PDN connection context the new bearer should be created. The dedicated bearer inherits key attributes from the default bearer, such as the PDN type (IPv4, IPv6) and the PGW/SMF IP address, but has its own distinct QoS profile defined by its QCI (QoS Class Identifier) and potentially GBR values.

Operationally, the LBI enables efficient bearer management and policy enforcement. All bearers sharing the same LBI belong to the same PDN connection and thus share the same UE IP address(es). This linkage allows the network to apply aggregate policies, like the Aggregate Maximum Bit Rate (APN-AMBR), across all bearers of that PDN connection. The APN-AMBR limits the total bit rate a UE can consume across its default and all linked dedicated bearers for a given APN. When the default bearer is modified or released, the LBI provides the necessary context to manage all its linked dedicated bearers accordingly. For example, releasing the default bearer (e.g., during PDN disconnection) triggers the release of all dedicated bearers linked via that LBI.

In the 5G System, the concept evolves but retains similar principles. Instead of an EPS bearer with an LBI, 5G uses QoS Flows within a PDU Session. The equivalent linking is achieved implicitly as all QoS Flows belong to a specific PDU Session, which has a unique PDU Session ID. However, the LBI parameter itself is still used in interworking scenarios between 4G and 5G (e.g., during handovers with N26 interface) to map EPS bearers to 5G QoS Flows and vice versa, maintaining service continuity. Thus, the LBI remains a critical element for maintaining the logical hierarchy and context of user plane connections in 3GPP networks.

Purpose & Motivation

The Linked Bearer Identity was created to address the complexity of managing multiple concurrent data flows with different QoS requirements for a single UE connection to a single APN. Early packet data systems provided a single, monolithic data pipe per APN connection. As services diversified, there was a need to support simultaneous applications—like web browsing (best-effort), VoIP (low latency), and video streaming (high bandwidth)—each requiring distinct QoS treatment within the same IP session. Creating entirely separate PDN connections for each service would be inefficient, consuming extra IP addresses and signaling overhead.

The LBI enables the elegant and efficient 'bearer hierarchy' model. It solves the problem of how to associate multiple specialized data paths (dedicated bearers) with a common control and connectivity anchor (the default bearer). Without the LBI, the network would have no way to group bearers logically, making it impossible to enforce aggregate policies like APN-AMBR or to perform coordinated lifecycle management (e.g., deleting all bearers for an APN when the user roams out of coverage). The LBI provides this essential grouping mechanism.

Furthermore, the LBI simplifies policy control and charging. The PCRF/PCF can authorize multiple dedicated bearers for different service data flows (e.g., based on deep packet inspection or application request) and link them to the same default bearer context using the LBI. This allows for granular, per-flow QoS and charging rules while maintaining a unified billing and policy context for the user's session with a specific service provider (APN). It also facilitates mobility; during handovers, the network can efficiently transfer the state of an entire PDN connection by identifying the default bearer and its linked dedicated bearers via the LBI. In essence, the LBI is the glue that allows 3GPP networks to offer sophisticated, multi-service IP connectivity with differentiated quality, which is a cornerstone of the All-IP network vision.