Binding Identification Number

Description

The Binding Identification Number (BID) is a fundamental component within the 3GPP architecture for managing IP flow mobility and multi-access connectivity. It operates within the framework defined for the Evolved Packet Core (EPC) and later the 5G Core (5GC), specifically in scenarios involving the IP Flow Mobility and Seamless Offload (IFOM) and Multi-Access Packet Data Network (PDN) Connectivity (MAPCON) features. The BID serves as a unique handle or reference number that binds a specific IP flow or set of flows to a particular access network connection (e.g., a specific PDN connection over LTE or a trusted non-3GPP Wi-Fi access). This binding is managed by network entities, primarily the Policy and Charging Rules Function (PCRF) in EPC or the Policy Control Function (PCF) in 5GC, in conjunction with the User Equipment (UE).

Architecturally, the BID is used in signaling messages between the UE and the network core. When a UE is capable of connecting to multiple access networks simultaneously (e.g., LTE and WLAN), it can establish multiple PDN connections or bearers. Each of these connections is assigned a unique BID. The UE and the network (PCRF/PCF) use the BID to correlate policy decisions, charging rules, and routing information for IP flows with the specific access bearer they are using. For instance, a video streaming flow might be bound via a BID to a Wi-Fi connection, while a voice call flow is bound via a different BID to the LTE connection, allowing for optimized traffic steering based on policy.

The BID's role is integral to the IP Flow Mobility (IFOM) procedures defined in 3GPP specifications. IFOM allows the movement of specific IP data flows from one access path to another without disrupting the application. The BID provides the necessary identifier to track which flow is mapped to which access path during these handover or offload events. The network uses the BID, along with other parameters like the Traffic Flow Template (TFT), to install appropriate packet filters in the UE and the network gateways (e.g., PGW in EPC, UPF in 5GC) to ensure packets are routed correctly. This mechanism enables seamless user experience and efficient network resource utilization across heterogeneous access networks.

In the 5G system, the concept evolves but retains similar principles. The BID can be used in the context of ATSSS (Access Traffic Steering, Switching, and Splitting) to manage multi-access connectivity. The Session Management Function (SMF) and PCF utilize identifiers analogous to the BID to steer specific traffic descriptors over different access types (3GPP vs. non-3GPP). The BID, therefore, represents a persistent logical binding that abstracts the underlying access-specific details, providing a stable reference for policy enforcement and mobility management across different 3GPP releases and network generations.

Purpose & Motivation

The BID was created to address the growing need for seamless mobility and traffic management between diverse access technologies. As mobile operators began integrating Wi-Fi and other non-3GPP access networks with their cellular infrastructure, a mechanism was required to allow a single UE to maintain simultaneous connections and to intelligently steer specific application flows to the most appropriate access. Prior to standardized mechanisms like IFOM and MAPCON, offloading to Wi-Fi was often a simple, all-or-nothing break-before-make switch, which could disrupt ongoing services. The BID provides the foundational identifier to enable make-before-break flow mobility, enhancing user experience.

The historical context stems from 3GPP Release 10 and the work on network-based mobility and offload. The primary problem solved is service continuity during access switching. For example, a user starting a video call on LTE could have the video flow seamlessly handed over to a Wi-Fi network upon entering coverage, without dropping the call, because the binding (via the BID) allows the network to re-route the specific IP flow. This solves limitations of earlier approaches where the entire PDN connection (and all its IP flows) would have to be moved, or where simultaneous multi-access connectivity was not efficiently managed. The BID enables granular, flow-level policy control.

Furthermore, the BID supports advanced charging and policy enforcement. By binding flows to specific accesses identified by a BID, operators can apply different charging rates or quality of service (QoS) policies per flow per access type. This granularity was not possible with earlier bearer-centric models where all traffic on a bearer received uniform treatment. The creation of the BID was thus motivated by the need for finer-grained network control, efficient spectrum utilization through traffic offload, and the commercial flexibility to create differentiated service offerings based on access technology.