Corresponding node

Description

The term 'Corresponding node' is defined in 3GPP TS 38.420 and TS 38.425 for the purpose of flow control in Integrated Access and Backhaul (IAB) networks. In an IAB network, the IAB-Donor hosts the Packet Data Convergence Protocol (PDCP) entity for a given data radio bearer (DRB), while the IAB-DU (Distributed Unit) serves the User Equipment (UE) for that DRB. The Corresponding node is the node that interacts with the PDCP-hosting node for flow control purposes. Specifically, in the IAB context, the Corresponding node is the IAB-DU that serves the UE for the corresponding DRB.

Flow control is essential in IAB networks to manage the data flow between the IAB-Donor and the IAB-DU, ensuring that the backhaul links are not congested and that the UE receives data at an appropriate rate. The Corresponding node, as the IAB-DU, provides feedback to the PDCP-hosting node (the IAB-Donor) regarding the status of the radio link and buffer occupancy, allowing the donor to adjust the flow of downlink data. This interaction is governed by the F1 Application Protocol (F1AP) and the Flow Control procedures defined in TS 38.425.

The concept of a Corresponding node is introduced in Release 19 as part of the ongoing enhancements to IAB in 5G-Advanced. It provides a clear terminology for the node that participates in flow control with the PDCP entity. The Corresponding node may also be involved in other procedures such as data forwarding during handover, but its primary role is in flow control. The definition is intentionally broad to cover various scenarios where a node interacts with a PDCP-hosting node for flow control, but the IAB-specific example clarifies its application.

In an IAB network, the IAB-DU is responsible for the radio interface with the UE and for the backhaul link with its parent IAB-node or the IAB-Donor. The IAB-DU hosts the Radio Link Control (RLC), Medium Access Control (MAC), and Physical (PHY) layers, while the PDCP layer is hosted at the IAB-Donor. This split architecture requires careful flow control to avoid buffer overflow at the IAB-DU and to ensure timely delivery of data. The Corresponding node (IAB-DU) sends flow control feedback to the IAB-Donor, which then adjusts the amount of data it sends over the backhaul link.

The introduction of the term 'Corresponding node' in Rel-19 reflects the maturity of IAB specifications and the need for precise terminology in flow control procedures. It is expected that this term will be used in future releases as IAB evolves, particularly with the integration of new features like multi-hop backhauling and advanced scheduling. The definition is concise but sufficient for engineers familiar with IAB architecture to understand the role of the node in flow control.

Purpose & Motivation

The term 'Corresponding node' was introduced to provide a clear and unambiguous reference to the node that interacts with a PDCP-hosting node for flow control in IAB networks. Prior to its definition, specifications might have used descriptive phrases or relied on context to identify this node, which could lead to confusion in multi-vendor implementations. By defining the term, 3GPP ensures that all parties have a common understanding of which node is responsible for flow control feedback and how it should interact with the PDCP entity.

In IAB networks, flow control is critical because the backhaul links can be wireless and subject to varying channel conditions. The IAB-DU must manage its buffer and request more or less data from the IAB-Donor based on the radio conditions and the amount of data pending for transmission to the UE. The Corresponding node is the entity that performs this function. Without a standardized term, engineers might misinterpret which node should send flow control messages, leading to interoperability issues.

The definition also serves to distinguish the Corresponding node from other nodes in the IAB architecture, such as the parent IAB-node or the IAB-Donor. While the IAB-Donor hosts the PDCP, the Corresponding node is the one that directly serves the UE and thus has the most accurate information about the radio link quality and buffer status. This clarity is essential for the correct implementation of flow control procedures as specified in TS 38.425.