Backhaul Adaptation Protocol

Description

The Backhaul Adaptation Protocol (BAP) is a key protocol layer within the IAB architecture, operating above the Radio Link Control (RLC) layer and below the Service Data Adaptation Protocol (SDAP) and Packet Data Convergence Protocol (PDCP) layers for user plane traffic. Its primary function is to provide routing and bearer mapping across the multi-hop wireless backhaul formed by IAB nodes. Each IAB node and the IAB donor contain a BAP entity. The BAP layer adds a BAP header to packets, which contains a BAP Routing ID. This ID is used to route the packet along the correct path through the backhaul topology towards its final destination, which could be another IAB node, the donor, or a UE attached to an IAB node.

BAP operates with two types of bearers: BAP bearers and radio bearers. A BAP bearer represents an end-to-end logical connection between the donor node's central unit (CU) and an IAB node or a UE served by an IAB node. It is identified by a BAP address and a BAP path ID. The BAP layer maps these BAP bearers onto radio bearers (e.g., RLC channels) on each individual wireless hop. This mapping is configurable and allows for traffic differentiation and QoS handling across the backhaul. The routing tables within each BAP entity, which map BAP Routing IDs to the next-hop link, are configured by the IAB donor's CU via F1-Application Protocol (F1-AP) signaling over the control plane.

For downstream traffic (from network to UE), the donor CU determines the BAP Routing ID. As the packet traverses each IAB node, the local BAP entity examines the BAP Routing ID, consults its routing table, and forwards the packet to the correct next-hop child node via the appropriate radio bearer. For upstream traffic, the IAB node where the UE is attached adds the BAP Routing ID. This ID is typically configured by the donor and directs the packet along the upstream path towards the donor. BAP also supports topology adaptation. When a link fails or a node is added, the donor CU can reconfigure the BAP routing tables in the affected nodes to establish new paths, enabling robust and self-healing backhaul networks.

Purpose & Motivation

BAP was created to address the challenge of building scalable and flexible wireless backhaul for 5G networks, specifically for the Integrated Access and Backhaul (IAB) feature. Traditional wired or point-to-point microwave backhaul is expensive and inflexible for dense, small-cell deployments. IAB allows 5G base stations (gNBs) to use part of their radio resources for backhauling traffic from other, more remote nodes, creating a wireless mesh. However, this required a new protocol to manage multi-hop routing within the gNB architecture without involving the core network.

The purpose of BAP is to provide a layer 2 routing mechanism that is tightly integrated with the 3GPP NG-RAN architecture. Prior to BAP, multi-hop networking would require IP routing at layer 3, which adds complexity, overhead, and is less optimal for the latency-sensitive and tightly synchronized RAN environment. BAP solves this by operating below the PDCP layer, allowing the IAB network to appear as a single, logical gNB to the 5G core. It enables efficient hop-by-hop forwarding, supports QoS differentiation across the backhaul by mapping to different RLC channels, and facilitates fast topology management under the control of the donor CU. This allows operators to rapidly deploy coverage, especially in areas where fiber is not available, by daisy-chaining nodes wirelessly.