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.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (18 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the BAP function was newly introduced with a focus on enhancing backhaul security protocols. This included the specification of implementation requirements for protecting backhaul and sidehaul interfaces using standard security protocols. Furthermore, the release introduced various corrections to the underlying security protocols and cryptography used for these adaptations.
In Release 16, the BAP function was enhanced with specific clarifications and corrections for Integrated Access and Backhaul in NR, as detailed in the 38.331 CR. These updates included alignment for the SBA network or transport layer protocol and provided corrections on the BAP address and default BAP configuration. Additionally, Release 16 defined stop conditions for timer T320 within the NR protocols to improve backhaul link management.
In Release 17, specific corrections and enhancements were made to the Backhaul Adaptation Protocol (BAP) function for Integrated Access and Backhaul (IAB). These included corrections to the protocol stack for IAB, refinements for topology adaptation procedures, and specific fixes for BAP entity release during MR-DC release procedures as detailed in TS 38.331. The updates focused on improving the reliability and specification clarity of IAB node operation and network-controlled topology management.
- Introducing Enhancements to Integrated Access and Backhaul for NR TS 38.331CR2811
- Resolution of editor notes related to protocol between NSSAAF and AAA TS 33.501CR1350
- Correction on protocol stack for IAB TS 38.401CR0242
- Corrections for Rel-17 IAB on topology adaptation TS 38.401CR0246
- Corrections on IAB inter-CU topology adaptation TS 38.401CR0250
- Alignment of NAS transport protocol for 5G-RG over Wireline TS 33.501CR1444
+ 1 more changes
In Release 18, the new work for the Backhaul Adaptation Protocol (BAP) introduced the IAB inter-CU topology adaptation procedure. This procedure enables the dynamic reconfiguration of the backhaul topology between Integrated Access and Backhaul (IAB) nodes and their controlling Central Units. The enhancement allows for more flexible and efficient network management within the 5G radio access network.
In Release 19, the BAP function was enhanced to introduce support for Wireless Access Backhaul, expanding its application to new deployment scenarios. This addition allows the protocol to operate over wireless backhaul links, integrating with the standard security and authentication frameworks already defined for the 5G system. The update leverages existing 5G security protocols and procedures, such as those within the NAS and PDCP layers, to ensure secure and reliable backhaul connectivity.
- Support for Wireless Access Backhaul TS 38.401CR0439
Explore further
Broader topics and technologies where BAP plays a role.
Defining Specifications
3GPP specifications that define or reference BAP, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 33.501 vk00 | 5G Security Architecture and Procedures | Rel-20 |
| TR 33.824 vh00 | Security Study for NR Integrated Access & Backhaul | Rel-17 |
| TS 38.306 vj00 | NR UE Radio Access Capability Parameters | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |
| TS 38.401 vj10 | NG-RAN Architecture Specification | Rel-19 |