Directed Acyclic Graph

Description

In 3GPP specifications, particularly TS 38.300, a Directed Acyclic Graph (DAG) is a fundamental architectural concept for the 5G Integrated Access and Backhaul (IAB) framework. It defines the logical topology and routing relationships between IAB nodes and the IAB donor, which connects to the core network. A DAG is a graph structure consisting of vertices (nodes) and directed edges (links), with the key property that no cycles exist—meaning there is no path that starts and ends at the same node following the direction of the edges. This acyclic property is crucial for preventing routing loops and ensuring deterministic, efficient packet forwarding in the multi-hop wireless backhaul.

The DAG architecture in IAB organizes nodes in a hierarchical manner. The IAB donor acts as the root of the DAG. IAB nodes are parent or child nodes relative to each other, forming a tree-like structure where each node (except the donor) has one or more parent nodes for upstream connectivity toward the donor and core network. Each IAB node also functions as a potential parent for downstream child nodes or User Equipment (UE). The DAG structure is established and maintained by routing protocols and topology management functions within the IAB system, which adapt to changes such as node addition, failure, or varying link qualities.

Key operational components within the DAG include the Backhaul Adaptation Protocol (BAP) layer and routing identifiers. The BAP layer, introduced at IAB nodes and the donor, is responsible for routing packets across the backhaul network based on a BAP Routing ID. This ID maps to the DAG topology, allowing packets to be directed along the appropriate path through parent-child relationships. The DAG enables efficient resource utilization through spatial reuse and multi-hop communication. It supports features like redundant paths (where a node may have multiple parents for reliability) and load balancing, though the primary DAG structure avoids cycles to simplify routing decisions and management.

The role of the DAG in the 5G RAN is to facilitate scalable and cost-effective network densification. By allowing wireless multi-hop backhaul, it reduces the dependency on fiber trenching for every base station. The DAG provides a controlled topology that supports QoS management, path selection, and network resilience. It is integral to the IAB's operation, enabling dynamic reconfiguration and efficient use of radio resources across access and backhaul links within a unified 5G NR air interface.

Purpose & Motivation

The Directed Acyclic Graph (DAG) concept was introduced in 3GPP Release 16 to address the challenge of economically deploying dense 5G networks, especially in areas where laying fiber optic backhaul is prohibitively expensive or logistically difficult. Prior to IAB and DAG-based topologies, cellular networks relied predominantly on star-topology backhaul with each base station requiring a direct, high-capacity wired (fiber or microwave) connection to the core network. This approach limited rapid deployment and scalability. The DAG enables a multi-hop, wireless self-backhauling network where nodes can relay traffic for each other, dramatically reducing the need for fiber penetration.

Historically, mesh networking and multi-hop routing existed in ad-hoc and wireless sensor networks, but they were not integrated into standardized cellular architectures with stringent requirements for latency, reliability, and QoS. The IAB work in 3GPP aimed to bring this flexibility into 5G NR. The DAG was chosen as the underlying topological model because its acyclic property inherently prevents routing loops, which are detrimental to network stability and performance. It simplifies routing protocols, reduces control overhead, and ensures predictable packet delivery—essential for meeting 5G service level agreements.

The DAG solves the problem of efficient topology management in a dynamic wireless environment. It allows the network to automatically organize IAB nodes into a hierarchical structure, optimize paths based on link metrics, and recover from failures by re-parenting. This enables operators to deploy networks in a plug-and-play manner, supporting use cases like temporary events, urban coverage enhancement, and fixed wireless access. The DAG is thus a key enabler for the cost-effective and scalable 5G network rollout envisioned by operators worldwide.

Key Features

• Acyclic topology preventing routing loops for stable packet forwarding
• Hierarchical organization with IAB donor as root and parent-child relationships between nodes
• Support for multi-hop wireless backhaul using the 5G NR air interface
• Dynamic topology adaptation and route management via BAP and routing protocols
• Enables spatial reuse of spectrum resources across different hops in the graph
• Facilitates network redundancy and load balancing through multiple parent nodes

Evolution Across Releases

Defining Specifications