Section: 4.7.1 Architecture
Content: Integrated access and backhaul (IAB) enables wireless relaying in NG-RAN. The relaying node, referred to as IAB-node, supports access and backhauling via NR. The terminating node of NR backhauling on network side is referred to as the IAB-donor, which represents a gNB with additional functionality to support IAB. Backhauling can occur via a single or via multiple hops. The IAB architecture is shown in Figure 4.7.1-1. The IAB-node supports the gNB-DU functionality, as defined in TS 38.401[ NG-RAN; Architecture description ]  [4], to terminate the NR access interface to UEs and next-hop IAB-nodes, and to terminate the F1 protocol to the gNB-CU functionality, as defined in TS 38.401[ NG-RAN; Architecture description ]  [4], on the IAB-donor. The gNB-DU functionality on the IAB-node is also referred to as IAB-DU. In addition to the gNB-DU functionality, the IAB-node also supports a subset of the UE functionality referred to as IAB-MT, which includes, e.g., physical layer, layer-2, RRC and NAS functionality to connect to the gNB-DU of another IAB-node or the IAB-donor, to connect to the gNB-CU on the IAB-donor, and to the core network. The IAB-node can access the network using either SA mode or EN-DC. In EN-DC, the IAB-node connects via E-UTRA to a MeNB, and the IAB-donor terminates X2-C as SgNB (TS 37.340[ Evolved Universal Terrestrial Radio Access (E-UTRA) and NR; Multi-connectivity; Overall Description; Stage-2 ]  [21]). Figure 4.7.1-1: IAB architecture; a) IAB-node using SA mode with 5GC; b) IAB-node using EN-DC All IAB-nodes that are connected to an IAB-donor via one or multiple backhaul hops and controlled by this IAB-donor via F1AP and/or RRC form an IAB topology with the IAB-donor as its root (Fig. 4.7.1-2). In this IAB topology, the neighbour node of the IAB-DU or the IAB-donor-DU is referred to as the child node and the neighbour node of the IAB-MT is referred to as the parent node. The direction toward the child node is referred to as downstream while the direction toward the parent node is referred to as upstream. The IAB-donor performs centralized resource, topology and route management for its IAB topology. Figure 4.7.1-2: Parent- and child-node relationship for IAB-node