Section: 4.3.32.1 IAB architecture and functional entities
Content: Integrated access and backhaul (IAB) enables wireless in-band and out-of-band relaying of NR Uu access traffic via NR Uu backhaul links. At high level, IAB has the following characteristics: - IAB uses the CU/DU architecture defined in TS 38.401[ NG-RAN; Architecture description ]  [90], and the IAB operation via F1 (between IAB-donor and IAB-node) is invisible to the EPC. - IAB performs relaying at layer-2, and therefore does not require a local S/P-GW; - IAB supports multi-hop backhauling; - IAB supports dynamic topology update, i.e. the IAB-node can change the parent node, e.g. another IAB-node, or the IAB-donor, during operation, for example in response to backhaul link failure or blockage. Figure 4.3.32.1-1 shows the IAB reference architecture with two backhaul hops, when connected to EPC, where both the IAB-node and the UE connect to the EPC with Dual Connectivity as defined in TS 37.340[ Evolved Universal Terrestrial Radio Access (E-UTRA) and NR; Multi-connectivity; Overall Description; Stage-2 ]  [85]. Figure 4.3.32.1-1: IAB architecture for EPS, with IAB-node and UE both connected to EPC Each relay, referred to as IAB-node, consists of a gNB-DU function and a UE function (referred to as IAB-UE). The gNB-DU in the IAB-node is responsible for providing NR Uu access to UEs and child IAB-nodes. The corresponding gNB-CU function resides on the IAB-donor gNB (referred to as IAB-donor-CU), which controls IAB-node gNB-DU via the F1 interface. When a UE connects to EPC via an IAB-node, the gNB-DU of the IAB-node appears as a normal secondary gNB to the UE. When the IAB-UE of another IAB-node connects to EPC via the IAB-node, the gNB-DU of the IAB-node also appears as a secondary gNB to the IAB-UE. The IAB-UE function reuses UE procedures to connect to: - the gNB-DU on a parent IAB-node or IAB-donor for access and backhauling; - the gNB-CU on the IAB-donor via RRC for control of the access and backhaul link; - EPC, e.g. MME, - OAM system via a PDN connection (based on implementation). NOTE: The EPC, e.g. MME, may detect that a PDN connection for the IAB-UE is for the OAM system access, e.g. by checking the APN and/or configuration. It is up to the operator configuration to choose whether to use 1 or multiple EPS bearers for OAM traffic and the appropriate QoS parameters, e.g. using QCI=6 for software downloading, and QCI=80 or a pre-configured QCI for alarm or control traffic.