Section: 5.35.1 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. In this Release of the specification, NR satellite access is not applicable. The serving PLMN may provide the mobility restriction for NR satellite access as specified in clause 5.3.4.1 The Uu backhaul links can exist between the IAB-node and: - a gNB referred to as IAB-donor; or - another IAB-node. The part of the IAB node that supports the Uu interface towards the IAB-donor or another parent IAB-node (and thus manages the backhaul connectivity with either PLMN or SNPN it is registered with) is referred to as an IAB-UE. In this Release of the specification, the IAB-UE function does not apply to the NR RedCap UE. At high level, IAB has the following characteristics: - IAB uses the CU/DU architecture defined in TS 38.401[ NG-RAN; Architecture description ]  [42], and the IAB operation via F1 (between IAB-donor and IAB-node) is invisible to the 5GC; - IAB performs relaying at layer-2, and therefore does not require a local UPF; - 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 5.35.1-1 shows the IAB reference architecture with two backhaul hops, when connected to 5GC. Figure 5.35.1-1: IAB architecture for 5GS 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, which controls IAB-node gNB-DU via the F1 interface. IAB-node appears as a normal gNB to UEs and other IAB-nodes and allows them to connect to the 5GC. The IAB-UE function behaves as a UE, and 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; - 5GC, e.g. AMF, via NAS; - OAM system via a PDU session or PDN connection (based on implementation). NOTE: The 5GC, e.g. SMF, may detect that a PDU session for the IAB-UE is for the OAM system access, e.g. by checking the DNN and/or configuration. It is up to the operator configuration to choose whether to use 1 or multiple QoS Flows for OAM traffic and the appropriate QoS parameters, e.g. using 5QI=6 for software downloading, and 5QI=80 with signalled higher priority or a pre-configured 5QI for alarm or control traffic. The IAB-UE can connect to 5GC over NR (SA mode) or connect to EPC (EN-DC mode). The UE served by the IAB-node can operate in the same or different modes than the IAB-node as defined in TS 38.401[ NG-RAN; Architecture description ]  [42]. The operation mode with both UE and IAB-node connected to EPC is covered in TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [26]. Operation modes with UE and IAB-node connected to different core networks are described in clause 5.35.6.