access link IAB-node IAB-node 5 NG-RAN Evolution IAB-donor gNB-CU IAB-donor gNB-CU IAB-donor IAB-donor IAB-donor gNB-DU IAB-donor gNB-DU NR Uu NR Ur NG-U, N4 gNB-DU IAB-node IAB-node NR Uu NR Uu NG-U, N4 gNB-DU IAB-node IAB-node NG-RAN NG-RAN NR Uu NR Uu NR Uu NR Uu NR Uu NR Uu Layer-2 architecture Layer-3 architecture Figure 5.2.4 Examples for layer 2 and layer 3 IAB architectures. 3GPP decided to use the layer 2 architecture (left). support of IAB. Alternatively, the IAB donor gNB can be split into gNB-DU and gNB-CU, and the gNB-CU can be further split into gNB-CU-CP and gNB-CU-UP in compliance with Release-15 CU/DU split architecture. These split architectures are described in more detail in Sections 4.2 and 4.4. The IAB donor therefore has the same deployment flexibility as a conventional gNB. 3GPP considered multiple layer-2 and layer-3 relaying architectures and decided in favor of the layer-2 shown in Figure 5.2.4. Another architecture, which uses layer-3 relaying, is also shown in the figure and discussed further below. In the layer-2 relaying architecture, the IAB node holds a gNB-DU and a mobility termi- nation (MT) function. The IAB node gNB-DU interfaces to the gNB-CU on the IAB donor via the F1 interface (with relevant extensions), which was originally defined for the gNB CU/DU split architecture. The MT replicates UE functionality to establish connections to a gNB and the core network. In this manner, IAB node MTs can connect to gNB-DUs on other IAB nodes or on the IAB donor. The gNB-CU on the IAB donor further becomes the central control function for all IAB- node gNB-DUs and MTs defining an interconnected IAB topology. The advantages of this architecture are: The IAB node is lightweight and low in complexity since it only supports a gNB-DU function. 5.2 Wireless Relaying in 5G The IAB donor gNB-CU offers centralization of control and management tasks allow- ing performance optimization and reducing need for specification of distributed functionality. The RAN to core network interface is terminated