with a dedicated NGC for IAB nodes (Figure 5.2.5). The layer-3 relaying architecture shown in Figure 5.2.4 is in line with one of the 4G relay- ing solutions (3GPP TS 36.300, clause 4.7). In this architecture, each backhaul link supports an IP connection via a self-contained Protocol Data Unit (PDU) session. For this purpose, the IAB donor and each IAB node have to hold a full gNB function and a user-plane function (UPF) in addition to the MT functionality. While this solution can reuse existing procedures for PDU session management, it requires core network signaling for every reconfiguration of the backhaul link. The amount of core network signaling was one of the main concerns. The layer-3 relaying architecture further lacks compatibility with wireline CU/DU split deployments. For these reasons, the layer-2 architecture was chosen. One of the objectives during the IAB standardization introduced in Release-16 was to allow Release-15 UEs to connect to the network via IAB. For this reason, all access-related protocol layers were retained in the IAB protocol stack (i.e. protocol layers color-coded in white in Figure 5.2.6). This includes Radio Link Control (RLC) channels including PHY, Medium Access Control (MAC) and RLC sub-layers between a UE and an IAB node DU, which are extended via F1 connections between the IAB node DU and the IAB donor 5 NG-RAN Evolution IAB-donor IAB-donor IAB-donor donor-CU donor-CU donor-CU donor-DU donor-DU donor-DU NR Uu NR Uu NR Uu LTE Uu gNB-DU gNB-DU gNB-DU IAB-node IAB-node IAB-node NR Uu NR Uu NR Uu LTE Uu LTE Uu IAB-node MT: SA mode IAB-node MT: SA mode IAB-node MT: EN-DC UE: SA mode UE: EN-DC UE: EN-DC Figure 5.2.5 Various options to integrate IAB with EPC and/or 5GC. CU across the wireless multi-hop backhaul plane, and Packet Data Convergence Protocol (PDCP), Service Data Adaptation Protocol (SDAP), and radio resource control (RRC) lay- ers between the UE and the IAB donor CU. For details about RRC, SDAP, PDCP, RLC, and MAC layers, please refer to Section 3.4. Since an IAB node