 MT operates like a UE, it supports the same access radio channels to its parent node DU and the IAB donor CU. This includes the RRC connection with the IAB donor CU, and it may include one or more data radio bearers, for example, to carry PDU sessions for its own traffic (see the OAM discussion below). The protocol layers for the IAB node MT's access traffic are therefore the same as those for a UE, shown in Figure 5.2.6. In addition to the RLC channels for access, the IAB node MT also supports RLC chan- nels for backhauling. While these backhaul RLC channels provide full RLC functionality on each hop, for example, such as Automatic Repeat Request (ARQ) for RLC Acknowledged Mode (AM) operation mode, they are not mapped to a specific PDCP entity. Instead, they carry the Backhaul Adaptation Protocol (BAP) as an upper protocol layer to enable forward- ing across the backhaul topology (the backhaul-related layers are shown as "BH RLC" in Figure 5.2.6). The BAP carries the native F1 protocol stack including IP (shown as "F1-C" in Figure 5.2.6), which is used for transport across wireless and wireline networks. The F1 interface can be security protected via a Network Domain Security framework (3GPP TS 33.210) in compliance with 5G security architecture (3GPP TS 33.501), which inserts an IPsec layer on top of IP. 5.2.3.2 Backhaul Transport and QoS Support for fine granular QoS and fairness on the wireless backhaul is critical since traffic aggregation may lead to congestion high up in the topology close to the IAB donor DUs. For QoS differentiation, multiple RLC channels can be established on each backhaul link. IAB supports 1 1 mapping between UE bearers and backhaul RLC channels, where each UE bearer is carried on a separate backhaul RLC channel (Figure 5.2.7, top). This enables IAB-node 2 IAB-node 1 IAB-donor DU IAB-donor CU-UP GTP-U GTP-U GTP-U GTP-U PHY/MAC PHY/MAC L1/L2 L1/L2 NR-Uu IPsec IPsec PHY/MAC PHY/MAC PHY/MAC PHY/MAC BH RLC channel BH RLC channel IAB-node 2 IAB-node 1 IAB-donor DU IAB-donor CU-CP NG-AP NG