. Backhaul transport is subject to packet loss on the wireless links as well as congestion-related packet drop. Packet loss on the wireless access and backhaul links is mitigated via Hybrid ARQ (HARQ) on the MAC layer and ARQ on the RLC layer. Packet drop due to congestion may occur if the capacity on the ingress link is much larger than that of the egress link. This phenomenon is also encountered on wireline networks and generally handled by flow and congestion control mechanisms on higher protocol layers, such as TCP. On the F1 interface, congestion-related packet drops have an aggravating impact since the UE's PDCP layer on top of F1 applies packet reordering and stops packet delivery to upper layers if a packet is missing. Congestion is likely to occur at the wireline-to-wireless interface (i.e. the IAB donor DU for IAB) since the wireline capacity is likely substantially higher than that of the wireless interface. For that reason, a flow and congestion control mechanism referred to as the NR user-plane protocol (3GPP TS 38.425) is used on F1, which embeds feedback on downlink congestion-related packet loss. In the upstream direction, congestion is likely to occur on the wireless part of the backhaul close to the IAB donor DU due to traffic aggregation. Such congestion, however, can be throttled by the MAC layer scheduler on the upstream side of the wireless links by limiting uplink grants to its child nodes and UEs in case it experiences high buffer load. 5.2.3.3 Resource Coordination For access systems, duplexing schemes are typically handled by the scheduler. For IAB, the gNB-DU scheduler needs to consider half-duplexing among access links and backhaul links to child nodes. Additional duplexing constraints need to be considered between links to parent and child nodes and among multiple parent nodes. Due to the interconnectedness of backhaul links throughout the IAB topology, it is necessary to enforce topology-wide resource coordination, which translates into a graph edge coloring problem. Figure 5.2.10 sh