 at the IAB donor gNB and therefore shielded from the IAB nodes while control and management are retained to the RAN. IAB is compliant with conventional wireline CU/DU split architecture, where the gNB-CUs can be virtualized and moved into the cloud. It is therefore relatively easy to upgrade an IAB node to a regular gNB-DU by furnishing it with a fiber connection. The backhaul link reuses the existing NR-Uu interface (with relevant extensions) reduc- ing specification effort and allowing easy integration of backhaul links with access links. The dual personality of the IAB node, represented by gNB-DU and MT functionality, imposes a hierarchical structure onto the IAB topology, where the MT connects to an upstream parent node (which may be another IAB node or the IAB donor) while the gNB-DU connects to a downstream child IAB node. A spanning tree topology is formed when multiple IAB node MTs connect to the same parent node gNB-DU on multiple hops. The topology becomes a directed acyclic graph, when one or more IAB nodes also connect to multiple parent nodes. The restriction to hierarchical topologies was motivated by the nature of the traffic flow in a backhaul network, which moves between access nodes and IAB donor, and it makes loop-free signaling and transport easier to enforce. IAB node DU and IAB donor CU together appear as a conventional gNB to the UE. In this manner, UEs can use standalone (5GC) mode or EN-DC (for details on EN-DC and more generalized MR-DC variant, see Chapter 3) to connect to the network. Since the IAB node MT uses Uu procedures, it can also use standalone mode and integrate with an 5GC, or EN-DC mode to integrate with an EPC. IAB nodes using EN-DC have to support an additional Long-Term Evolution (LTE) link to an eNB for control-plane signaling while keeping backhauling confined to NR (for details about multi-connectivity architectures, refer to Section 4.3). The operation mode of a UE and an IAB node MT are independent from each other. It is therefore possible to combine an EPC for UEs 