Section: 4.7.4.3 Topological Redundancy
Content: The IAB-node may have redundant routes to the IAB-donor-CU(s). For IAB-nodes operating in SA-mode, NR DC can be used to enable route redundancy in the BH by allowing the IAB-MT to have concurrent BH links with two parent nodes. The parent nodes may be connected to the same or to different IAB-donor-CUs, which control the establishment and release of redundant routes via these two parent nodes. Either parent node's gNB-DU functionality together with the respective IAB-donor-CU assumes the role of the IAB-MT's master node or secondary node. The NR DC framework (e.g., MCG/SCG-related procedures) is used to configure the dual radio links with the parent nodes (TS 37.340[ Evolved Universal Terrestrial Radio Access (E-UTRA) and NR; Multi-connectivity; Overall Description; Stage-2 ]  [21]). The procedure for establishment of topological redundancy for IAB-nodes operating in SA-mode is captured in TS 38.401[ NG-RAN; Architecture description ]  [4]. An IAB-node operating in NR-DC may also use one of its links for BH connectivity with an IAB-donor and the other link for access-only connectivity with a separate gNB that does not assume IAB-donor role. The IAB-donor can assume the MN or the SN role. The IAB-node may exchange F1-C traffic with the IAB-donor via the backhaul link and/or via the access link with the gNB. In the latter case, the F1-C messages are carried over NR RRC between the IAB-node and the gNB, and via XnAP between the gNB and the IAB-donor. IAB-nodes operating in EN-DC can exchange F1-C traffic with the IAB-donor via the MeNB. The F1-C message is carried over LTE RRC using SRB2 between IAB-node and MeNB and via X2AP between the MeNB and the IAB-donor. The procedures for establishment of redundant transport of F1-C for IAB-nodes using NR-DC and EN-DC are captured in TS 37.340[ Evolved Universal Terrestrial Radio Access (E-UTRA) and NR; Multi-connectivity; Overall Description; Stage-2 ]  [21] and TS 38.401[ NG-RAN; Architecture description ]  [4].