Content for  TS 38.401  Word version:  18.4.0

During the inter-CU topology adaptation for a single-connected IAB-node, the IAB-MT migrates from an old parent node to a new parent node, where the old and the new parent nodes are served by different IAB-donor-CUs. Without loss of generality, the old parent node is referred to as source parent node, and the new parent node is referred to as target parent node. Figure 8.17.3.1-1 shows an example of the topology adaptation procedure, where the IAB-MT is migrated from a source IAB-donor-CU to a target IAB-donor-CU. In this procedure, the migrating IAB-node becomes a boundary IAB-node since its IAB-DU retains F1AP with the source IAB-donor-CU while its IAB-MT obtains RRC connectivity with the target IAB-donor-CU.

The target IAB-donor-CU may request the modification of the L2 transport for the offloaded traffic in the target IAB-donor-CU's topology using the IAB TRANSPORT MIGRATION MODIFICATION REQUEST message. The source IAB-donor-CU reconfigures the UL BH mappings accordingly, and acknowledges the modification via the IAB TRANSPORT MIGRATION MODIFICATION RESPONSE message. The target IAB-donor-CU may further reconfigure the TNL addresses of the migrating IAB-node via RRC. The traffic offload through the inter-CU topology adaptation procedure for the migrating IAB-node can be fully revoked. The non-F1-terminating IAB-donor-CU can initiate the full revoking of traffic offload by executing the XnAP Handover Preparation procedure for the migrating IAB-MT. After the migrating IAB-MT is handed over in reverse direction, i.e., from the non-F1-terminating IAB-donor-CU to the F1-terminating IAB-donor-CU, the traffic of the migrating IAB-node's IAB-DU is routed again along the former source path. The F1-terminating IAB-donor-CU can initiate the full revoking of traffic offload by requesting the release of all offloaded traffic from the non-F1-terminating IAB-donor-CU by sending the IAB TRANSPORT MIGRATION MANAGEMENT REQUEST to the non-F1-terminating IAB-donor-CU. This message may trigger the XnAP Handover Preparation procedure for the migrating IAB-MT towards the F1-terminating IAB-donor-CU. The F1-terminating IAB-donor-CU may request full or partial release of the offloaded traffic from the non-F1-terminating IAB-donor-CU via the IAB TRANSPORT MIGRATION MANAGEMENT REQUEST message.

Figure 8.17.3.2-1 shows an example of the topology adaptation procedure where the migrating IAB-MT is migrated from a source IAB-donor-CU to a target IAB-donor-CU, and where the migrating IAB-node has a descendant IAB-node which retains both its RRC connection and F1 connection with the source IAB-donor-CU.

The target IAB-donor-CU may trigger the modification of the L2 transport of the offloaded traffic in the target IAB-donor-CU's topology using the IAB TRANSPORT MIGRATION MODIFICATION REQUEST message. Based on this message, the source IAB-donor-CU may reconfigure the UL BH mappings on the descendant nodes, the routing entries and BH RLC channel mappings on the migrating node and the descendant nodes, and the BAP header rewriting entries on the migrating node, and acknowledges the modification via the IAB TRANSPORT MIGRATION MODIFICATION RESPONSE message. The target IAB-donor-CU may further provide updated TNL address information for the descendant IAB-node to the source IAB-donor-CU. The full or partial release or revoking of traffic offload pertaining to the descendant IAB-nodes and their served UEs follows the same procedure as defined for the partial migration in clause 8.17.3.1.