Section: 8.9.8 BH RLC channel establishment procedure
Content: NOTE: The general principles and procedures described in this clause does not apply to ng-eNB The BH RLC channel is an RLC channel used for backhauling between IAB-node and IAB-donor-DU, or between different IAB-nodes. The BH RLC channel establishment may be triggered by a UE accessing the network and establishing a DRB. Figure 8.9.8-1: Signalling flow for IAB BH RLC channel establishment procedure 1. The IAB-donor-CU sends to the IAB-donor-DU a UE CONTEXT MODIFICATION REQUEST message for setting up the parent-node IAB-DU side of the BH link between IAB-donor-DU and IAB-node 1. This step is optional and is required only when a new BH RLC channel needs to be established on the BH link between IAB-donor-DU and IAB-node 1. 2. The IAB-donor-DU sends the UE CONTEXT MODIFICATION RESPONSE message to the IAB-donor-CU. 3. The IAB-donor-CU sends to the IAB-donor-DU a DL RRC MESSAGE TRANSFER message encapsulating the RRCReconfiguration message for configuring the IAB-MT of IAB-node 1. This step is optional and is required only when a new BH RLC channel needs to be established on the BH link between IAB-donor-DU and IAB-node 1. 4. The IAB-donor-DU decapsulates and forwards the RRCReconfiguration message to the IAB-MT of IAB-node 1. This step is optional and is required only when a new BH RLC channel needs to be established on the BH link between IAB-donor-DU and IAB-node 1. 5. The IAB-MT of IAB-node 1 sends to the IAB-donor-DU an RRCReconfigurationComplete message destined to the IAB-donor-CU. This step is optional and is required only when a new BH RLC channel needs to be established on the BH link between IAB-donor-DU and IAB-node 1. 6. The IAB-donor-DU sends the UL RRC MESSAGE TRANSFER message encapsulating the RRCReconfigurationComplete message to the IAB-donor-CU. This step is optional and is required only when a new BH RLC channel needs to be established on the BH link between IAB-donor-DU and IAB-node 1. 7. The IAB-donor-CU sends to the IAB-DU of IAB-node 1 a UE CONTEXT MODIFICATION REQUEST message for setting up the parent node IAB-DU side of the BH link between IAB-node 1 and IAB-node 2. This step is optional and is required only when a new BH RLC channel needs to be established on the BH link between IAB-node 1 and IAB-node 2. 8. The IAB-node 1 sends the UE CONTEXT MODIFICATION RESPONSE message to the IAB-donor-CU. 9. The IAB-donor-CU sends to the IAB-DU of IAB-node 1 a DL RRC MESSAGE TRANSFER message encapsulating the RRCReconfiguration message for configuring the IAB-MT of IAB-node 2. This step is optional and is required only when a new BH RLC channel needs to be established on the BH link between IAB-node 1 and IAB-node 2. 10. The IAB-DU of IAB-node 1 decapsulates and forwards the RRCReconfiguration message to the IAB-MT of IAB-node 2. This step is optional and is required only when a new BH RLC channel needs to be established on the BH link between IAB-node 1 and IAB-node 2. 11. The IAB-MT of IAB-node 2 sends to IAB-node 1 an RRCReconfigurationComplete message destined to the IAB-donor-CU. This step is optional and is required only when a new BH RLC channel needs to be established on the BH link between IAB-node 1 and IAB-node 2. 12. IAB-node 1 sends the UL RRC MESSAGE TRANSFER message encapsulating the RRCReconfigurationComplete message to the IAB-donor-CU. This step is optional and is required only when a new BH RLC channel needs to be established on the BH link between IAB-node 1 and IAB-node 2. The IAB-donor-CU uses the existing CU-DU split principles and the UE Context Setup procedure or UE Context Modification procedure to configure the parent IAB-DU side of the BH RLC channel. The IAB-donor-CU uses RRC signaling (which is encapsulated in the DL RRC MESSAGE TRANSFER message terminating at the parent node IAB-DU side of the BH RLC channel) to configure the child node IAB-MT side of the BH RLC channel. The IAB-donor-CU configures the IAB-DU with a mapping to the BH RLC channel to be used for a specific UL F1-U tunnel during the UE Context Setup procedure or the F1-AP UE Context Modification procedure for the UE.