Section: 4.3.32.2 System enhancements to support IAB
Content: In IAB operation, the IAB-UE interacts with the EPC using procedures defined for UE. The IAB-node gNB-DU only interacts with the IAB-donor-CU and follows the CU/DU design defined in TS 38.401[ NG-RAN; Architecture description ]  [90]. For the IAB-UE operation, the existing UE authentication methods as defined in TS 33.401[ 3GPP System Architecture Evolution (SAE); Security architecture ]  [41] apply. For EPC, only USIM based methods are allowed. The following aspects of EPS are enhanced to support the IAB operation: - the Attach procedure as defined in clause 5.3.2 is enhanced to indicate IAB-node's capability to the MME: - the IAB-node provides an IAB-indication to the eNodeB when the IAB-node establishes the RRC connection. If the IAB-indication is received, the eNodeB selects an MME that supports IAB, and includes the IAB-indication in the INITIAL UE MESSAGE so that the MME can perform IAB authorization; - the UE Subscription data as defined in clause 5.7.1 is enhanced to include the authorization information for the IAB operation; - Authorization procedure during the UE attach procedure is enhanced to perform verification of IAB subscription information; - If the IAB operation is not authorized, the MME may reject the IAB-UE's attach request or detach the IAB-UE, or the MME may initiate UE Context setup/Modification procedure by providing IAB authorized indication with the value set to "not authorized" to the eNodeB, but the IAB-UE is in the EMM-REGISTERED state. - If the IAB operation is authorized, UE Context setup/modification procedure is enhanced to provide IAB authorized indication with the value set to "authorized" to eNodeB and IAB-donor gNB. After attached or registered, the IAB-node remains in ECM-CONNECTED state if the IAB operation is authorized. In the case of radio link failure, the IAB-UE uses existing UE procedure to restore the connection with the network. The IAB-UE uses the Detach procedure defined in clause 5.3.8 to disconnect from the network.