name: E1
full_form: Interface between gNB-CU and gNB-DU for control plane
context: The procedure for changing the UE state from RRC-connected to RRC-inactive is shown in Figure 8.9.6.1-1. Figure 8.9.6.1-1: RRC Connected to RRC Inactive state transition. 1. The gNB-CU-CP sends BEARER CONTEXT SETUP REQUEST message with UE/PDU session/DRB level inactivity timer. 2. The gNB-CU-UP sends BEARER CONTEXT SETUP RESPONSE message. 3. The gNB-CU-UP sends BEARER CONTEXT INACTIVITY NOTIFICATION message with inactivity monitoring results. 4. The gNB-CU-CP determines that the UE should enter RRC-inactive (e.g., after receiving Bearer Context Inactivity Notification procedure). 5. The gNB-CU-CP sends BEARER CONTEXT MODIFICATION REQUEST message with a Bearer Context Status Change to the gNB-CU-UP, which indicates that the UE is entering RRC-inactive state. The gNB-CU-CP keeps the F1 UL TEIDs. 6. The gNB-CU-UP sends the BEARER CONTEXT MODIFICATION RESPONSE message including the PDCP UL and DL status that may be needed for e.g., data volume reporting. The gNB-CU-UP keeps the Bearer Context, the UE-associated logical E1-connection, the NG-U related resource (e.g., NG-U DL TEIDs) and the F1 UL TEIDs. 7. The gNB-CU-CP sends the UE CONTEXT RELEASE COMMAND message to the gNB-DU serving the UE, together with the RRCRelease message to be sent to the UE. NOTE: step 5 and step 7 can be performed at the same time. 8. The gNB-DU sends the RRCRelease message to the UE. 9. The gNB-DU sends the UE CONTEXT RELEASE COMPLETE message to the gNB-CU-CP.
description: The E1 interface is a critical communication link between the gNB-CU (Central Unit) and gNB-DU (Distributed Unit) for control plane operations. It enables the exchange of control messages, such as bearer context setup requests and responses, inactivity notifications, and UE context release commands, facilitating the management of UE states, including transitions from RRC-connected to RRC-inactive.
explanation: E1 is classified as an Interface because it represents a specific communication pathway between gNB-CU and gNB-DU for control plane operations, aligning with the 3GPP's specifications for efficient network operations.
section_title: 8.9.6.1 RRC Connected to RRC Inactive
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