參考資料：3GPP 38.463
gNB-CU：gNB Central Unit，一個邏輯節點，承載gNB的RRC、SDAP、PDCP協議，或en-gNB的RRC和PDCP協議。
gNB-DU：gNB Distributed Unit ，一個邏輯節點，承載gNB或en-gNB的RLC、MAC和PHY。
gNB-CU-CP：gNB-CU-Control Plane，一個邏輯節點，承載gNB或en-gNB的RRC以及PDCP控制面。
gNB-CU-UP：gNB-CU-User Plane，一個邏輯節點，承載en-gNB的PDCP用戶面，或gNB的PDCP用戶面和SDAP。
1. E1AP服務
E1AP提供gNB-CU-CP節點與gNB-CU-UP節點之間的信令服務。服務分爲兩類：
- 非UE相關服務：在gNB-CU-CP與gNB-CU-UP之間建立E1AP接口實例。
- UE相關服務：爲UE接入核心網提供信令和數據連接。
2. E1AP一般過程分類
（1）E1AP一般過程按是否有響應，可分爲兩類：
- Class 1：應答類消息，需要等待響應消息。若在規定的時間內未等到相應的消息，認爲失敗。
| 一般過程 | 發起消息 | 成功響應 | 失敗響應 |
|---|---|---|---|
| gNB-CU-UP E1 Setup | GNB-CU-UP E1 SETUP REQUEST | GNB-CU-UP E1 SETUP RESPONSE | GNB-CU-UP E1 SETUP FAILURE |
| gNB-CU-CP E1 Setup | GNB-CU-CP E1 SETUP REQUEST | GNB-CU-CP E1 SETUP RESPONSE | GNB-CU-CP E1 SETUP FAILURE |
| gNB-CU-UP Configuration Update | GNB-CU-UP CONFIGURATION UPDATE | GNB-CU-UP CONFIGURATION UPDATE ACKNOWLEDGE | GNB-CU-UP CONFIGURATION UPDATE FAILURE |
| gNB-CU-CP Configuration Update | GNB-CU-CP CONFIGURATION UPDATE | GNB-CU-CP CONFIGURATION UPDATE ACKNOWLEDGE | GNB-CU-CP CONFIGURATION UPDATE FAILURE |
| E1 Release | E1 RELEASE REQUEST | E1 RELEASE RESPONSE | |
| Reset | RESET | RESET ACKNOWLEDGE | |
| Bearer Context Setup | BEARER CONTEXT SETUP REQUEST | BEARER CONTEXT SETUP RESPONSE | BEARER CONTEXT SETUP FAILURE |
| Bearer Context Modification (gNB-CU-CP initiated) | BEARER CONTEXT MODIFICATION REQUEST | BEARER CONTEXT MODIFICATION RESPONSE | BEARER CONTEXT MODIFICATION FAILURE |
| Bearer Context Modification Required (gNB-CU-UP initiated) | BEARER CONTEXT MODIFICATION REQUIRED | BEARER CONTEXT MODIFICATION CONFIRM | |
| Bearer Context Release (gNB-CU-CP initiated) | BEARER CONTEXT RELEASE COMMAND | BEARER CONTEXT RELEASE COMPLETE |
- Class 2：通知類消息，無響應消息。
| 過程 | 消息 | 作用 |
|---|---|---|
| Error Indication | ERROR INDICATION | |
| Bearer Context Release Request (gNB-CU-UP initiated) | BEARER CONTEXT RELEASE REQUEST | |
| Bearer Context Inactivity Notification | BEARER CONTEXT INACTIVITY NOTIFICATION | |
| DL Data Notification | DL DATA NOTIFICATION | |
| UL Data Notification | UL DATA NOTIFICATION | |
| Data Usage Report | DATA USAGE REPORT | |
| gNB-CU-UP Counter Check | GNB-CU-UP COUNTER CHECK | |
| gNB-CU-UP Status Indication | GNB-CU-UP STATUS INDICATION | |
| MR-DC Data Usage Report | MR-DC DATA USAGE REPORT |
（2）E1AP一般過程按功能劃分，可分爲兩類：
| 功能 | 過程 |
|---|---|
| 接口管理 | gNB-CU-UP E1 Setup gNB-CU-CP E1 Setup gNB-CU-UP Configuration Update gNB-CU-CP Configuration Update Reset Error Indication E1 Release gNB-CU-UP Status Indication |
| 承載上下文管理 | Bearer Context Setup Bearer Context Modification (gNB-CU-CP initiated) Bearer Context Modification Required (gNB-CU-UP initiated) Bearer Context Release (gNB-CU-CP initiated) Bearer Context Release Request (gNB-CU-UP initiated) Bearer Context Inactivity Notification DL Data Notification UL Data Notification gNB-CU-UP Counter Check gNB-CU-UP Counter Check Data Usage Report MR-DC Data Usage Report |