参考资料：3GPP 38.463
gNB-CU：gNB Central Unit，一个逻辑节点，承载gNB的RRC、SDAP、PDCP协议，或en-gNB的RRC和PDCP协议。
gNB-DU：gNB Distributed Unit ，一个逻辑节点，承载gNB或en-gNB的RLC、MAC和PHY。
gNB-CU-CP：gNB-CU-Control Plane，一个逻辑节点，承载gNB或en-gNB的RRC以及PDCP控制面。
gNB-CU-UP：gNB-CU-User Plane，一个逻辑节点，承载en-gNB的PDCP用户面，或gNB的PDCP用户面和SDAP。
1. E1AP服务
E1AP提供gNB-CU-CP节点与gNB-CU-UP节点之间的信令服务。服务分为两类：
- 非UE相关服务：在gNB-CU-CP与gNB-CU-UP之间建立E1AP接口实例。
- UE相关服务：为UE接入核心网提供信令和数据连接。
2. E1AP一般过程分类
（1）E1AP一般过程按是否有响应，可分为两类：
- Class 1：应答类消息，需要等待响应消息。若在规定的时间内未等到相应的消息，认为失败。
| 一般过程 | 发起消息 | 成功响应 | 失败响应 |
|---|---|---|---|
| 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：通知类消息，无响应消息。
| 过程 | 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一般过程按功能划分，可分为两类：
| 功能 | 过程 |
|---|---|
| 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 |
| 承载上下文管理 | 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 |