NG Interface是核心网和接入网之间的一个逻辑接口，是NG-RAN节点和AMF间的互联。按照协议分两种：
1.NG User Plane（NG用户面）；
2.NG Control Plane（NG控制面），
下面分别介绍这两种接口的功能:
一.NG User Plane，如下是协议对用户面协议的描述：
The NG user plane interface (NG-U) is defined between the NG-RAN node and the UPF. The user plane protocol stack of the NG interface is shown on Figure 4.3.1.1-1. The transport network layer is built on IP transport and GTP-U is used on top of UDP/IP to carry the user plane PDUs between the NG-RAN node and the UPF.
上面描述的是NG用户接口是在NG-RAN和UPF间的一个逻辑接口，传输网络层建立在IP传输之上，GTP-U用于UDP / IP之上用以承载用户平面PDUs数据在NG-RAN节点和UPF之间，如下图：
NG User Plane
二.NG Control Plane,如下协议描述
The NG control plane interface (NG-C) is defined between the NG-RAN node and the AMF. The control plane protocol stack of the NG interface is shown on Figure 4.3.1.2-1. The transport network layer is built on IP transport. For the reliable transport of signalling messages, SCTP is added on top of IP. The application layer signalling protocol is referred to as NGAP (NG Application Protocol). The SCTP layer provides guaranteed delivery of application layer messages. In the transport, IP layer point-to-point transmission is used to deliver the signalling PDUs.
上面协议大致意思是NG控制面也是NG-RAN节点与AMF间的一个逻辑接口，传输网络层是建立在IP传输之上，为了可靠地传输信令消息，在IP之上添加了SCTP，应用层信令协议被称之NGAP(NG应用协议),SCTP层保证了应用程序层消息的传递。 在传输中，IP层点对点传输用于传输PDU信令，如下图：
NG Control Plane
NG-C provides the following functions(NG-C提供了如下功能):
- NG interface management; //NG接口管理
- UE context management; //UE上下文管理
- UE mobility management; //UE移动性管理
- Transport of NAS messages; //传输NAS消息
- Paging; //寻呼消息
- PDU Session Management; //PDU会话管理
- Configuration Transfer; //配置传输
- Warning Message Transmission.//告警消息传输
Xn Interface 接口用于连接两个NG-RAN节点之间的逻辑接口，也分两种：
1.Xn User Plane (Xn 用户面)；
2.Xn Control Plane（Xn控制面）
下面分别介绍这两种功能的作用：
一.Xn User Plane协议介绍如下：
The Xn control plane interface (Xn-C) is defined between two NG-RAN nodes. The control plane protocol stack of the Xn interface is shown on Figure 4.3.2.2-1. The transport network layer is built on SCTP on top of IP. The application layer signalling protocol is referred to as XnAP (Xn Application Protocol). The SCTP layer provides the guaranteed delivery of application layer messages. In the transport IP layer point-to-point transmission is used to deliver the signalling PDUs.
Xn控制面接口是接连两个NG-RAN的接口，传输网络层是建立在IP之上的SCTP上，应用层信令协议被称之XnAP，SCTP层保证了应用程序层消息的传递。 在传输中，IP层点对点传输用于传输PDU信令，如下图：
Xn User Plane
Xn-U provides non-guaranteed delivery of user plane PDUs and supports the following functions:(Xn-U提供非保证的用户平面PDU传输，它支持如下功能)
- Data forwarding; //数据转发
- Flow control.//流控制
Further details of Xn-U can be found in TS 38.420 [17].更多的Xn-U接口信息需要看TS 38.420协议.
二.Xn Control Plane,协议介绍如下：
The Xn control plane interface (Xn-C) is defined between two NG-RAN nodes. The control plane protocol stack of the Xn interface is shown on Figure 4.3.2.2-1. The transport network layer is built on SCTP on top of IP. The application layer signalling protocol is referred to as XnAP (Xn Application Protocol). The SCTP layer provides the guaranteed delivery of application layer messages. In the transport IP layer point-to-point transmission is used to deliver the signalling PDUs
从上面协议可以看到和Xn用户面相似，不在赘述，具体结构如下图：
Xn Control Plane
The Xn-C interface supports the following functions(Xn-C接口支持如下功能):
- Xn interface management;//Xn接口管理
- UE mobility management, including context transfer and RAN paging;//UE 移动性管理，包括上下文传输和RAN（无线接入网下的寻呼消息）
- Dual connectivity. //双链接
Further details of Xn-C can be found in TS 38.420 [17].有关详细的Xn-C接口信息需要查看TS 38.420 协议.
