1.先来两个5G架构图
专有名词解释：
5GC: 5G(generation) core Network 5G核心网络
NG-RAN: Next generation radio access network 下一代无线接入网络
AMF: Access and Mobility Management Function 接入和流动性管理功能
UPF: User Plane Function 用户控制功能
NG: NG（下一代），一种接口，连接基站（有4G,ng-eNB和5G,gNB基站）与5GC核心网络
gNB: next generation node B，5G基站专用名, node providing new radio(NR) user plane and control plane protocol terminations towards the UE(用户设备), connected via the NG interface to the 5GC.
ng-eNB: not next generation evolved Node B（LTE 基站）， 4G基站专用名
Xn: 一种接口，连接不同基站
gNB-CU: control unit 控制单元
F1: 一种接口，连接CU和DU
gNB-DU: distributed unit 分布式单元
PDCP: packet data convergence protocol 分组数据集中协议
UL PDCP: uplink packet data convergence protocol
RLC: radio link control 无线连接控制
2.一些说明
The NG-RAN consists of a set of gNBs connected to the 5GC through NG interface.
NG-RAN由一组gNB组成，它们通过NG接口连接到5GC
An gNB can support FDD mode, TDD mode or dual mode operation.
gNB可以支持FDD(Frequency Division Duplexing, 频分双工)模式， TDD(Time Division Duplexing, 时分双工)模式或双模式操作。
gNBs can be interconnected through the Xn interface.
gNB之间可以通过Xn接口互连
A gNB may consist of a gNB-CU and one or more gNB-DU(s). A gNB-CU and a gNB-DU is connected via F1 interface.
一个gNB可以由一个CU和一个或多个DU组成，CU和DU之间通过F1接口连接。
One gNB-DU is connected to only one gNB-CU.
一个DU仅连接一个CU（因为一个基站只有一个CU，这是一般情况）
For resiliency, a gNB-DU may be connected to multiple gNB-CUs by appropriate implementation.（可能接多个CU）
NG, Xn, F1 are logical interfaces.
NG, Xn, F1都是逻辑接口
For NG-RAN, the NG and Xn-C interfaces for a gNB consisting of a gNB-CU and gNB-DUs, terminate in the gNB-CU. For EN-DC, the S1-U and X2-C interfaces for a gNB consisting of a gNB-CU and gNB-DUs, terminate in the gNB-CU. The gNB-CU and connected gNB-DUs are only visible to other gNBs and 5GC as a gNB.
对于NG-RAN， gNB的NG和Xn-C接口终止于gNB-CU。 对于EN-DC（4G和5G共存系统），gNB的S1-U和X2-C接口终止于gNB-CU。 gNB-CU和其连接的gNB-DUs只对其他gNBs可见， 而5GC可以看作gNB。
The node hosting user plane part of NR PDCP (e.g. gNB-CU, gNB-CU-UP, and for EN-DC, MeNB or SgNB depending on the bearer split) shall perform user inactivity monitoring and further informs its inactivity or (re)activity to the node having C-plane connection towards the core network(e.g. over E1, X2). The node hosting NR RLC (e.g. gNB-DU) may perform user inactivity monitoring and further inform its inactivity or (re)activation to the node hosting control plane, e.g. gNB-CU or gNB-CU-CP.
支持用户面（PDCP, packet data convergence protocol 分组数据集中协议的一部分）的节点（用户面节点）（5G: 为gNB-CU和gNB-CU-UP，双系统4G+5G为MeNB和SgNB）会监视用户的无操作并需要通知它的无操作或者重新开始活动给连接到5GC的节点（控制面节点），可通过E1，X2接口。支持NR RLC（radio link control 无线链接控制）的节点(e.g. gNB-DU)会监视用户的无操作并需要通知它的无操作或者重新开始活动给支持控制面的节点e.g. gNB-CU or gNB-CU-CP
参考文献
NG Radio Access Network (NG-RAN)Balazs Bertenyi1, Richard Burbidge2, Gino Masini3,Sasha Sirotkin4and Yin Gao5
3GPP TS 38.401 V15.2.0 (2018-06)
