The 5G system (5GS) consists of the 5G core network (5GC), which is also called Next Generation Core (NGC), and NG Radio Access Network (NG-RAN), as shown in the picture.
The NG interface is a logical interface between an NG-RAN and 5GC. The Xn interface is a logical interface between gNodeBs, between ng-eNodeBs, or between a gNodeB and ng-eNodeB. To better understand the NG and Xn interfaces, the following briefly describes the 5GC and NG-RAN.
1- 5GC
The 5GC hosts functions such as authentication, authorization, accounting, and end-to-end connection establishment. 5GC NEs include the access and mobility management function (AMF), session management function (SMF), user plane function (UPF), policy control function (PCF), unified data management (UDM), authentication server function (AUSF), network exposure function (NEF), network repository function (NRF), and network slice selection function (NSSF). The AMF and UPF are connected to the NG-RAN. The AMF is a control-plane function and the UPF is a user-plane function.
2- NG-RAN
The NG-RAN mainly provides functions related to radio access. An NG-RAN node can be either of the following: gNodeB, which provides services for UEs in accordance with NR user-plane and control-plane protocols. Generally, its network entity is an NR base station. ng-eNodeB, which provides services for UEs in accordance with LTE user-plane and control-plane protocols. Generally, its network entity is an enhanced LTE base station (that is, an eLTE eNodeB). In the NSA networking architecture of Option 4 or Option 7, the LTE base station must be upgraded to support eLTE and interconnect with the 5GC. This upgraded LTE base station is called eLTE eNodeB.
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