The NG-RAN is layered into a Radio Network Layer (RNL) and a Transport Network Layer (TNL). The NG-RAN architecture, i.e. the NG-RAN logical nodes and interfaces between them, are defined as part of the RNL.
The NG-RAN architecture consists of a set of gNBs and ng-eNBs which are connected to the 5GC through the NG interface and is specified in TS 38.300
The NG-RAN may have several NG access points towards the 5GC. As a minimum, each NG access point
(in NG-RAN or 5GC) shall independently fulfil the requirements of the relevant NG specifications (3GPP 38.41x series - see clause 7
NG is a logical interface.
There may be multiple NG-C logical interfaces towards the 5GC from any one NG-RAN node. The selection of the NG-C interface is then determined by the NAS Node Selection function as described in clause 5
There may be multiple NG-U logical interfaces towards the 5GC from any one NG-RAN node. The selection of the NG-U interface is done within the 5GC and signalled to the NG-RAN node by the AMF.