The 5G system consists of a 5G Core Network(5GC) & 5G Radio Access Network (NG-RAN). In further details 5GC consists of functional components AMF for authentication & mobility functions, UPF for user plane function & SMF for session management functions.
-An NG-RAN node can either be a gNB that provides NR user plane & control plane protocol terminations to the UE or ng-eNB that provides E-UTRAN user plane protocol & control plane terminations to the UE.
-The gNB & ng-eNBs are connected with each other by means of the Xn interface, which is a logical interface that uses a specific protocol structure.
-The Xn interface can be split into an Xn-U interface for the user plane data exchanged between gNB in the case of handover or dual connectivity scenarios.
–Xn-C interface for control information exchange between gNBs.
-The protocol stack for the user plane Xn-U consists of the protocol layers PDU, GTP-U, UDP, IP & data link with physical layer.
-Control plane Xn-C consists of the Xn-AP, SCTP, IP & data link with physical layer.
–gNB can be divided into a gNB-CU centralize unit & a gNB-DU distributed unit. They are connected to one another via the F1 interface.
-The gNB & ng-eNB are connected to the 5G core network via the NG interface. There is a connection to the AMF via the NG-C interface to allow the exchange of control messages.
-There is an interface NG-U to the UPF that contains user plane data.
-The protocol structure on the NG-U interface is similar to the Xn-U interface & contains the protocol layers.
-The protocol structure on the NG-C interface is similar to the Xn-C & consists of the protocol layers.
Note 1: NodeB, eNB, gNB & ng-eNB represent a protocol anchor so that the protocol layer perspective is used when describing the related functions, while terms such as base station refer to the hardware & RF resources linked to radio connectivity.
Note 2: 3GPP has introduce the term ng-eNB for the case in which a EUTRA eNB is connected to the 5GC instead of the EPC in case of LTE.
