The Xn User plane (Xn-U) interface is defined between two NG-RAN nodes.  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.

The Xn User plane (Xn-U) interface is defined between two NG-RAN nodes.  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.

Xn-U provides non-guaranteed delivery of user plane PDUs and supports the following functions:

– Data forwarding;

– Flow control.

The Xn control plane interface (Xn-C) is defined between two NG-RAN nodes. 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. The Xn-C interface supports the following functions:

– Xn interface management;

– UE mobility management, including context transfer and RAN paging;

– Dual connectivity.