Editing My 3GPP 38.300 Notes (section)

=== 4.3.2 Xn Interface ===

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

==== 4.3.2.1 Xn User Plane ====

The Xn User plane (Xn-U) interface is defined between two NG-RAN nodes. The user plane protocol stack on the Xn interface is shown in following figure. 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.

<center>[[File:Protocol stack Xn-U.png|Protocol stack Xn-U]]</center>
 
Xn-U provides non-guaranteed delivery of user plane PDUs and supports the following functions:
* Data forwarding;
* Flow control.

Further details of Xn-U can be found in TS 38.420.

==== 4.3.2.2 Xn Control Plane ====

The Xn control plane interface (Xn-C) is defined between two NG-RAN nodes. The control plane protocol stack of the Xn interface is shown on following figure. 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.
 
[[File:Protocol stack Xn-C.png|center|Protocol stack Xn-C]]

The Xn-C interface supports the following functions:
* Xn interface management;
* UE mobility management, including context transfer and RAN paging;
* Dual connectivity.

Further details of Xn-C can be found in TS 38.420.