5 Functions of the Xn interface

38.4203GPPNG-RANRelease 17TSXn general aspects and principles

5.1 General

The following clauses describe the functions supported in Xn interface.

5.2 Functions of Xn-C

5.2.1 Xn-C interface management and error handling functions

5.2.1.1 General

These functions allow for managing of signalling associations between NG-RAN nodes, surveying the Xn interface and recovering from errors.

5.2.1.2 Xn Setup function

This function allows for the initial setup of an Xn interface between two NG-RAN nodes, including exchange of application level data.

5.2.1.3 Error Indication function

This function allows the reporting of general error situations on application level.

5.2.1.4 Xn reset function

This function allows an NG-RAN node to inform a second NG-RAN node that it has recovered from an abnormal failure and that either all or some of the contexts (except the application level data) related to the first node and stored in the second shall be deleted, and the associated resources released.

5.2.1.5 Xn configuration data update function

This function allows two NG-RAN nodes to update application level data at any time.

5.2.1.6 Xn removal function

This function allows two NG-RAN nodes to remove the respective Xn interface.

5.2.2 UE mobility management functions

5.2.2.1 Handover preparation function

This function allows the exchange of information between source and target NG-RAN nodes in order to initiate the handover of a certain UE to the target.

5.2.2.2 Handover cancellation function

This function allows informing an already prepared target NG-RAN node that a prepared handover will not take place. It allows releasing the resources allocated during a preparation.

5.2.2.3 Retrieve UE Context function

The Retrieve UE context function is used for a NG-RAN node to retrieve UE context from another one.

5.2.2.4 RAN Paging function

The RAN paging function allows a NG-RAN node to initiate the paging for a UE in the inactive state.

5.2.2.5 Data Forwarding control function

The data forwarding control function allows establishing and releasing transport bearers between source and target NG-RAN nodes for data forwarding.

5.2.2.6 Handover Success Indication Function

This function allows informing a source NG-RAN node that the UE has successfully accessed a target NG-RAN node.

5.2.2.7 Conditional Handover cancellation function

This function allows informing a source NG-RAN node that resources reserved for candidate target cell(s) during a conditional handover preparation are about to be released by the target NG-RAN node.

5.2.3 Dual connectivity function

The dual connectivity function enables usage of additional resources in a secondary node in the NG-RAN.

5.2.4 Energy saving function

This function enables decreasing energy consumption by indication of cell activation/deactivation over the Xn interface.

5.2.5 Resource coordination function

This function enables coordination of cell resource usage between two NG-RAN nodes.

5.2.6 Secondary RAT Data Volume Report function

This function enables the NG-RAN node to report Secondary RAT usage data information in case of MR-DC with 5GC, either with a dedicated procedure or by including Secondary RAT usage data information in other messages.

5.2.7 Trace function

The Trace function provides means to control trace sessions for a UE over Xn interface.

5.2.8 Load management function

This function allows exchanging resource status and traffic load information between NG-RAN nodes, such that the NG-RAN node can control the traffic load appropriately.

5.2.9 Data exchange for self-optimisation function

This function allows two NG-RAN nodes to exchange information in order to support self-optimization functionality.

5.2.10 IAB support function

5.2.10.1 F1-C Traffic Transfer function

This function is used to deliver F1-C traffic between the M-NG-RAN node and the S-NG-RAN node serving a dual-connected IAB-node, where the F1-C traffic is either received from the IAB-node or sent to the IAB-node.

5.2.10.2 IAB Transport Migration function

This function allows the exchange of information between the F1-terminating IAB-donor-CU and the non-F1-terminating IAB-donor-CU of a boundary IAB-node, for the purpose of managing the migration of the boundary and descendant IAB-node traffic between the topologies managed by the two IAB-donor-CUs.

5.2.10.3 IAB Resource Coordination function

This function is used to exchange information between the F1-terminating IAB-donor-CU and the non-F1-terminating IAB-donor-CU of a boundary IAB-node in order to support resource multiplexing between the IAB-MT and the IAB-DU of the boundary IAB-node.

5.2.11 Small data transmission function

5.2.11.1 General

This function supports small data transmission sessions in RRC_INACTIVE both with and without anchor relocation, in case the UE is served by a new NG-RAN node.

5.2.11.2 Partial UE Context Transfer function

The Partial UE Context Transfer function is used for the last serving NG-RAN node to provide part of the UE Context to the receiving gNB.

5.2.12 QMC support function

The QMC function provides means to support the mobility of QMC sessions over the Xn interface.

5.2.13 MBS management support function

This function is used to support the management of MBS Sessions, including the addition of MBS related information in interface management and mobility procedures, and the support of RAN Multicast paging.

5.3 Functions of Xn-U

5.3.1 Data transfer function

The data transfer function allows the transfer of data between NG-RAN nodes to support dual connectivity or mobility operation.

5.3.2 Flow control function

The flow control function enables a NG-RAN node receiving user plane data from a second NG-RAN node to provide feedback information associated with the data flow.

5.3.3 Assistance information function

The assistance information function enables a NG-RAN node receiving user plane data from a second NG-RAN node to provide assistance information to the second node (e.g. related to radio conditions).

5.3.4 Fast retransmission function

The fast retransmission function provides coordination between PDCP-hosting node and corresponding node in case of outage in one of the nodes, to enables the node in good RF conditions to handle data previously forwarded to the node in outage.