The I-RNTI provides the new NG-RAN node a reference to the UE context in the old NG-RAN node. How the new NG-RAN node is able to resolve the old NG-RAN ID from the I-RNTI is a matter of proper configuration in the old and new NG-RAN node. Table C-1 below provides some typical partitioning of a 40bit I-RNTI, assuming the following content:

• UE specific reference: reference to the UE context within a logical NG-RAN node;
• NG-RAN node address index: information to identify the NG-RAN node that has allocated the UE specific part;
• PLMN-specific information: information supporting network sharing deployments, providing an index to the PLMN ID part of the Global NG-RAN node identifier.

The SPID values are defined in Annex I of TS 36.300. From the SPID reference values, only the SPID=253 also applies for 5GC.

Each NG-RAN node serving a cell identified by a Cell Identity associated with either a subset of PLMNs, or a subset of SNPNs, or a subset of PNI-NPNs is connected to another NG-RAN node via a single Xn-C interface instance. Each Xn-C interface instance is setup and removed individually. Xn-C interface instances terminating at NG-RAN nodes which share the same physical radio resources may share the same signalling transport resources. If this option is applied:

• Non-UE associated signalling is associated to an Xn-C interface instance by including an Interface Instance Indication in the XnAP message;
• Node related, non-UE associated Xn-C interface signalling may provide information destined for multiple logical nodes in a single XnAP procedure instance once the Xn-C interface instance is setup;
• A UE associated signalling connection is associated to an Xn-C interface instance by allocating values for the corresponding NG-RAN node UE XnAP IDs so that they can be mapped to that Xn-C interface instance.

The I-RNTI provides an NG-RAN node with a reference to the UE context and a reference to the NG-RAN node that allocated the UE context. To support an NG-RAN node in resolving the Local NG-RAN ID of the NG-RAN node that allocated the UE context, the I-RNTI structure is as follows:

• the first part (starting from the MSB of the I-RNTI) identifies the I-RNTI profile. It is 2 bits long for a Full I-RNTI or 1 bit long for a short I-RNTI, and it indicates the length of a Local NG-RAN Node Identifier of the NG-RAN Node that allocated the I-RNTI;
• the second part, immediately following the first part, is a Local NG-RAN Node Identifier;
• the third part, immediately following the second part, identifies the UE context stored in the NG-RAN node that allocated the I-RNTI.

In case a NG-RAN node takes an additional Local NG-RAN Node identifier into use or removes a Local NG-RAN Node identifier currently in use it informs its neighbour NG-RAN nodes about this change. The I-RNTI profiles for Full I-RNTI are described in Table F-1.

The I-RNTI profiles for Short I-RNTI are described in Table F-2.