P address (FQDN) TAI 1 UPF IP address (FQDN) TAI 1 SUPI/SUPI C-RNTI PEI/IMEI I-RNT 5G-TMSI 5G UE PDU session ID/DNN, Data network TAI 1 Source: 3GPP TS 23.003 (Release 15) TAI 2 Figure 1.63 NG-RAN/5GC entities and bearers and identifiers. An application protocol identity (AP ID) is assigned when a new UE-associated logical con- nection is created in either a gNB or an AMF. An AP ID uniquely identifies a logical con- nection with a UE over the NG interface or Xn interface within a node (gNB or AMF). Upon receipt of a message that has a new AP ID from the originating node, the receiving node stores the corresponding AP ID for the duration of the logical connection. The defini- tion of AP IDs used over the NG, Xn, or F1 interface are as follows [15]: gNB UE NG application protocol (NGAP) ID is used to uniquely identify a UE over the NG interface within a gNB. This identifier is stored by the AMF for the duration of the UE association through the logical NG connection. This identifier is included in all UE associated NGAP signaling. AMF UE NGAP ID is allocated to uniquely identify the UE over the NG interface within an AMF. This identifier is stored for the duration of the UE-associated logical NG connection by the gNB. This identifier is included in all UE-associated NGAP sig- naling once known to the gNB. 132 Chapter 1 Old gNB UE XnAP ID is used to uniquely identify a UE over the Xn interface within a source gNB and it is stored by the target gNB for the duration of the UE association over the logical Xn connection. This identifier is included in all UE-associated XnAP signaling. New gNB UE XnAP ID is used to uniquely identify a UE over the Xn interface within a target gNB. When a source gNB receives a new gNB UE XnAP ID, it stores it for the duration of the UE association over logical Xn connection. This identifier is included in all UE-associated XnAP signaling. MgNB UE XnAP ID is allocated to uniquely identify the UE over Xn interface within an MgNB for dual-connectivity. The SgNB stores this identity for the du