In LTE the eNodeB connects to the core via the S1 interface, split into S1-MME for control signaling using S1AP over SCTP and S1-U for user data using GTP-U over UDP. In 5G the gNB connects to the core via the NG interface, similarly split into N2 for control using NGAP over SCTP toward the AMF and N3 for user data using GTP-U over UDP toward the UPF. Between base stations, the LTE X2 interface becomes the Xn interface, with XnAP replacing X2AP for control signaling. The user-plane transport protocol GTP-U remains unchanged across both generations.