GTP (GPRS Tunneling Protocol) has been the workhorse of mobile data since 3G, and it continues in 5G. It creates a "tunnel" between the base station and the core network, encapsulating your IP packets inside another IP packet with a GTP header. GTP-U (User Plane): Carries actual user data between gNB and UPF (N3 interface) and between UPFs (N9 interface). GTP-C (Control Plane): Used in 4G for session management. Replaced by HTTP/2 in 5G core, but PFCP is used on N4.