The infrastructure of an LTE cellular network comprises the E-UTRA/E-UTRAN air interface and the Evolved Packet Core (EPC), which includes the gateways, mobility management and subscriber databases. The air interface and packet core enable the LTE mobile user to connect to external data networks such as the Internet. In contrast to 2G and 3G networks, which separates voice and data networks, all media are transmitted as IP packets in an LTE network. The LTE base station (Evolved Node B) connects to the EPC via the Serving Gateway (S-GW), and the EPC connects to the packet network via the Packet Data Network Gateway (PDN-GW). The Mobility Management Entity (MME) tracks the user's movement, while the Home Subscriber Server (HSS) database provides subscriber information similar to the Home Location Register (HLR) in 3G/UMTS networks. Contrast with 2G/3G architecture. See E-UTRAN, LTE and cellular generations.