The infrastructure of 2G and 3G cellular networks are similar. They comprise an air interface between the user's mobile device and the base station and two core networks; one for circuit-switched voice and another for packet-switched data. In the subsequent 4G/LTE architecture, voice and data are both based on IP packets.The Air InterfaceThe GERAN (GSM EDGE Radio Access Network) is the 2G air interface, and the UTRAN (Universal Terrestrial Radio Access Network) is the 3G interface. The air interface comprises the base stations (cell towers) and controlling equipment. The 2G base station is a Basic Transceiver Station (BTS) controlled by the Base Station Controller (BSC). The 3G base station is a Node B controlled by the Radio Network Controller (RNC).Circuit SideFor the circuit side, the BSC or RNC connects to the Mobile Switching Center (MSC), which sets up and tears down the calls, handles text messages (SMS) and tracks users as they move from cell to cell. When a user arrives within an MSC's jurisdiction, subscriber information is sent from the Home Location Register (HLR) database to the Visitor Location Register (VLR) within the MSC. The Gateway MSC (GMSC) connects the MSC to the external circuit-switched networks.Packet SideThe counterpart to the MSC on the packet side is the Serving GPRS Support Node (SGSN), which manages the packet connection for the user. The Gateway GPRS Support Node (GGSN) provides the connection to the external packet networks. The GGSN also receives subscriber information from the HLR. Contrast with LTE architecture. See UMTS and cellular generations.