Cellular technology based on the 3G network is widely available and allow communicating data without relying on an internet connection.
WiFi is a useful, low-cost choice where wireless internet connectivity is readily available.
Ethernet capability provides for low-cost, low-power, wired internet connectivity. This is a suitable choice for facilities where wireless connectivity is not possible such as nuclear power plants or sensitive sites.
RFID (Radio Frequency IDentification) are low-cost stickers and cards. They provide a passive way of transmitting data within a proximity ranging from contact to up to 300 feet away from the reader depending on the RFID frequency.
Any number of sensors can be connected to the main device either as an integrated component or by radio-based connectivity.