Radio Frequency Identification is used to transfer data and commands from tags on objects bidirectionally over a short distance to an interrogator station by means of radio frequency technology. This is done through an air interface in the typical frequency bands: LF, HF or UHF. In the case of LF, this also works in water, on metal and in other harsh or dirty environments, or out of sight. In active systems the tag carries a battery to power the chip in the tag. This increases the distance. The type of application defines if a Read-Only (R/O) or Read-Write (R/W) device is required. An RFID system always consists of two elements, the tag and the interrogator, which is commonly called the "reader", even though it does not only receive and read data, but also transmits energy, data, and commands to the tag. Passive solutions operate within the LF frequency range of 100 to 150kHz. Active solutions operate at 100 to 150kHz in the downlink path, and at UHF frequencies (315MHz or 433MHz, depending on country regulations) in the uplink path.