Radio Frequency Identification (RFID) tags are becoming increasingly common.  RFID tags that include sensing capabilities have emerged as a generally inexpensive and effective means of addressing many wireless sensor applications in both indoor and outdoor sensing applications.  Purely passive sensors, such as RFID tags, when actively interrogated by an RF transceiver/reader, receive energy to power themselves up so that they can acquire readings from their attached sensing elements.  Generally, RFID tags equipped with one or more sensors require a source of energy to measure and store their acquired information at times other than during active interrogation by a reader.  Standard passive (battery-less) RFID tags provide no means of acquiring sensor information unless they are being actively interrogated by a reader. 
Next generation sensor networks may be powered by energy harvesting techniques to avoid requiring battery maintenance.  Energy harvesting is a process by which energy is derived from external sources (e.g., radio frequency energy, solar power, thermal energy, wind energy, salinity gradients, or kinetic energy), captured and stored. 
Energy may be harvested from radio frequency signals propagating wirelessly.  With RF harvesting, wireless energy comes from a radio frequency transmitting device that is some distance away from a device that harvests energy from the radio frequency transmission. 
One of the more popular forms of RF used today is Wi-Fi (also referred to as IEEE 802.11a/b/g/n etc.)  communications.  Today, most Wi-Fi communications are in the 2.4 GHz and 5.8 GHz frequency bands and there are many local area networks that are based on Wi-Fi in which access points enable Wi-Fi clients to gain access to networks such as the Internet.  Furthermore, the 2.4 GHz and 5.8 GHz bands also support other networking standards, such as Zigbee and Bluetooth, and other proprietary networks, each transmitting energy by communicating in this same frequency band.  Additionally there are other frequency bands that support different communication protocols, each of which transmit energy when they are communicating.