1.    Definition

The acronym RFID refers to radio-frequency identification: establishing the identification of an object using radio communications.  This single acronym is used to describe a variety of technologies with different principles of operation and characteristics.  

A typical RFID system consists of an interrogator, more often known as a reader, and a transponder, commonly referred to as a tag (Figure 1).  The tag is usually attached to an object that is to be identified.  Radio transmissions are used by the reader to send a query to the tag, and by the tag to return an answer, generally containing identifying information.  (The tag may not have its own radio transmitter, but may reflect the transmissions of the reader to communicate -- more about that below.)   The reader sends the identifying information to a network, or sometimes directly to a host computer,  where it can be displayed in human-readable form, or incorporated into a database to track objects and guide the activities of people and machines.

RFID system overview
Figure 1:  schematic depiction of a simplified RFID system

Radio-frequency identification has been around for a long time.  Early uses include the development of radar transponders (Figure 2), employed to identify the radar echo of an otherwise-anonymous airplane.  The basic technology was established in the mid-1950's, though the cost and size of the implementing equipment have fallen with the passing of the years.  A radar transponder is mounted in an airplane.  In operation, it is interrogated by a radar transmitter, operating at a frequency around 1 GHz, and responds with a message containing the identifying number of the airplane, as set by the pilot using the numbered dials on the front panel.  The resulting identifying information can be displayed for a radar operator, providing the identification and location of an airplane within that operator's controlled airspace.  The radar transponder is an example of an ultra-high-frequency, active RFID system that also provides location information.  

radar transponder

Figure 2:  typical aviation transponder

Radar transponders cost thousands of US$ and consume large amounts of power.  Their use is necessarily restricted to applications where identification serves critical purposes: in this case, ensuring the safety of air transport.  Wider application of RFID in the last two decades has been the result of two powerful trends: huge reduction in the cost and size of complex electronics due to the advances of integrated circuit technology, and the consequent increase in networked information systems, increasing the benefits of digitized information about the physical world.  Today, RFID is used to identify and in some cases locate not only airplanes but cars, cows, cartons, pallets, pets, and even people.  In this brief introduction we'll learn a bit about how this is done and what the consequences might be.