Radio frequency identification (RFID) is an automatic identification system which consists of readers and tags. An RFID reader recognizes an object through reading the identification number (ID) of the RFID tag attached to it . To read tag IDs, the reader sends out a signal supplying instructions to tags. The tag transmits its own ID to the reader, and then the reader consults an external database with the ID to recognize the object. RFID is fast replacing bar code-based identification mechanisms because (1) communication between a reader and a tag is not limited by the requirement of “line-of-sight” reading and (2) each tag is allowed to have a unique ID.