Editing Radio-frequency identification (section)

===Signaling===
[[File:A3tag.jpg|thumb|RFID hard tag]]
Signaling between the reader and the tag is done in several different incompatible ways, depending on the frequency band used by the tag. Tags operating on LF and HF bands are, in terms of radio wavelength, very close to the reader antenna because they are only a small percentage of a wavelength away. In this [[Near and far field|near field]] region, the tag is closely coupled electrically with the transmitter in the reader. The tag can modulate the field produced by the reader by changing the electrical loading the tag represents. By switching between lower and higher relative loads, the tag produces a change that the reader can detect. At UHF and higher frequencies, the tag is more than one radio wavelength away from the reader, requiring a different approach. The tag can [[backscatter]] a signal. Active tags may contain functionally separated transmitters and receivers, and the tag need not respond on a frequency related to the reader's interrogation signal.<ref name="Dobkin08">Daniel M. Dobkin, ''The RF in RFID: Passive UHF RFID In Practice'', Newnes 2008 {{ISBN|978-0-7506-8209-1}}, chapter 8</ref>

An [[Electronic Product Code]] (EPC) is one common type of data stored in a tag. When written into the tag by an RFID printer, the tag contains a 96-bit string of data. The first eight bits are a header which identifies the version of the protocol. The next 28 bits identify the organization that manages the data for this tag; the organization number is assigned by the EPCGlobal consortium. The next 24 bits are an object class, identifying the kind of product. The last 36 bits are a unique serial number for a particular tag. These last two fields are set by the organization that issued the tag. Rather like a [[Uniform resource locator|URL]], the total electronic product code number can be used as a key into a global database to uniquely identify a particular product.<ref>John R. Vacca ''Computer and information security handbook'', Morgan Kaufmann, 2009 {{ISBN|0-12-374354-0}}, page 208</ref>

Often more than one tag will respond to a tag reader. For example, many individual products with tags may be shipped in a common box or on a common pallet. Collision detection is important to allow reading of data. Two different types of protocols are used to [[Singulation|"singulate"]] a particular tag, allowing its data to be read in the midst of many similar tags. In a [[ALOHAnet|slotted Aloha]] system, the reader broadcasts an initialization command and a parameter that the tags individually use to pseudo-randomly delay their responses. When using an "adaptive binary tree" protocol, the reader sends an initialization symbol and then transmits one bit of ID data at a time; only tags with matching bits respond, and eventually only one tag matches the complete ID string.<ref>Bill Glover, Himanshu Bhatt,''RFID essentials'', O'Reilly Media, Inc., 2006 {{ISBN|0-596-00944-5}}, pages 88–89</ref>
[[File:RFID search environment.png|thumb|right|An example of a binary tree method of identifying an RFID tag]]
Both methods have drawbacks when used with many tags or with multiple overlapping readers.{{Citation needed|reason=See talk page|date=July 2021}}