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Near-field communication
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== Design == NFC is a set of short-range wireless technologies, typically requiring a separation of {{cvt|10|cm|in|frac=8}} or less. NFC operates at 13.56 [[MHz]] on [[ISO/IEC 18000-3]] air interface and at rates ranging from 106 kbit/s to 424 kbit/s. NFC always involves an initiator and a target; the initiator actively generates an [[Radio frequency|RF]] field that can power a passive target. This enables NFC targets to take very simple form factors such as unpowered tags, stickers, key fobs, or cards. NFC peer-to-peer communication is possible, provided both devices are powered.<ref name="PopSciNFC" /> NFC tags contain data and are typically read-only, but may be writable. They can be custom-encoded by their manufacturers or use NFC Forum specifications. The tags can securely store personal data such as debit and credit card information, loyalty program data, PINs and networking contacts, among other information. The NFC Forum defines five types of tags that provide different communication speeds and capabilities in terms of configurability, memory, security, [[data retention]] and write endurance.<ref>{{cite web|website=nfc-forum.org|url=https://nfc-forum.org/build/specifications/|title=The Building Blocks of Contactless, Core Specifications|date=12 February 2024|access-date=12 February 2024}}</ref> As with [[proximity card]] technology, NFC uses [[inductive coupling]] between two nearby [[Electromagnetic coil|loop antennas]] effectively forming an air-core [[transformer]]. Because the distances involved are tiny compared to the [[wavelength]] of [[electromagnetic radiation]] (radio waves) of that frequency (about 22 metres), the interaction is described as [[Near and far field|near field]]. An alternating [[magnetic field]] is the main coupling factor and almost no power is radiated in the form of [[radio wave]]s (which are electromagnetic waves, also involving an oscillating [[electric field]]); that minimises interference between such devices and any radio communications at the same frequency or with other NFC devices much beyond its intended range. NFC operates within the globally available and unlicensed [[radio frequency]] [[ISM band]] of 13.56 MHz. Most of the RF energy is concentrated in the Β±7 kHz bandwidth allocated for that band, but the emission's [[Bandwidth (signal processing)|spectral width]] can be as wide as 1.8 MHz<ref name="patauner2007">{{cite conference | author=Patauner, C. | author2=Witschnig, H. | author3=Rinner, D. | author4=Maire, A. | author5=Merlin, E. | author6=Leitgeb, E. | url=http://www.eurasip.org/Proceedings/Ext/RFID2007/pdf/s1p4.pdf | title=High Speed RFID/NFC at the Frequency of 13.56 MHz | conference=RFID 2007 | conference-url=http://rfid07.ftw.at/ | location=Vienna, Austria | date=24 September 2007<!--from http://rfid07.ftw.at/program.html--> | access-date=10 December 2010 | archive-date=1 May 2011 | archive-url=https://web.archive.org/web/20110501030235/http://www.eurasip.org/Proceedings/Ext/RFID2007/pdf/s1p4.pdf | url-status=live }}</ref> in order to support high data rates. Working distance with compact standard antennas and realistic power levels could be up to about {{cvt|20|cm|in|frac=8}} (but practically speaking, working distances never exceed {{cvt|10|cm|frac=8|disp=or}}). Note that because the pickup antenna may be quenched in an [[eddy current]] by nearby metallic surfaces, the tags may require a minimum separation from such surfaces.<ref>{{cite report|last=Lee|first=Eric|title=Investigation of using Radio Frequency Identification (RFID) System for Gear Tooth Crack Detection|publisher=Australian Government Aerospace Division DSTO Defence Science and Technology Organisation|date=June 2014|citeseerx = 10.1.1.1011.4228|url=https://apps.dtic.mil/sti/tr/pdf/ADA612338.pdf|access-date=22 October 2024}}</ref> The ISO/IEC 18092 standard supports data rates of 106, 212 or 424 [[kbit/s]]. The communication takes place between an active "initiator" device and a target device which may either be: ; Passive: The initiator device provides a carrier field and the target device, acting as a [[transponder]], communicates by modulating the incident field. In this mode, the target device may draw its operating power from the initiator-provided magnetic field. ; Active: Both initiator and target device communicate by alternately generating their own fields. A device stops transmitting in order to receive data from the other. This mode requires that both devices include power supplies. {| class="wikitable" |- ! scope="col" | Speed (kbit/s) ! scope="col" | Active device ! scope="col" | Passive device |- ! scope="row" | 424 | Manchester, 10% [[Amplitude-shift keying|ASK]] | Manchester, 10% ASK |- ! scope="row" | 212 | Manchester, 10% ASK | Manchester, 10% ASK |- ! scope="row" | 106 | Modified Miller, 100% ASK | Manchester, 10% ASK |} NFC employs two different [[coding theory|coding]]s to transfer data. If an active device transfers data at 106 kbit/s, a modified [[Miller coding]] with 100 percent [[modulation]] is used. In all other cases [[Manchester coding]] is used with a modulation ratio of 10 percent. Every active NFC device can work in one or more of three modes: ; NFC card emulation: Enables NFC-enabled devices such as smartphones to act like smart cards, allowing users to perform transactions such as payment or ticketing. See [[Host card emulation]] ; NFC reader/writer: Enables NFC-enabled devices to read information stored on inexpensive NFC tags embedded in labels or smart posters. ; NFC peer-to-peer:Enables two NFC-enabled devices to communicate with each other to exchange information in an [[Ad hoc network|ad hoc]] fashion. NFC tags are passive data stores which can be read, and under some circumstances written to, by an NFC device. They typically contain data ({{As of|2015|lc=y}} between 96 and 8,192 bytes) and are read-only in normal use, but may be rewritable. Applications include secure personal data storage (e.g. [[Debit card|debit]] or [[credit card]] information, [[loyalty program]] data, [[personal identification number]]s (PINs), contacts). NFC tags can be custom-encoded by their manufacturers or use the industry specifications.
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