How NFC Works: Tap-to-Pay and Tags Explained

Near-field communication, or NFC, lets two devices exchange small pieces of data when they are almost touching. It is what runs a tap-to-pay payment, a transit card tap or a tap on a smart poster. It works by magnetic coupling between two small coils, so it needs no pairing, and a simple tag does not even need a battery.

The basic physics

NFC uses a radio frequency of 13.56 MHz, in a band set aside for industrial, scientific and medical use. Each device has a loop antenna. When one device makes a changing magnetic field in its loop, the other loop picks it up, the way a transformer passes energy between two coils. Wikipedia calls this inductive coupling.

This only works over a very short distance. The NFC Forum, the industry body, says the typical range is up to 2 cm and the certified range is 5 mm. Wikipedia gives about 4 cm as a maximum, and notes other figures in other parts of its article. The sources differ, but all describe a distance of centimetres, not metres.

Why a tag needs no battery

The device that starts the exchange is the initiator, such as a phone. It produces the field. A passive target, such as a sticker, key fob or card, takes a little power from that field and uses it to send its reply. So a tag can sit unpowered for years and wake only when a reader comes near. Two phones swapping data are different: both must be powered, according to Wikipedia.

Three ways devices use it

• Reader/writer: a phone reads or writes a tag, for example a sticker that opens a web page or logs a device into Wi-Fi.
• Card emulation: the phone acts like a contactless card, for payments or transit tickets.
• Peer-to-peer: two devices touch to share a small amount of data.

The NFC Forum also lists wireless charging as a mode, delivering up to 1 W over up to 2 cm, enough for small items such as headsets or watches. Wikipedia contrasts this with the Qi standard, which handles up to 15 W over up to 4 cm.

What the data looks like

Tags store information in a common format called NDEF, short for NFC Data Exchange Format. A record might hold a web address, a phone number or Wi-Fi details. When a phone reads it, the phone decides what to do: open a link, start a call and so on. The NFC Forum defines five tag types, which differ inside but look the same to the reading device.

Speeds are modest. Wikipedia says ISO/IEC 18092 data rates are 106, 212 or 424 kilobits per second. The NFC Forum cites a broader span, from about 46 kbit/s to 1.7 Mbit/s. This is fine for a payment token or a web address, but it is far slower than Bluetooth for larger files. Wikipedia notes that NFC sets up in under 0.1 second without pairing, which is the real advantage.

How payments are kept safe

The NFC Forum describes two ways a phone can act as a card. With a secure element, a separate secure chip or SIM handles the transaction. With host card emulation, an app on the phone handles it and the NFC controller passes commands along. The radio link itself has no built-in protection against eavesdropping, Wikipedia warns, and relay attacks, where a signal is forwarded between devices, are possible. It says applications should add their own cryptography on top, which payment systems do.

Dates worth knowing

These dates come from Wikipedia. Philips and Sony agreed a technical specification on 25 March 2002. It was approved as an ISO/IEC standard on 8 December 2003. Nokia, Philips and Sony formed the NFC Forum on 18 March 2004. The Nokia C7, released in 2010, is named as the first NFC smartphone, and Apple Pay launched in the United States on 20 October 2014.

Sources

• NFC Technology, NFC Forum
• Near-field communication, Wikipedia