What are the three NFC operating modes?
Three modes exist in the standards: reader writer, where the phone powers and reads a tag; card emulation, where the phone behaves like a card in front of a terminal; and peer to peer, where two powered devices talk directly. The first two matter daily. The third is history.
The three modes
| Mode | The phone | The other side | Everyday example |
|---|---|---|---|
| Reader writer | Supplies the field and asks | A passive tag or card | Tapping a sticker or a poster |
| Card emulation | Answers, powered by the terminal | A powered reader | Paying, or opening a door |
| Peer to peer | Either side may start | Another powered device | Historically, phone to phone transfer |
What runs on your phone every day
Reader writer mode is the one you notice. It is what happens when you hold a phone against a tag: the phone becomes the power source and the interrogator, and the tag answers.
Card emulation is the one you use without noticing. Paying with a phone, opening an office door with it, presenting a transport pass: the phone stops being a reader and becomes the thing being read, drawing its cue from a terminal that supplies the field.
The role reversal is complete. In one mode the phone gives energy, in the other it receives instructions.
Which ones you can actually use
This is where the practical answer diverges sharply from the specification.
Reader writer mode is open. Any application may read and write tags, within what each platform allows, and this is the mode where you have real freedom.
Card emulation is not open in the same way. What your phone may pretend to be is controlled by the operating system and, for payment, by the card schemes and the hardware. This is the answer to the very common question of why you cannot simply copy your office badge onto your phone.
Peer to peer is not available in any form either platform offers to ordinary users today.
How the phone knows which to use
It does not decide in advance. While idle it polls, emitting brief pulses to detect a tag, and it also listens for the field of an external reader.
Whichever happens first determines the role for that exchange. Bring a tag close and the phone becomes a reader. Bring the phone to a terminal and it becomes a card. The switch is automatic and invisible, which is why most people never realise their phone does two opposite things with the same antenna.
Frequently asked questions
Can I choose which mode my phone uses?
Not directly. The role is determined by what the phone meets: a passive tag makes it a reader, an external field makes it a card. Applications can influence what happens within a mode, not which mode is entered.
Why can I not copy my access badge to my phone?
Because card emulation is controlled by the platform rather than by applications. Even where the technical path exists, presenting an arbitrary credential is restricted, and the badge itself may use a scheme the phone cannot reproduce.
Is peer to peer mode gone for good?
It remains in the standards and has disappeared from the platforms. Nothing either mobile operating system currently offers to users relies on it, and file sharing moved to Bluetooth discovery with Wi-Fi transfer.
Does the mode change the read distance?
Not meaningfully. Distance is governed by antenna geometry and by which side is producing the field, and in card emulation the terminal usually has a larger antenna than a phone, which can make the tap feel more forgiving.