What happens, step by step, when a tag enters the field?
Six things happen in sequence: the phone detects a change in its field, the tag powers up, the two agree on a protocol, the reader singles out one tag, the content is read, and the operating system decides what to do with it. A failed tap failed at one specific stage.
The six stages
Most descriptions of NFC stop at "the phone reads the tag", which hides everything useful. The sequence below is what actually runs between contact and action.
| Stage | What happens | What it looks like when it fails |
|---|---|---|
| 1. Detection | The phone's polling pulse is loaded by something in the field | Nothing at all, no sound, no vibration |
| 2. Power up | The tag rectifies enough energy to boot its chip | Nothing, or an intermittent read at the same spot |
| 3. Protocol | The two sides settle on a technology and a bit rate | The phone reacts but reports an unknown tag |
| 4. Selection | One tag is singled out from any others present | Erratic behaviour with two tags in range |
| 5. Reading | The structure is read, then the stored records | Detected but empty, or a partial read |
| 6. Dispatch | The operating system decides which app or action handles it | A read with no action, or the wrong action |
What each stage can go wrong at
Stages one and two look identical from the outside and account for most complaints. Both produce complete silence. The difference is that stage one fails on geometry, wrong spot or too far, while stage two fails on energy, usually because the tag is small or sitting on something that absorbs the field.
Stage three is where a tag that is simply not supported shows up. The phone knows something is there and cannot talk to it.
Stage five is the one people misread most often. A tag can be detected, identified and still contain nothing a phone can act on, either because it was never written in a standard format or because it was formatted and left empty. "The phone sees it but nothing happens" almost always means the read worked and there was nothing worth dispatching.
Stage six is not NFC at all. It is the operating system applying its own rules, and it is where the two mobile platforms differ the most.
Why this is worth knowing
Because "it does not work" is not a diagnosis, and the fixes for the six stages have nothing in common. Moving the phone fixes stage one. A larger tag fixes stage two. Rewriting the content fixes stage five. Nothing you do to the tag will fix stage six.
A single question separates the top half from the bottom half: does the phone react at all, with a sound, a vibration or a banner? If it does, the radio side worked and the problem is in the content or in the platform. If it does not, stop looking at the content.
Frequently asked questions
Why does my phone vibrate but nothing opens?
The read succeeded and the dispatch stage found nothing to act on. Either the tag holds no content in a standard format, or it holds a record type the platform ignores. The radio side is fine, which is why moving the phone will not help.
How long does the whole sequence take?
Fast enough to feel instantaneous when everything works, which is why a failure feels like nothing happened rather than like something timed out. The transfer of your data is the shortest part of it.
What happens if I pull the phone away halfway through?
The tag loses power and the exchange stops. For a read that is harmless, you simply get nothing. For a write it can leave the tag holding partial content, which is one reason a write should be verified rather than assumed.
Does the phone read the whole tag every time?
No. It reads the structure first to learn how the memory is laid out, then reads only what that structure points at. This is why a mostly empty tag reads as fast as a full one.