Send a file phone-to-phone as a live QR stream — screen to camera, no network.
The file is split into pieces, each drawn as a QR code, and the sender plays them as a video loop while the receiver films the screen. Cameras drop frames constantly, so the stream is fountain-coded: every frame is a random combination of pieces rather than piece number 47, which means the receiver just needs enough frames rather than a specific set. Nothing has to be retransmitted and the two sides never talk back.
None. This is light travelling from one screen to a lens. Both phones can be in airplane mode with Wi-Fi and Bluetooth off. Load the page once beforehand and the transfer itself needs nothing.
Photos, documents and small audio — think tens to a few hundred kilobytes rather than video. A screen and a camera is a genuinely narrow pipe. Use Wi-Fi Beam when both phones can reach one network.
When there is no shared network and you cannot make one: locked-down corporate or hotel Wi-Fi that isolates devices, a plane, or a machine deliberately kept off every network. Light crosses gaps a network cannot.
Turn both screens to full brightness, hold the phones steady about a hand's width apart, and avoid direct sunlight or a screen protector that scatters light. Anything that helps the camera read cleanly means fewer wasted frames.