Is it mandatory to have a WiFi connection during iPhone-to-iPhone data transfer?

While transferring data from iPhone to iPhone, is it mandatory to connect to WiFi ? (I meant after transferring part gets started both the devices are for sure supposed to be connected to the WiFi ?) if yes, for some reason my WiFi keeps getting disconnected in the middle and I have to restart the whole thing again. Which then takes hours yet fails. What do I do ?

[Re-Titled by Moderator]