What is Fax over IP?

Fax over IP (FoIP) is not the scanning of an image and transmission of the results to a remote end via the Internet. That would be e-mailing (the resulting TIFF file), or FTPing it, or sending the file via HTTP PUT. Or whatever. No, that would be very easy; the Internet was born for it, but that is not faxing.

Fax over IP is actually to interact via the Internet with a remote, regular (T30, PSTN) fax machine. The problem is, to exactly reproduce the characteristics of a PSTN electrical circuit via a packet network is almost impossible. Packets get delayed, they arrive out-of-order (for example, the third packet arrives after the fifth), they get lost (for example, the fourth and seventh packets do not arrive at all), delays are not constant as a transmission proceeds (for example, jitter), and so on. All of this is not a problem for file transfer protocols like HTTP, and is a minor nuisance for voice transmission (both software correction and the human ear are very adaptable...