For occasional fax use, it may be reasonable to use either traditional fax machines or PCs equipped with individual phone lines that talk directly either to physical fax machines or to modems and fax software. Most commonly, this means you will receive faxes as T.37-defined e-mail attachments that you can print. And -- for fax output -- you can simply treat the fax capability as a printer in the menu of printers you can select. Receiving does require transmission to your phone number.

Rather than use an analog PSTN line, you may send the signal over VOIP lines that use a high-bit-rate codec compatible with fax. Current analog fax machines are Group 3 (G3), and use the V.34bis protocol for transmission. These fax machines can send at 33Kbit/sec., but can fall back to lower speeds on bad lines. The fallback speeds are still faster than the fallback speeds of earlier generations.

General fax standards

The ITU T.30 standard involves the negotiation of capabilities between real or virtual fax machines, while ITU T.4 specifies the actual scanning and reproduction of fax images, including things such as paper size, and is agnostic about being carried over analog or digital packet facilities. These protocols have become substantially more complex since they were first introduced for analog transmission, and their timing needs are responsible for much of the quality of service requirements for FOIP.

While FOIP is less sensitive to message delay than is VOIP, fax protocols do have more sensitivity to network impairments than does, say, browsing Web documents. As with VOIP calls, both the network and processing add delay. Absolute delay, in the absence of keep-alive mechanisms, can cause the fax session to drop. While FOIP is still sensitive to variable delay (i.e., jitter), it is less sensitive to it than is VOIP, because packets can be time-stamped and put into the correct sequence. Were VOIP packets to be reordered, the voice call sound like Yoda would.