FOIP, or Fax-over-IP, relates to sending and receiving faxes over a network (often the internet).

The acronym is very similar to VOIP (Voice-over-IP) and has probably gained more traction through its similarity to VOIP.

Although FOIP and VOIP rely on many of the same technologies, there are important differences. For example, during a VOIP call, if one or both parties lose some of the audio information, this might not even be noticed, and even a noticeably degraded VOIP call can be understood and useful.

However, in some FOIP implementations, losing any of the data can result in an unrecoverable error, sometimes meaning a fax image comes through partially or in a corrupted way.

Increasingly, VOIP/FOIP hardware and software support technologies to help mitigate the caveats of early FOIP tech. Protocols like T.38, when implemented end-to-end, can maintain much higher reliability than rockier "conventional real-time over a VOIP connection" approaches.