Protocols and standards
V.17
V.17 is the ITU-T modem standard designed for fax, sending the image at up to 14.4 kbit/s, with fallbacks to 12,000, 9,600 and 7,200 bit/s.
Adopted in 1991, it uses trellis-coded modulation, which makes it more resistant to noise than earlier standards at the same speed. It is used only for image transfer: negotiation between machines always happens at 300 bit/s. It is the speed almost every Group 3 fax machine in service can reach.
Why it matters for fax
V.17 is the reference compromise: fast enough for a page in about ten seconds, robust enough to survive a mediocre line. When a transmission fails, machines step down speed by speed, which is why the same page can take very different times.