Protocols and standards
V.34
V.34 is the ITU-T modem standard that reaches 33.6 kbit/s over a phone line, used in fax by so-called Super G3 machines.
Published in 1994, it gets the most out of an analogue line by probing its characteristics before choosing the transmission parameters. Its speeds approach the theoretical limit of the voice channel. In return, it needs a clean line and a path with no untimely signal processing.
Why it matters for fax
A V.34 fax sends a page roughly twice as fast as V.17. On a packet network, that sensitivity comes at a price: many gateways and operators deliberately cap fax at V.17 or lower, which is slower but far more tolerant.