Fax Command Sets; Defining The Fax Command Sets - ZyXEL Communications U-336E User Manual

You modem allows for fax speeds up to 14400 bps when
transmitting to a fax machine which complies with the V.17 fax
standard. Speeds will fall back to 12000, 9600, or 7200 bps in poor
line conditions. When connecting to a G3 fax device, your modem
allows for fax speeds up to 9600 bps and will automatically fall
back to 7200, 4800, and 2400 bps if the line quality is poor.

Fax Command sets

The U-336E supports four command sets for fax operation:
Class 1 command set
TIA PN-2388 Class 2 command set
TIA 592 Class 2.0 command set
ZyXEL Extended Fax AT command set

Defining the Fax Command Sets

The EIA Class 1 and Class 2 fax commands are a set of AT fax
commands defined by EIA/TIA (Telecommunications Industry
Association) for controlling fax/modems from a computer through
the serial RS-232 interface. All fax/modems and fax software
supporting this standard will be compatible with each other.
Class 1 commands control on-line modem negotiation while Class 2
commands allow the modem to do many negotiations
simultaneously. The Class 1 protocol uses the modem to transmit
fax data only. The complete organizational overhead for this
protocol is handled by the connected computer. The Class 1
command set is also called the TIA-578 standard.
Several revisions of the class 2 standard exist. Implementations
conforming to different revisions may not work together. A
Fax Operation 47