ZyXEL Communications U-336SA - Manual (Page 80)

66 Fax Operation
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-336S/SA 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.