During a fax transmission, data is sent in "frames" (or blocks) of data. On a mechanical fax machine, the print head will have physical limitations on how fast a line can be printed. Due to the Modified Hufman encoding of faxes, it is possible to send a single black pixel line in only a few bits. Because the fax machine has to physically move the printer head on the paper, there is a minimum line sending time required while sending data. Fax modems must send the data frames at a speed and timed to accommodate the receiving fax machine.

A Class 1 modem only sends frames. The fax software is required to build the frames from the fax data, and then send them with respect to the timing issues for the remote fax machine. In a multitasking environment, the computer may be busy with another task, and miss the timing on one of these frames. When this happens, the fax is aborted.

A Class 2 or 2.0 modem handles all the fax framing issues. The software simply gives the modem a command to send the fax data, and then streams the fax image to the modem. Class 2 modems are much better suited to multitasking environments.

Note to shoppers: Many modem makers are dropping Class 2/2.0 features from their consumer modems. I suspect that this is due to the fact that most consumers only need Class 1 support for Windows, combined with the push to 56k .90 protocols. (By providing 56K, the manufacturer consumes eprom space on the modem board that was previously used by class 2 procedures.)

find out what class my fax modem supports:

Type the following in Windows (Hyper)Terminal, Procomm or Kermit:
AT+FCLASS=?
The modem reply will be of the form:
0,1,1.0,2,2.0
Where 0 is for data, 1 is class 1 fax, 1.0 is for class 1.0 fax, 2 is class 2 fax and 2.0 is class 2.0 fax.