There are several general types of fax modems available, mainly grouped by Class. The modem class describes the way in which the computer talks to the modem via serial communications.These include:
For the purpose of this document, the author will generalize between class 2 and class 2.0 for brevity sake. The reader should be aware that there are specific command syntax differences between a class 2 and 2.0 modem.
Most software packages support Class 2/2.0. Many consumer products also support Class 1, however it is not recommended for a multi-tasking operating system. Hylafax will support 2 or 2.0 modems.
Note on Group 3:
Group 3 fax ability refers to the way in which the modem talks to the remote fax machine. Almost all fax machines and fax modems use Group 3, so you do not need to be concerned with compatibility issues of Group 3. To the author's knowledge the only non-group 3 fax currently available is a digital adaptation for sending a fax via a digital ISDN connection. (not analog over ISDN)
Durring a fax transmission, data is sent in "frames" (or blocks) of data. On a mechanical fax machine, the print head will have physical limitations to 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 acomodate 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 isuses. 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 X2 and K56Flex protocols. (By providing 56K, the manufacturer consumes eprom space on the modem board that was previously used by class 2 procedures.)
Yes. Highly recommended!
Use a serial communications program, (procomm, windows terminal, etc)
Type: AT
Get: OK
Now you know your modem is working. Proceed with:
Type: at+fclass=?
Get: 0,1,2
This means you have a modem that will do the following:
data (0)
class 1 fax
class 2 fax
Some modems including US Robotics will report 2.0 rather than 2.
If you want to know more about fax programming, read The Fax Modem Sourcebook by Andrew Margolis.