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The Class of a fax modem defines the set of commands to which the modem responds. These commands are given by the controlling software, so the modem and the software must obviously be compatible. There are currently three such classes: 1, 2 and 2.0. Class 2.0 was ratified in July '95 and is very similar to 2, but not identical. The present release of FaxIt supports both Class 2 and Class 2.0 modems. For more information on modem classes, see the section on Classes below.

To determine what type of fax modem you have, firstly consult the manual. If that doesn't give the information, try the following. Connect to the modem using the Psion Comms application, at 19,200 baud, 8 data bits, 1 stop bit, no parity. These are the settings used by FaxIt. Type:

        AT      return

The modem should respond with OK. Now type:

        AT+FCLASS=?       return

The modem should now list the available classes to which it conforms, separated by commas. It may say something like 0,1,2 or 0,1,2.0. We are interested in the 2 or 2.0 part of this. If your modem responds with ERROR to the second command, or if it returns a list of classes that does not include 2 or 2.0, then unfortunately, it will not work with FaxIt.

Class 1 and Class 2 Fax Modems

It is easy to get confused between the Class of a fax modem and the Group of a fax machine.

Group

The group of a fax machine or fax modem specifies the communications protocol across the telephone link. Almost all fax machines these days are Group 3. Think of the group as a network. Any G3 fax machine can talk to any other G3 machine, at least that's how the theory goes.

Class

Class applies to fax modems, not fax machines and refers to the commands supported by the modem. Sending a fax is a two stage process. Firstly, the text (and any graphics) must be converted into a compressed bitmap in a form suitable for fax transmission. This format is known as T.4. Next, the call must be placed to the receiving station and appropriate negotiations performed between the two machines. These negotiations might include:

• Exchange of station identifiers (CUI)
• Exchange of station capabilities (paper size, scanline timings etc.)
• Proposed transmission speed based on machine capabilities
• Modified transmission speed based on prevailing line conditions

This negotiation protocol is known as T.30. Once these negotiations are complete, the T.4 data is transmitted. There are currently two approaches to controlling the fax modem in order to send the fax and these are known as Class 1 and Class 2. Both methods leave the conversion of the image into T.4 to the host program. The difference lies in how the T.30 protocol is implemented.

Class 1

Class 1 (or EIA-578) provides a minimal set of commands (4) for handling T.30, little more than send and receive data in fact. Consequently, the host program must have full knowledge of the protocol and implement it completely. Additionally, data is normally returned from the modem in HDLC frames which require an extra level of decoding.

Class 2

Class 2 provides a much higher level of control. Because most of the T.30 protocol is implemented inside the modem, the controlling software is an order of magnitude simpler. There are 30-40 commands available. Data is returned from the modem in logical strings which need very little extra processing.

FaxIt

FaxIt has been written to support Class 2/2.0 modems. The modem driver was developed as a separate module so that it could easily be replaced in the future by a Class 1 driver (or indeed anything else that might become standard). Having said that, a combination of the amount of work to do on the mainline software coupled with the huge development effort to write a Class 1 driver, plus the fact that Class 1 modems are few and far between, add up to only a very slim chance of such a driver ever being written.