nicke@mizar.tde.lth.se (02/19/91)
The subject says it. We have a telex system that we would like to extend with fax. First: are there any standard for fax modems? Second: Can I send text to the modem, or do I have to send some graphic format? In which format do incoming faxes come in? Any answers or pointers will be greatly appreciated.
tnixon@hayes.uucp (02/22/91)
In article <NICKE.91Feb18173153@mizar.tde.lth.se>, nicke@mizar.tde.lth.se writes: > The subject says it. We have a telex system that we would like to > extend with fax. First: are there any standard for fax modems? Second: Can I > send text to the modem, or do I have to send some graphic format? In which > format do incoming faxes come in? Until recently, there was no standard interface to fax modems. Each manufacturer designed their own, based on the creativity and ingenuity of their engineers (I'm thinking positively here!) Unfortunately, this also meant that users were restricted to using only the software that came with their modem, and it was very difficult to write your own software to use a fax modem. The industry recognized this as a problem that was holding back the growth in sales of fax modems, and joined together in standards committee TIA TR-29.2 to develop a series of fax modem command set standards. The first such standard, EIA/TIA-578, defines "Service Class 1" for fax modems. Class 1 provides the minimal, hardware-level support necessary to send faxes -- Group 3 modulation schemes (V.21, V.27ter, V.29), async-to-sync conversion, and HDLC framing. All of the Group 3 fax protocols and image processing must be done in the PC software, which makes it difficult to support a Class 1 modem in the background or in a multitasking environment. It adds about 10 new AT commands to the modem. The second standard, defining Service Class 2, is about to be completed; I expect it to be adopted this summer. It moves most of the T.30 fax protocol processing out into the modem, freeing up the computer from most time-critical processing. This makes it much easier to do fax support in a TSR, in background, on a multiuser or multitasking system, or across a LAN. It adds more than 40 new AT commands to control all of the various parameters and protocols associated with Group 3 fax. Service Class 3, the next major task for the committee, is still far away, controversial, and speculative. There are significant disagreements on the scope of this class of service. Generally, it is envisioned as moving much of the actual fax image processing from the PC out to the modem, making the modem in effect an image conversion co-processor. The modem would understand particular data file formats such as PCX, GIF, TIFF, RTF, HPCL, PostScript, and text (or some of these); the application would basically just point the Class 3 device at a file, and off it would go. Again, remember that this purely conjecture at this time, since the scope of the project is not yet really defined. It is not clear at this time whether this level of service is even appropriate to be handled on a "modem"; to me, it seems more like an API, with the division of processing between the main CPU and the modem being ill-defined or left up to the implementor. To directly answer your questions, a program wishing to send a fax using Class 1 or Class 2 fax commands must, at a minimum, convert from text or graphic data into CCITT T.4 compressed image format, which is also the format in which incoming data is received. -- Toby Nixon, Principal Engineer | Voice +1-404-840-9200 Telex 151243420 Hayes Microcomputer Products Inc. | Fax +1-404-447-0178 CIS 70271,404 P.O. Box 105203 | UUCP uunet!hayes!tnixon AT&T !tnixon Atlanta, Georgia 30348 USA | Internet hayes!tnixon@uunet.uu.net