At the hardware level, the two standards that govern the exchange of commands between a host computer and a fax modem are EIA-578 (-Class 1-) and EIA-592 (-Class 2-).
At the software level there is one "official" standard and a number of "industry standards". The one "official" standard is ITU-T T.611 ([need more information on this standard]).
<h3>Class 1</h3>The Class 1 fax modem standard describes an extension to the "Hayes Modem Command Set" to permit computers to send and receive faxes using fax modems. The Class 1 standard is a low-level specification in which most of the protocol work (i.e. T.30) as well as image generation (rasterising and T.4 compression) must be done by the computer (in software) while the modem only handles the basic modulation as well as converting the asynchronous data from the computer into the synchronous packets used in fax communications.
The primary advantage of Class 1 modems is that fax protocol is implemented in software which means that new extensions to the fax protocol standard (i.e. T.30) can be implemented without requiring a ROM change in the modem (or without waiting for the modem manufacturer to get around to supporting the new feature). Also software developers are not dependent on the quality of the T.30 firmware in the modem (as are developers who use Class 2 modems).
The primary disadvantages are (a) the software vendor has to handle the complexity of the T.30 protocol and (b) Class 1 is very sensitive to timing and multi-tasking operating systems (such as *IX) have great difficulty in reliably meeting the tight timing constraints and maintaining the fax connection. Lifting this timing limitation is the primary motivation behind the new proposed Class 4 standard.
The official standard for Class 1 is EIA/TIA-578. (Note that if you are purchasing a copy of EIA/TIA-578 be sure to purchase also the TIA/EIA Telecommunications Systems Bulletin 43 (TSB43) which provides additional information missing from, or incorrect in, the original EIA/TIA-578 standard.
The ITU-T has approved an international version of the Class 1 standard and designated it T.31. Note that T.31 includes a number of things not in Class 1.
Although the official standard is copyright EIA/TIA/ANSI, a draft version has been published electronically by Supra and is available from their BBS and FTP sites (see sections I.10, and I.15) and from Sam Leffler at SGI (retrieve his HylaFAX package, described in section P.1).
<h3>Class 2</h3>
The Class 2 fax modem standard describes an extension to the "Hayes Modem Command Set" to permit computers to send and receive faxes using fax modems. The Class 2 standard is a higher-level specification in which most of the protocol work (i.e. T.30) is done by the modem while the computer is responsible for managing the session and providing the image data in the appropriate format (i.e. T.4).
The priimary advantage of Class 2 is that the low-level detail work is handled by the modem. Not only does this mean that software developers do not have to be burdened with having to support the T.30 protocol, it also relieves the host computer of all of the time-critical aspects of fax communications, making support of Class 2 modems under *IX systems possible.
The biggest headache for software developers is that the Class 2 standard took a long time to be approved (more for political than technical reasons, IMHO) and many companies did not wait for the final version to be approved before shipping modems. As a result we have a situation (as of 93Q4) in which all shipping Class 2 modems adhere (more or less) to the first draft of the TR29.2 committee (document SP-2388) and not to the standard as it was approved. To compensate for this, the "new" Class 2 is referred to as Class 2.0 and the "old" as plain Class 2.
(Warning - flame from a frustrated fax programmer on...)
Even more disconcerting is the fact that most companies who have implemented (the old) Class 2 have done one or more things wrong (they must have been smoking *and* inhaling) so we have a further division of the standard into "true, old Class 2" (which includes the Everex 24/96D and MultiTech modems) and everything else (mostly based on the Rockwell chip which differs from SP-2388 in a number of ways, although some other chip makers, such as EXAR, have found even more ways than Rockwell to depart from SP-2388). It's so bad that most modem companies now implement the Rockwell version of Class 2 just because so many of the *%#& things have been shipped (i.e. Multitech has a special command which switches their modem from proper Class 2 operation to Rockwell-like operation just so they can interoperate with DOS software that expects Rockwell-like operation). And of course no one at Rockwell or EXAR or the other companies bothered to write down the difference between their version of Class 2 and the TR29.2 document. (Flame off.)
The draft standard for the "old" Class 2 is SP-2388, Document TR-29/89-21R8, dated March 21, 1990. This is available by contacting the EIA/TIA directly. This is the standard implemented by all Class 2 modems on the market prior to the end of 1993.
The official standard for the "new" Class 2 (also referred to as Class 2.0) is EIA/TIA/ANSI-592. This document is available from Global Engineering Documents (see below).
As of 94Q4 the only modems known to the editor of this FAQ that support 2.0 are those produced by USRobotics and ZyXEL.
Note that although many modems that implement Class 2 also support Class 1, Class 1 is *not* a subset of Class 2. Also, there are some modems that only support Class 2 and many that only support Class 1.
Although the official standard is copyright EIA/TIA/ANSI, a draft version has been published electronically by Supra and is available from their BBS and FTP sites (see sections I.10, and I.15) and from Sam Leffler at SGI (retrieve his HylaFax package, described in section P.1).
The ITU-T has approved an international version of the Class 2.0 standard (along with the TIA/EIA-605 Data Link Protocol) and designated it T.32. Note that T.32 includes a number of things not in Class 2.
Ben , Voor meer info kijk op :
http://www.faximum.com/faqs/fax of op
http://www.faximum.com/faqs/fax.glossary
:strip_exif()/u/10073/compu.gif?f=community)