comp.dcom.fax FAQ
comp.dcom.fax FAQThe 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).
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.
There has been a proposal to change the Group 3 standard to mandate CNG on all fax calls.
In some sense DID can be viewed as the opposite of Caller ID. With Caller ID the signal indicates which number placed the call (i.e. the phone number of the originator of the call). With DID the signal indicates which number was dialled (i.e. the phone number of the destination of the call).
Note, however, that the signalling mechanism used for Caller ID is different from the method used for DID. In other words, equipment that can decode the Caller ID signals will not work on a DID trunk.
Historically DID has been used by PBXs that provided direct dialling to internal extensions. For example, dialling 555-1201 would ring on extension 101. Dialling 555-1202 would come in on the same trunk to the PBX but the PBX would route the call to extension 102.
Now DID is also used with fax modems and boards to provide routing of inbound faxes. Each employee or department is given a different fax number but all of the calls come in on the same DID trunk. The fax board (or external DID decode box) decodes the signal from the telco central office which indicates which number was dialled and uses this number to route the fax to the appropriate user or department.
Normally the T.4/T.30 protocol is error detecting but not error correcting. The receiving fax machine can usually tell when an error has impaired the image but cannot selectively request retransmission of the damaged portions of the image. The only options are to (a) ignore the errors (if few in number), (b) request that the page be resent (ignored by most fax machines), or (c) give up.
The ITU is an organization, a union, of Member countries. As of 1993 there were 166 Members. The Union's headquarters are in Geneva, in the Place des Nations.
Before 1993, the ITU consisted organizationally of five permanent organs: the General Secretariat, the International Frequency Registration Board (IFRB), the International Radio Consultative Committee (CCIR), the International Telegraph and Telephone Consultative Committee (CCITT) and the Telecommunications Development Bureau (BDT).
In early 1993, the ITU was reorganized into the General Secretariat and three Sectors: Radiocommunication, Telecommunication Standardization and Telecommunication Development. The standards-making activities of the CCITT and CCIR have been consolidated into the Telecommunication Standardization Sector (ITU-T). The remainder of CCIR activities were integrated with the activities of the IFRB into the Radiocommunication Sector (ITU-R). The Development Sector (ITU-D) facilitates telecommunications development by offering technical cooperation and assistance. The ITU General Secretariat supports the activities of the three Sectors.
(This description has been taken from material published by the ITU.)The standards promulgated by the ITU-T are called Recommendations and the recommendations of relevance to the fax world are the T series which govern the fax protocols and the V series which govern modem operation. (See also T.*, and V.*, below.)
For more information on the ITU and the publications available from them, see the description of ITUDOC in section I.10 in Part 2 of this FAQ.
First, we added G3 fax compatability to printers. The device accepts PS jobs from the Mac, PC, and Unix hosts (we have host driver support for all three) rasterizes these jobs at G3 resolutions with optional cover pages and captions, and then sends the fax. You get all of the usual bells and whistles like broadcast, delay, whatever. This differs from a "normal" fax machine in that you avoid printing and rescanning and thus get to transmit very high quality without extra effort. It differs from a PC fax modem because it is network shareable thus saving hardware and phone line charges. Our testing shows that our imaging is higher quality than popular PC fax programs, but there's no intrinsic reason they couldn't do just as well. Also, the fax modems tend to drag down the PC while sending or receiving, whereas we offload the really hard work of controlling the modem to the printer (yes, you can still print while transmitting).
When we receive a G3 we automatically print it out, scaled to fit the available paper if necessary. This plain paper output is much nicer than a roll-fed device can produce. There's a trade-off vs. a PC fax modem. With PS Fax you don't have to leave your PC on to receive faxes, just your printer (which probably has a sleep mode), and you don't have to deal with the very slow printing speed that many fax modem packages seem to suffer from. But, if you wanted that file on the PC so you could edit it or re-transmnit it or... Well, we don't support receiving back to the PC yet.
One obvious difference from a traditional fax machine is that PS Fax printers do not yet offer a scanner. Unless you have a scanner for your PC, there's no way to fax clippings or handwritten documents with PS Fax. Obviously, the "wonder box" printer, fax, copier, scanner is our next target.
The other half of the equation is a thing called Postscript File Transfer. If both you and the person you're communicating with have PS Fax devices then the PS file gets sent rather than a G3. This usually results in a shorter phone call and it always results in significantly higher document quality including high resolution (ex 600 dpi), large format, color, etc. Compared to 30 million G3 units the PS Fax installed base is small, so the PSFT trick is only likely to work in closed environments. It's been most successful either in big corporations who use it to communicate between offices or for consultants who have a need to transmit very high quality output to their clients and can talk their clients into buying a PS Fax receiver.
Courtesy of mparker@mv.us.adobe.com (Mike Parker). You can also obtain more information on PostScript Fax from Adobe's WWW server: http://www.adobe.com/PS/PSFax.html.The idea is that the sending fax machine allows the user to enter the fax routing information after the fax phone by pressing the # key on the DTMF keypad (note that most machines do not currently support this so do not try this at home, kids).
The syntax of the subaddress field are spelled out in TIA/EIA IS-141 which defines the concept of fax extensions and fax forwarding.
For example, to send the fax to "fax extension" 1234 at fax number 1 604 926 8182 one would type on the fax machine's keypad "1 604 926 8182#1234"
According to IS-141 it is possible to request the receiving fax machine distribute copies of the fax to multiple destinations or even forward it to another fax machine.
The basic coding scheme (called in the recommendation "One-dimensional coding scheme" but also known in the industry as MH or Modified Huffman) takes each scan line of pixels and compresses it by (a) converting the raster in a sequence of run lengths (the number of white pixels followed by the number of black pixels followed by the number of white pixels etc. and etc. until the entire raster has been converted into runlengths) and (b) encoding each run length into a unique variable-length bit string. The code words used for white and black runlengths are different and have been chosen in order to do a reasonable job of compressing a "typical" fax page.
For example, in one dimensional encoding the following raster: OOOOOOOOOO****OOOOOO**OOOOOOO*OOOOOOOOO***... converted into run lengths: 10 4 6 2 7 1 9 3 ... encoded into MH bit strings: 00111 011 1110 11 1111 010 10100 10 (spaces have been added for readability and are not part of the MH bit string)
Since our example has unusually short white run-length it does not accurately illustrate the degree of compression which can be achieved. For example, a normal fine resolution fax image contains about 3,800,000 pixels (464K). Using one-dimensional encoding this can be reduced to between 20K - 50K.
In two-dimensional encoding, the first line of a group of lines is compressed using one-dimensional coding (see above) and subsequent lines are compressed using an algorithm that describes line n in terms of line n-1. Since there is usually a high-degree of correlation between the pixels of adjacent scan lines, this usually results in significant compression.
Since the basic fax protocol (T.30) is error detecting (but not error correcting), there is a limit on the number of two-dimensionally compressed scan lines that can follow a 1-D line. This is to limit the propagation of errors through an image. This limit is referred to as 'k' in the standard and is 2 for standard-resolution faxes and 4 for high-resolution faxes.
Unfortunately, this method of compression is computationally intensive and most (inexpensive) fax machines do not support it.
See Standards Related to Facsimile Communication for information on how to obtain a copy of the T.4 standard.
T.6 is essentially the two-dimensional compression algorithm from T.4 (see above) except that 'k' is infinite (i.e. all lines are two dimensionally compressed). This can be done because Group IV fax machines operate over an error-free communications channel.
See Standards Related to Facsimile Communication for information on how to obtain a copy of the T.6 standard.
At this point the machines go to one of phase B, C, or E depending on the exchange of commands and responses during phase D.
See Standards Related to Facsimile Communication for information on how to obtain a copy of the T.30 standard.
Gray Associates (manufacturers of fax protocol testing equipment) also have an in-depth discussion of fax protocols at http://www.grayfax.com/faxsminar.html.
See Standards Related to Facsimile Communication for information on how to obtain a copy of the T.41x standards.
See Standards Related to Facsimile Communication for information on how to obtain a copy of the T.434 standard.
See Standards Related to Facsimile Communication for information on how to obtain a copy of the T.611 standard.
The relevance of TIFF to fax is explained in the following entry.
A TIFF class defines the tags that are required to be written by TIFF writers (i.e. those tags that TIFF readers may depend upon) and defines those tags (and tag values) which all TIFF readers of that class must be able to handle.
TIFF Class F was developed by Joe Campbell while he was at Everex developing the first Class 2 fax modem. The Class F specification defines those tags (and by extension, those TIFF file formats) which ought to be used and supported by fax software.
Many fax software companies support TIFF F files and some use it as their native file format for received and transmitted faxes (i.e. Faximum Software).
A flat-text version of the TIFF-F specification may be obtained from
i
ftp://ftp.faximum.com/pub/documents/tiff_f.txt
-EOM-
Please mail comments to webmaster@www.faximum.com
Last updated: Fri Jun 16 11:24:17 PDT 1995
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