FAX Questions and Answers - Part 1
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http://www.whnet.com/fax1.html
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Sender: pajari@Faximum.COM (George Pajari)
Archive-name: fax-faq/part1
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This article contains the answers to some Frequently Asked Questions (FAQ)
often seen in the USENET newsgroup comp.dcom.fax relating to facsimile
standards, software, and hardware. It will be posted approximately monthly.
If you would like to make any submissions or corrections to the FAQ, please
contact faxfaq@faximum.com. Your input is greatly appreciated. Suggested
questions need not be accompanied by suggested answers. Areas that are in
particular need of contributions are marked "".
Flames and other comments (constructive or otherwise) are also welcomed.
Note that this FAQ is primarily concerned with fax standards in general
and computer-based fax in particular. It contains little information on
commercial fax machines and related paraphenalia. If someone else would
like to start and maintain such a section (or separate FAQ) they are more
than welcome to do so. Otherwise, please send contributions to this FAQ.
This FAQ is crossposted to news.answers. As a consequence, this text will
also be automatically archived on many FAQ servers all over the world
(e.g., look with anonymous ftp at rtfm.mit.edu in directory
/pub/usenet/news.answers). You'll also find there many other answers
to frequently asked questions. This faq is stored in the directory fax-faq.
regards
g.
pajari@Faximum.COM
George Pajari / Faximum Software / Tel: +1 (604) 925-3600 / Fax: ... 926-8182
1497 Marine Drive, Suite 300 / West Vancouver, BC / Canada V7T 1B8
Std. Disclaimers:
----------------
The content of this article is the sole responsibility of the author(s) and
contributors, and does not necessarily represent the opinions or policies
of their employers or other companies mentioned.
The information provided herein is believed to be correct but the author and
contributors cannot accept any liability for errors and omissions. Readers
are cautioned to verify any information before making decisions or taking
action based upon this information.
While every reasonable effort has been taken to maintain an objective and
unbiased approach in the collection and presentation of this information,
readers are advised that the author and possibly some of the contributors
work for or have an interest in commercial organisations involved in the
fax and/or computer industry.
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TABLE OF CONTENTS
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Note that this FAQ has grown to the point where it has been split into two
parts. Part 1 contains the front matter (introduction, disclaimer, etc.)
and the glossary. Part 2 contains everything else (questions and answers,
sources of information, and information on products).
----- Part 1 of 2 [This part] -----
G. Glossary and Background Information
----- Part 2 of 2 [The other part] -----
Q. Frequently Asked Questions
Q.1 Can I use my * data modem to send/receive faxes?
Q.1A Can my fax modem transmit data?
Q.2 How can I fax PostScript or PCL documents using computer-based fax?
Q.3 How can I view incoming faxes on my computer?
Q.4 How can I print incoming faxes on my computer?
Q.5 Can fax modems also handle data or voice calls?
Q.6 What resolution are fax images?
Q.7 Can I take a fax file and edit it?
Q.8 Is there a standard program interface (API) for fax communications?
Q.9 How can I share my single phone line with voice, fax, data, etc.
I. Sources of Information
I.1 Standards Related to Facsimile Communication
I.2 Where to Obtain Standards Documents and Related Information
I.3 Magazine Reviews of UNIX Fax Software
I.4 Magazine Reviews of DOS/Windows Fax Software
I.5 Magazine Reviews of Mac Fax Software
I.6 Magazine Reviews of Fax Modems (see also O/S specific reviews)
I.7 Magazine Reviews of Fax Machines
I.8 Publications Devoted to Fax and Telecommunications
I.9 Books on Fax
I.10 Other Sources of Information on Fax
I.11 Conferences on Fax
I.12 Associations Related to Fax Technology
I.13 Fax-on-Demand Phone Numbers
P. Product Information
P.1 List of UNIX Fax Software
P.2 List of MS-DOS Fax Software
P.3 List of MacIntosh Fax Software
P.4 List of Windows Fax Software
P.5 List of OS/2 Fax Software
P.6 List of Fax Modem Vendors
P.7 List of Fax Board Vendors
P.8 List of Vendors of Secure Fax Equipment
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Subject: G. GLOSSARY and BACKGROUND INFORMATION
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ANSI/AIIM MS53-1993
The American National Standard File Format for Storage and Exchange of
Images - Bi-Level Image File Format: Part 1 (ANSI/AIIM MS53-1993) was
approved in 1993. The standard defines a format for a file containing
one page with one image. Page sizes and image sizes can be specified.
Both definite length and indefinite length are supported. Clipping of the
image can be specified. Image coding may be according to ITU-T Recs. T.4
(one- and two-dimensional) and Rec. T.6. Bitmap may also be specified.
Both facsimile style least significant bit and industry style most
significant bit mapping are supported.
(Definition courtesy of hrs1@cbnewsi.cb.att.com (herman.r.silbiger))
BFT or
Binary File Transfer
A method of transferring files using fax modems (as an extension to the
fax protocol). This standard, which will be approved shortly, will be
referred to as T.434.
Brooktrout Patent (taken from a press release from Brooktrout)
Brooktrout's patent (number 4,918,722), issued by the US patent office
in 1990, covers generally any method for the selection of facsimile messages
and their deliver to a particular telephone number under control of commands
entered through a telephone, for example in the form of signals generated
from the telephone's touch-tone keypad. This method is employed in many
fax-on-demand systems, which provide business users and service providers the
ability to offer automated fax delivery of specified information in response
to requests from customers, subscribers or other callers.
[Editor's note: this patent is the subject of litigation and the current
status or validity of this patent is not known.]
CAS
An API for fax devices invented by Intel and DCA and tied to the Intel
and MS-DOS architectures.
CCITT
Comite Consultatif International Telegraphique et Telephonique (a.k.a. The
International Telegraph and Telephone Consultative Committee). The old
name for ITU-T, the body responsible for setting the international
standards for telecommunications equipment. See ITU below.
CED or
Called Station Identifier
The distinctive tone generated by a Group III fax machine when it
answers the phone (2100 Hz).
Class 1
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.
Although the official standard is copyright EIA/TIA/ANSI, a draft version
has been published electronically by Supra and is available from their
BBS (see section I.10 for the number) and from Sam Leffler at SGI (retrieve
his FlexFax package, described in section P.1).
Class 2
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 primary 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). There are rumours of 2.0 modems shipping but as
of January 1994 none of those tested by the editor of this FAQ were ready
for prime time.
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 (see section I.10 for the number) and from Sam Leffler at SGI (retrieve
his FlexFax package, described in section P.1).
Class 3
A class number reserved for a project to define a standard for fax modems
that would, in addition to handling the T.30 protocol (i.e. Class 2), also
handle the conversion of ASCII data streams into images (i.e. T.4). Although
there are a couple of fax modems that handle the ASCII to fax conversion,
no draft document has been circulated and the future of this project is in
doubt.
Class 4
Class 1 with intelligent buffering to reduce the need for the host
computer to respond instantly to the fax modem. This standard is expected
to go out to ballot in 94.
Class 8
Not a fax standard at all but an extension to the Hayes command set to
support voice.
CNG or
Calling Tone
The distinctive tone that a fax machine ought to generate when placing
a fax call (1100 Hz on for 1/2 second, off for 3 seconds). Note that the
Group 3 fax standard only requires fax machines in "automatic operation"
to generate this tone so that machines which require you to dial the
number (either on the keypad of the fax machine or using an attached
phone) need not generate this tone. The lack of CNG can cause some
fax switches (see Q.9 below) problems.
There has ben a proposal to change the Group 3 standard to mandate CNG
on all fax calls.
CSI or
Called Subscriber Information
The "name" of the answering fax machine. An optional frame of information
sent to the calling fax machine during Phase B (see T.30 below). Although
many fax machines permit ASCII information, the T.30 standard states that
this is to contain the international phone number of the fax machine,
including the plus symbol, the country code, the area code, and the
subscriber number using only digits, the plus symbol, and a space.
(i.e. the North American fax number (604) 926-8182 ought to be programmed
into the fax machine as +1 604 926 8182).
ECM or
Error Correcting Mode
An extension to T.30 to permit the receiving fax machine to request that
portions of an image that were received with errors be retransmitted.
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.
EIA/TIA
The Electronics Industry Association and the Telecommunications Industry
Association. The U.S. bodies responsible for the development of standards
related to telecommunications in general and for fax in particular.
EIA/TIA-465
The US version of T.4 (will probably be accepted as T.4 in the near
future).
EIA/TIA-466
The US version of T.30 (will probably be accepted as T.30 in the near
future).
EIA/TIA-530
The US version of something or other related to fax.
.
EIA/TIA-578
See the definition of Class 1 (above).
EIA/TIA-592
See the definition of Class 2 (above).
EIA/TIA-602
The ANSI/EIA/TIA standard for the "Hayes Command Set" for modems.
FaxBios
An industry consortium (including companies such as Everex, HP, WordPerfect,
etc.) that has published a specification for a FAX API. Versions for
DOS and WINDOWS have been developed and discussions continue on adapting
this API to other operating systems.
With the demise of Everex the association seems to have collapsed leaving
WordPerfect as the only significant company supporting and promoting the
FaxBios standard.
Group I Fax
An old (now obsolete) standard for fax machines in which a page was
transmitted in about six minutes at a resolution of 98 scan lines/inch.
Group I devices frequently worked by attaching the page to be transmitted
to a rotating drum (at 180 rpm) along which a photocell moves. Either
amplitude modulation (the blacker the pixel the louder the tone) or
frequency modulation (the blacker the pixel the higher the tone) can be used.
The gory details may be found in ITU-T Recommendation T.2.
Group II Fax
An old (now almost obsolete) standard for fax machines in which a page was
transmitted in about three minutes at a resolution of 100 scan lines/inch.
Group II uses vestigial sideband amplitude modulation with phase shifts.
A white pixel is represented by a louder tone.
Group III
One of the current standards for fax machines in which a page is transmitted
in about one minute. See the definition of T.30 (below) for more details.
Group IV
A standard for fax transmission using ISDN at 64kbps.
ITU and
ITU-T
The International Telecommunication Union (ITU) is the United Nations
specialized agency dealing with telecommunications.
The purposes of the ITU as defined in the Convention are:
- to maintain and extend international cooperation for the improvement
and rational use of telecommunication of all kinds;
- to promote the development of technical facilities and their most
efficient operation with a view to improving the efficiency of
telecommunication services, increasing their usefulness and making
them, so far as possible, generally available to the public;
- to harmonize the actions of nations in the attainment of those
common ends.
The ITU works to fulfil these basic purposes in three main ways:
1. international conferences and meetings;
2. technical cooperation;
3. publication of information, world exhibitions.
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.
MH or Modified Huffman compression
Also known as Group III one-dimensional compression. See T.4.
MR or Modified READ compression
Also known as Group III two-dimensional compression. See T.4.
One-Dimensional Compression
See T.4
SP-2388
The first draft standard for Class 2 that was implemented by many
companies while waiting for the final standard to be approved
(see also the definition for Class 2 above).
T.1
See Group I Fax. Not to be confused with T-1, a digital telephony
standard that runs at 1.544 Mb/s (at least in North America).
T.4
One of the recommendations (i.e. standard) for Group III fax.
In particular, this recommendation covers the page size, resolution,
transmission time, and coding schemes supported for Group III fax.
(See also the definition of T.30 below.)
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.
T.6
The recommendation that covers the image compression algorithm used for
Group IV fax machines.
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.
T.30
One of the recommendations (i.e. standard) for Group III fax.
In particular, this recommendation covers the protocol used to manage
the session and negotiate the capabilities supported by each fax machine.
The details of the image format are covered by the T.4 recommendation
(see above).
The protocol describes each fax call as proceeding through five phases:
A: Call Set-Up
This phase covers the placing of the call on the PSTN and the
distinctive tones the calling and called stations are to emit.
B: Pre-Message Procedure for Identifying and Selecting Facilities
During this phase the two fax machines:
- agree on whether to use tones or binary codes to exchange
information on capabilities (most current fax machines use
binary codes)
- (optionally) the called machine sends a CSI frame identifying
it to the calling machine.
- the called machine sends a DIS frame telling the calling
machine what capabilities it has (i.e. resolution, page
size, receiving speed, etc.)
- (optionally) the calling machine sends a TSI frame identifying
it to the called machine.
- the calling machine sends a DCS frame telling the called
machine what capabilities are in effect for this document
(based on the calling machine's capabilities and the information
received in the DIS frame).
- the two machines determine the maximum baud rate that the
communications link will reliable sustain (training & phasing)
C: Message Transmission
The fax is sent. The end of the last scan line is marked by a
RTC code (return to control).
D: Post-Message Procedure including End-of-message, Confirmation,
and Multi-Page Procedures
- the calling machine indicates what it wants to do next
(send another page, terminate the call, request operator
intervention, etc.).
- the called machine indicates its response to the page and command
just received (o.k., o.k. but retrain, not o.k., give up, etc.)
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.
E: Call Release
Hang up the phone.
T.434
The standard for Binary File Transfer Format (a method of encoding documents
and sending them by fax without converting them to image format first.
T.611
A standard for high-level fax API.
TSI or
Transmitting Subscriber Information
The "name" of the calling fax machine. An optional frame of information
sent by the calling fax machine during Phase B (see T.30 above). See
CSI (above) for details on the recommended format.
Two-Dimensional Compression
See T.4.
V.17
The ITU-T recommendation for 14,400 bps *synchronous* half-duplex
modems. Used during the image transmission phase of fax
communications. Optional (most fax machines do not support V.17).
V.22bis
The ITU-T recommendation for 2400 bps asynchronous full-duplex modems.
(Not used in fax but frequently supported by modems that handle fax.)
V.27ter
The ITU-T recommendation for 2400 and 4800 bps *synchronous*
half-duplex modems. Used during the image transmission phase of fax
communications.
V.29
The ITU-T recommendation for 7200 and 9600 bps *synchronous*
half-duplex modems. Used during the image transmission phase of fax
communications.
V.32
The ITU-T recommendation for 9600 bps asynchronous full-duplex modems.
(Not used in fax but sometimes supported by modems that also handle fax.)
V.32bis
The ITU-T recommendation for 14,400 bps asynchronous full-duplex
modems. (Not used in fax but sometimes supported by modems that
also handle fax.)
V.42
The ITU-T recommendation for error-checking and correction.
(Not used in fax but sometimes supported by modems that also handle fax.)
V.42bis
The ITU-T recommendation for data compression.
(Not used in fax but sometimes supported by modems that also handle fax.)
X.5
The ITU recommentation for a Fax PAD facility in a public data network.
X.38
The ITU recommentation for a Group 3 fax equipment/DCE interface for
equipment accessing the fax PAD facility in a public data network.
X.39
The ITU recommentation for procedures for the exchange of control information
and user data between a fax PAD facility and a packet mode DTE.
WH Networks
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To FAX or not to fax, that is the
question.
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