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Fax (comp.dcom.fax) Frequently Asked Questions (FAQ) [Part 1/2]

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comp.dcom.fax FAQ (Frequently Asked Questions) -- Part 1 of 2
=============================================================

Introduction
   
   The comp.dcom.fax FAQ is brought to you by Faximum Software Inc.
   
   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. Note that the World Wide Web (HTML) version
   contains additional information that is not part of the monthly
   posting. Also the WWW version is "master" copy and is updated
   regularly. The posted version may lag by several months. The WWW
   version may be reached at http://www.faximum.com/faqs/fax
   
   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 "[Need
   more information]". 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.

   My apologies for the somewhat strange format of this FAQ.  As mentioned
   elsewhere, this FAQ is now maintained in HTML (i.e. WWW) format and the
   posted version is generated automatically using lynx and a shell script.

   Obviously some more work is required in order to create a more visually
   pleasing flat-text version.
   
   
How to Get a Current Copy of 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., anonymous ftp to 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.
   
   Most FAQs (including the posted flat-text version of this FAQ) are
   available through Thomas Fine's WWW FAQ archive:
   http://www.cis.ohio-state.edu:80/hypertext/faq/usenet/FAQ-List.ht
   ml.
   
   This FAQ is also available on the WWW at
   http://www.faximum.com/faqs/fax . The WWW version is always the
   most current version. The text version can lag by up to a month from
   the html version.
   
   The current text version is available by FTP:
   ftp://ftp.faximum.com/pub/faqs/fax/fax-faq . You can also have
   the current text version of this FAQ mailed to you. Send any message
   (content not important) to sendfaxfaq@faximum.com. regards
   g.
   George.Pajari@Faximum.COM * http://www.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.
   
==============================================================================

TABLE OF CONTENTS
   
  A. INTRODUCTION
  
  G. GLOSSARY AND BACKGROUND INFORMATION
   
  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.
   Q.10 How can I send a fax over the Internet?
   Q.11 What legal restrictions are there on the use of facsimile
         devices?
   
   
  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
   I.14 Related FAQs
   I.15 Fax-Related URLs
   
   
  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
   P.9 List of Libraries and Related Information for Writing Fax
   Servers
   P.10 List of Vendors of Fax Protocol Test Equipment
   P.11 List of Vendors of UNIX-Based Fax-On-Demand Software/Systems
   P.12 List of Vendors of Fax-On-Demand Software/Systems
   P.13 List of Vendors of Products Delivered by Fax
   
   
==============================================================================

G. GLOSSARY and BACKGROUND INFORMATION

   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)
          
   APPLI/COM
          The name for the ITU-T API for computer-based facsimile. See
          T.611 below for more information.
          
   Bell 103
          A standard for 300 bps full duplex dial-up modems. Popular in
          the U.S. and Canada. In Europe the preferred standard is
          V.21. (Not used in fax but frequently supported by modems
          that handle fax.)
          
   BFT or
          
   Binary File Transfer
          A method of transferring files using fax modems (as an
          extension to the fax protocol). The ITU-T standard for BFT is
          T.434. The US version is TIA/EIA-614.
          
   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. The full text of the
          specification may be obtained from
          ftp://ftp.faximum.com/pub/documents/cas.txt.
          
   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. (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.
          
          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 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 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 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.
          
   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 been 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).
          
   DID or Direct Inward Dialling
          A special type of phone line (trunk) provided by the telco
          which associates multiple phone numbers with a single telephone
          line and which send a signal down the line when a call arrives
          which indicates which number was used to place this call.
          
          
          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.
          
   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. Need more
          information.
          
   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 for modems.
          
          
          
   EIA/TIA-614
          The ANSI/EIA/TIA standard for 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.
          
   IS-101
          The Interim Standard developed and published by the TIA 29.2
          committee for voice modems (a set of extensions to the AT modem
          command set to support voice recording and playback with
          modems).
          
   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
          
   PostScript Fax
          PostScript Fax has to be considered in two pieces:
          
          
          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.
          
   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.2
          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.3
          See Group II Fax.
          
   T.4
          One of the ITU-T 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.
          
          See Standards Related to Facsimile Communication for
          information on how to obtain a copy of the T.4 standard.
          
   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.
          
          See Standards Related to Facsimile Communication for
          information on how to obtain a copy of the T.6 standard.
          
   ITU-T 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:
                
               o agree on whether to use tones or binary codes to
                 exchange information on capabilities (most current fax
                 machines use binary codes)
               o (optionally) the called machine sends a CSI frame
                 identifying it to the calling machine.
               o the called machine sends a DIS frame telling the calling
                 machine what capabilities it has (i.e. resolution, page
                 size, receiving speed, etc.)
               o (optionally) the calling machine sends a TSI frame
                 identifying it to the called machine.
               o 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).
               o 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
                
               o the calling machine indicates what it wants to do next
                 (send another page, terminate the call, request operator
                 intervention, etc.).
               o 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.
                
   
          
          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.
          
   T.411 - T.418
          Open document architecture (ODA) and interchange format
          standards.
          
          See Standards Related to Facsimile Communication for
          information on how to obtain a copy of the T.41x standards.
          
   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.
          
          See Standards Related to Facsimile Communication for
          information on how to obtain a copy of the T.434 standard.
          
   T.611
          Programmable communication interface (PCI) APPLI/COM for
          facsimile group 3, facsimile group 4, teletex and telex
          services. (i.e. an API for fax services).
          
          See Standards Related to Facsimile Communication for
          information on how to obtain a copy of the T.611 standard.
          
   TIFF (Tagged Image File Format)
          
          The TIFF specification was developed by Aldus (now part of
          Adobe) and Microsoft as a general file format for storing
          raster images. A PostScript version of the specification may be
          obtained from ftp://ftp.faximum.com/pub/documents/TIFF6.ps 
          
          The relevance of TIFF to fax is explained in the following
          entry.
          
   TIFF/F (Tagged Image File Format, Class F)
          
          The TIFF specification is an extremely general and extensible
          one which makes it difficult to write programs which can
          dependable handle all possible TIFF files. To simplify the
          problem somewhat the authors of the TIFF specification have
          developed the concept of TIFF classes.
          
          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 
          
   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.21
          The ITU-T standard for 300 bps full duplex dial-up modems.
          Popular in Europe. In U.S. and Canada the preferred standard is
          Bell 103. (Not used in fax but frequently supported by
          modems that handle fax.)
          
   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-T recommentation for a Fax PAD facility in a public
          data network.
          
   X.38
          The ITU-T 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-T recommentation for procedures for the exchange of
          control information and user data between a fax PAD facility
          and a packet mode DTE.
   
==============================================================================

                             FAX FAQ LEGAL NOTICE
                                       
   COPYRIGHT 1994, 1995 George Pajari ALL RIGHTS RESERVED.
   
   The FAQ has been written in part, and compiled by, George Pajari.
   While reasonable efforts have been made to verify the information in
   this document, no liability can be accepted by George Pajari, Faximum
   Software, or the many contributors for the correctness of the
   information herein. Readers are cautioned to take such measures as
   they deem necessary to verify the information herein before taking any
   action based on this information.
   
   Limited permission is granted to copy or distribute this faq for
   non-commercial purposes as long as: (a) the FAQ is reproduced in its
   entirety (including this NOTICE) without any changes, and (b) the
   distribution is done for non-commercial purposes.
   
   Electronic distribution of this FAQ (in its entirely) as part of
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   permitted.
   
   Including this FAQ in a printed book or CD-ROM is specifically not
   permitted without prior permission from the author and copyright
   holder. Permission is almost always granted. It is just considered
   polite to ask.
   

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