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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)

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.)

 

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.]

 

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.

 

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.

 

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.

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).

 

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 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.

 

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. It is expected that the functionality which were to be covered by the Class 3 project will be rolled into the MFPI work (Multi-Function Peripheral Interface).

 

Class 4

Class 1 with intelligent buffering to reduce the need for the host computer to respond instantly to the fax modem. Although there is a draft of this standard, the editor has resigned and the project to complete this standard is on hold until a new project editor is found.

 

Class 8

Not a fax standard at all but an extension to the Hayes command set to support voice. The interim standard covering voice extensions to the AT command set is designated IS-101.

 

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).

The US version of T.30 (will probably be accepted as T.30 in the near future).

The US version of something or other related to fax. Need more information.

See the definition of Class 1 (above).

See the definition of Class 2 (above).

The ANSI/EIA/TIA standard for the "Hayes Command Set" for modems.

The ANSI/EIA/TIA standard for Binary File Transfer (see BFT above).

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.

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.

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.

A standard for fax transmission using ISDN at 64kbps.

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).

The Interim Standard developed and published by the TIA 29.1 committee for subaddressing. (See Subaddressing below.)

IS-650

The Interim Standard developed and published by the TIA 29.1 committee for Multi-Function Peripheral Interfaces (MFPI). This provides a method for communicating with devices that support multiple functions such as fax, printing, and scanning. Such devices are expected to be connected to the host computer using either a single serial link or a bi-directional parallel port.

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.

Also known as Group III two-dimensional compression. See T.4.

One-Dimensional Compression

See T.4

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).

Subaddressing

Subaddressing is a new feature added to the T.30 protocol to permit the sending fax to specify where the fax is to be delivered by the receiver. The subaddress consists of 20 characters limited to the digits 0-9, space, *, and #.

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.

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

 

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