FlexFAX Modem Bakeoff
November 2, 1993

ABSTRACT

This paper reports on an experiment to evaluate the performance of a number of commonly used contemporary facsimile modems. Nine modems were used to send a 3-page facsimile to 37 different facsimile machines and modems located in North America and Europe. The results are discussed and a metric is presented for comparing the modems.

[This document is converted from the original paper and is available in a PostScript format that is more condusive to the presentation of figures, tables, and equations.]

This report is Copyright Sam Leffler and Silicon Graphics, Inc.

Permission to copy this document for non-profit use is hereby granted without fee, provided that (i) this document is reproduced unchanged and in its entirety, (ii) the above copyright notices and this permission notice appear in all copies, and (iii) the names of Sam Leffler and Silicon Graphics may not be used in any advertising or publicity relating to the document without the specific, prior written permission of Sam Leffler and Silicon Graphics.

1. Introduction

This document is organized in six sections. In Section 2 the test environment and methodology is discussed. Section 3 provides a primer on the communication protocol used by Group 3 facsimile devices. Section 4 discusses the test results, while Section 5 presents the metric that was derived from this experiment. In Section 6 the conclusions that were drawn from the experiment are presented together with some advise to buyers of facsimile modems.

2. Test Environment and Methodology

Only sending was evaluated for several reasons. First, the Group 3 facsimile protocol is designed so that the sender is mainly responsible for the reliable transfer of facsimile data. Personal experience indicates that a modem that does well at sending facsimile also does well at receiving facsimile, while the reverse is not true. Second, most uses of facsimile modems are significantly more oriented toward sending than receiving facsimile. Consequently modem buyers are more concerned with a modem's ability to send than receive. (It should be noted however that because of just this reason, some modem vendors do not put as much effort into testing the receive capability of their modems as they should.) Finally, the logistics of running receive testing were too difficult to surmount in the time available to do the experiment.

The experiment was run on a Silicon Graphics Indigo machine under version 4.0.5H of the IRIX operating system. [The system was modified in one important way; it included a bug fix to the UART driver to correct a problem with the handling of DCD on devices that require RTS/CTS flow control. Without this bug fix the USR Sportster modem cannot be used with an SGI system.] The 3-page facsimile shown in Appendix B was used for testing. The document includes three different types of data, as shown in Table 1. While many facsimile are either mostly text or simple graphics (e.g. charts of tables), the selected matter represents a more difficult test of the Group 3 encoding algorithms and, consequently, of the performance of the sender and receiver. The pages were encoded and transmitted exclusively using the CCITT Group 3 1D encoding algorithm. FlexFAX also supports the 2D encoding algorithm but it was not used because some fax machines have been observed to falsely advertise support for 2D-encoded data. Thus, rather than complicate the tests it was decided to use only 1D-encoded data. As shown in Table 1, a 2D-encoded test document is 12% smaller than a 1D-encoded version of the same data. This is significant and should be taken into account when evaluating a modem's capabilities since not all facsimile modems support the transmission of 2D-encoded data.

The sending site was located in Berkeley, California USA (+141552687xx) and the phone line was a normal unconditioned 2-wire hookup. [The phone numbers have been altered by replacing the last few digits with characters. The country and city/area codes have been retained.] The line is normally shared with a telephone, but for these tests it was isolated to avoid any possible interactions with the telephone set. All the tests were run at night or on the weekend in order to obtain lower utility rates.

2.1 Facsimile Modems Tested

It is critical to note the firmware revision that each modem was running during the testing. While some modem problems are due to hardware design limitations, many problems are caused by faulty firmware. Vendors routinely claim that problems that exist in one firmware revision are corrected in a subsequent version. This often is true, but when evaluating a modem one must always consider both the hardware platform and operating firmware together.

Class 2 modems differ from Class 1 modems in that more of the facsimile protocol is implemented in firmware in the modem. Class 1 modems provide four basic fax-specific operations: send/receive an HDLC protocol frame, and send/receive T.4-encoded message data. Class 2 modems do not expose these low-level calls, instead presenting higher-level interfaces that shield the host software from many of the timing critical aspects of the facsimile protocol. Class 1 modems place significant demands on the host computing resources while Class 2 modems are less sensitive to the performance of the host computer. Consequently Class 2 modems have been desirable for most UNIX systems. FlexFAX however supports both Class 1 and Class 2 modems and Class 1 modems function very well on some UNIX hosts. Modems that support both the Class 1 and Class 2 interfaces were used only in Class 2.

Table 3 shows the method by which the modems were connected to the host computer. All modems were run using RTS/CTS flow control whenever possible. Some modems were found to not correctly implement RTS/CTS flow control when sending facsimile or simply did not support hardware flow control for facsimile operation. Note that RTS/CTS flow control is not required by the Class 1 or Class 2 specifications. Using software flow control between a host and modem at rates above 9600 baud can be problematic.

The Indigo serial port can operate up to 38.4 Kb/s without an external clock. All modems were run at the maximum possible rate. Many modems based on the Rockwell RC144DP lock the serial port rate to 19.2 Kb/s when sending or receiving facsimile; this is why the Supra and Twincom modems were run at 19.2 Kb/s. The Everex modem does not operate reliably at 38.4 Kb/s and so was run at 19.2 Kb/s.

2.2 Receiving Devices

There were 20 different facsimile machines and 17 modems, with 10 different modem models. The sites with modems were all running the FlexFAX software except for one site that was running Hayes SmartFax software on a 486 machine. The one non-FlexFAX site crashed early on in testing while receiving a facsimile and was unable to recover in time to participate in the majority of the testing. For this reason the tests include results for only 36 receivers: 20 machines and 16 modems.

Table 4 also lists some of the capabilities of the remote systems that can affect the results of the transmission tests. These capabilities are received by the sender in the initial facsimile protocol handshake. Several interesting trends can be observed from this data:

1. Only 2 of the 20 facsimile machines support V.17 communication at 14400 b/s.
2. Only 2 of the 20 facsimile machines support a minimum scanline time of 0 ms.
3. Most facsimile machines support 2D-encoded data (15 of 20), but only 22 of 37 total receivers advertised themselves as capable of receiving 2D-encoded data.
4. Only 10 of 37 receivers support the optional error correction mode (ECM); none of which are modem-based receivers.

Items 1 and 2 directly affect the transmission time of a facsimile in unavoidable ways. Obviously the higher the signaling rate the less time it takes to transfer the page data. Less obvious however is that using a V.17 modem can reduce other overhead in the protocol through the use of ``short training''; this is discussed in Section 3. The minimum scanline time is the minimum amount of time the receiver requires to process a row of pixels. Facsimile machines that immediately print received facsimile typically require a non-zero minimum scanline time so that they have time to mark and move the paper. 1D versus 2D encoding refers to the algorithm by which pixel data is encoded into a digital bit stream. 2D-encoding is more compact than 1D-encoding and is especially desirable for pages comprised of text and simple graphics. It is interesting that so many modems do not indicate a capability for the transfer of 2D-encoded data because doing so requires only trivial support in the modem firmware. [Unless the modem implements copy quality checking.] Item 4 is interesting from the perspective that using ECM can potentially reduce transmission time when poor line conditions are encountered. This issue was not explored in the experiment.

2.3 Test Methodology

All communication was recorded using the standard FlexFAX session logging mechanism. Additional information logged via the system logging facility was also recorded as was the information recorded in the FlexFAX accounting file.

The participants were asked to collect the received facsimile and grade the quality of each page received based on a scale of 0-9, where 0 meant that a page was not received and 9 meant that a page was received completely and without any discernible errors. Some of the received facsimile were not available for this grading due to the usual circumstances associated with a communal facsimile device. For example, one person that was not involved with the experiment apparently decided there was no reason to keep duplicate pages and so discarded all but one good page for each of the pages that were received. Also some of the receivers were uncommunicative for various reasons (e.g. modems being turned off). This information was used only as a ``sanity check'' on the information collected at the sending site.

Note that the intent of the experiment was to simulate normal usage of FlexFAX. As such, manual intervention was minimized. The jobs were just entered into the system and permitted to run to completion.

3. Group 3 Facsimile Primer

Facsimile are transmitted through a fax modem device that accepts digital information, encodes it according to CCITT Recommendation T.4, and then modulates it to form an analog signal that is transmitted on a telephone line. Facsimile machines include, in addition to the modem and associated digital logic, a scanner that converts document pages to a digital form, and a printer that converts digital data to a printed form. Facsimile modems by themselves are not useful; they must either be combined with a computer or integrated into a facsimile machine.

Data is communicated between facsimile modems according to a set of rules specified in CCITT Recommendation T.30. These rules comprise what we call the Group 3 facsimile protocol. The protocol deals with negotiating acceptable characteristics for the transmitted page data (page width, page length, data encoding, output resolution, etc.) and the correct communication of the negotiated page data. In particular, the standard protocol includes an acknowledgement by the receiver that each page has been correctly received and is of acceptable quality.

The facsimile protocol actually operates using two different signaling schemes. A 300 b/s low speed carrier is used to carry handshaking information while a higher speed carrier is used to carry the T.4-encoded page data. The speed of the high speed carrier is determined during the handshaking procedure according to the capabilities of the sender and receiver and according to perceived quality of the telephone link between sender and receiver. All fax devices are required to support a V.27ter modem for sending page data at 4800 and 2400 b/s. Most devices also support a V.29 modem for transmitting page data at 7200 and 9600 b/s. Newer devices also support a V.17 modem which is a half-duplex trellis-coded modem that runs at 14400 b/s with fallback speeds of 12000, 9600, and 7200 b/s. Note that the techniques by which page data are transmitted by a facsimile modem are different from those of a normal data modem.

The basic flow of the Group 3 protocol is illustrated by the single page transfer shown in Figure 1. There are five phases of communication:

Phase A. Call establishment.

Phase B. Pre-message procedure.

Phase C. Message transmission.

Phase D. Post-message procedure.

Phase E. Call release.

Phase A. Call establishment.

Phase B. Pre-message procedure.

Phase C. Message transmission.

There are two aspects to the image data transfer that are noteworthy. First, as part of the capabilities negotiation done in Phase B a minimum scanline time is selected. This value specifies the minimum amount of time that the receiver requires to process a row of image data. Receiving devices that immediately print a facsimile may need time to mark black on a page and advance the paper to the next row on the page. Senders must honor the requested minimum scanline time by zero-padding each scanline that so that it takes at least the requested time. Class 2 modems do this scanline padding automatically in the modem and only the raw scanline data must be transferred from the host. Class 1 modems typically do the zero-padding on the host and transfer the padded scanline data to the modem.

A second issue is the duration of the training signal sent prior to the page data. V.27 and V.29 modems use a longer training signal than V.17 modems. This also can affect the time spent in Phase C during a facsimile transmission.

Phase D. Post-message procedure.

• followed by another page in the same document (MPS),
• followed by more pages in a new document (EOM), or
• the last page in the last document (EOP).

When the receiver receives RTC it listens on the low speed carrier for the post-page message and then responds with a message that acknowledges reception of the page or that indicates the page was received in error. In addition the Post-Page Response (PPR) returned by the receiver can request that the sender redo training if it believes that signal quality has degraded. Note that the sender is free to retrain at the same signaling rate or at a lower rate in response to a request to retrain.

The post-page handshake involves critical timing to insure the sender and receiver remain synchronized. If the sender does not receive a response to its post-page message within approximately 3 seconds it will retransmit the message. The sender will transmit the post-page message at most three times before aborting Phase D. Also, the delay between switching from the high speed message carrier to the low speed carrier is tricky. If this delay is made too short the receiver may miss the start of the post-page message and discard the data. If the delay is made too long the receiver may timeout waiting for a post-page message before the sender has had an opportunity to send its full complement of three messages. The timing characteristics of different facsimile devices varies widely in Phase D. Some machines are notorious for being slow to switch from the high speed message carrier to the low speed message carrier resulting in their only seeing the second or third transmit of the post-page message.

Phase E. Call release.

4. Test Results

Results are summarized in a number of tables that have the following column headings: The Tries column gives the number of times that FlexFAX attempted to send a facsimile to the destination, while the Calls column indicates the number of times the modem reported that the receiver picked up the phone. These numbers are different when calls failed because the line was busy, because the receiver did not answer, or because the sending and receiving modems were unable to establish carrier. ConnTime is the total amount of time spent on the phone processing facsimile jobs--this number should be identical to the connect time charged by the phone company. Note that connect time does not include the time spent on the phone waiting for the phone to be answered. [ The software was configured to wait up to 60 seconds for carrier. Busy signals were recognized in significantly less than this time. Modems that reported ambiguous results for dialing were assumed to reach the receiver but be unsuccessful establishing carrier. ] The Pages column indicates the total number of pages that were successfully transmitted. There were initially 37 receivers but four were dropped from the experiment because of repeated failures. This left 33 receivers, so the total number of pages that a sender transmits successfully should be 3 x 33 = 99. (And each receiver should receive 3 pages from 10 different modems for 3 x 10 = 30 pages.) The final column marked TypRate is the most frequent signaling rate used to transmit page data.

4.1 Receiver Test Results

Some observations can be made regarding the performance of the receiving devices:

1. The facsimile machines functioned significantly better than the standalone facsimile modems. Of course it is difficult to fully assess this as the quality of the modems is questionable (especially when the results of this experiment are taken into account) and their operation is open to confusion--i.e. a facsimile machine is significantly easier to operate correctly than a modem. Nonetheless one should recognize that since a facsimile machine is a self-contained system that can be fully tested for correct operation, it is possible to achieve a higher degree of reliability than a standalone modem unit that functions with a variety of hardware and software systems.
2. International phone calls were significantly more prone to failure. In 58% of the international calls, carrier was successfully established; compared to 71% of the continental calls (83% if we discount +130597073xx and +160278929xx).

Tables 5c and 5d show the average time spent in each phase of the Group 3 facsimile protocol, collated by receiver. Phases C and D are broken down by page: ``C[2]'' is the time spent in Phase C delivering page 2 of the test facsimile. The numbers were calculated from the timestamps recorded in the FlexFAX session log files. Only phases that were completed successfully were counted in the calculations. Phase E was not calculated because it is difficult to deduce the timing for Class 2 modems. Note that the per-phase timings are based on the time at which status messages were received by the host from the modem. If a modem, in particular a Class 2 modem, delays the delivery of a status message then these numbers will be skewed. Per-phase times are averaged over all the tests and given in ``minutes:seconds'' (beware that the calculation of the per-phase times effectively average transfer rates for V.29 and V.17 modems.) Appendix A shows the complete set of per-phase times for each modem and receiver.

4.2 Sender Test Results

There are two entries in Table 6 for the Digicom Scout+ modem. These entries correspond to the modem operating with two different versions of firmware: one that supports V.27, V.29, and V.17; and one that supports only V.27 and V.29. Section 4.3.2 explains the reason for the additional tests.

4.3 Sending Modem In-depth Results

4.3.1 AT&T DataPort 14.4/FAX

4.3.2 Digicom Systems Scout+

The calls to the Pitney-Bowes 9300 at +141696989xx and the Multi-Tech 1432 at +41142227xx failed because no response was received to the PPM sent after the first page. After three successive attempts to send a page FlexFAX will abort the job and return the untransmitted contents to the sender. The call to the Zoom 14.4X at +446127561xx failed because the two modems were unable to succeed in training.

The first call to the Supra modem at +150866368xx, the ZyXEL modem at 170836718xx, and the Sportster at +170889876xx, failed because of training; with the subsequent call going perfectly. Other calls failed either because of failure to train the modems or because of failure to receive a response to the post-page message for the first page. The call to the Olivetti OFX 430 at +4956180442xx took a long time because of retraining and having to make an extra call to retransmit the last page.

In general it appeared that the Scout+ performed poorly in the face of line noise and/or low signal quality. Discussions with informed parties confirmed this observation: firmware revision 3507/3506 has problems with line noise and poor signal quality. For this reason the tests were rerun with a previous version of the firmware that did not include V.17 support but which was thought to perform well otherwise. The results for firmware revision 2A19/2931 are shown in Table 8a. Problems with misrecognized carrier (+FCERROR) were encountered for many of the same receivers. Otherwise the older firmware did a better job of communicating with facsimile machines (as opposed to modems); but performed worse than the newer firmware that includes support for V.17.

4.3.3 Everex Systems EverFax 24/96D

Typically when the modem was able to establish a connection communication was very reliable. Calls to the GUIS ETFax 7 at +49428616xx, the Pitney-Bowes 9300 at +141696989xx, and the Intel SatisFaxion 100 at +181830557xx required an additional call because the receiver failed to respond to a post-page message.

4.3.4 Multi Tech Systems MT1432BA

4.3.5 Supra SupraFAXModem V.32bis

4.3.6 Telebit Systems T3000

The Pitney-Bowes 9300 at +141696989xx, the Multi-Tech 1432 at +41142227xx, and the Canon FAX-L770 at +446127560xx failed with what looks to be an oversight in the modem firmware. The modem sent a ``NO CARRIER'' result string without first sending a Class 2 ``+FHNG:'' result:

The calls to the Brother InstaFax 390 at +171864874xx failed with another problem that is definitely a firmware bug:

4.3.7 Twincom 14.4/DF

A significant test was to the Multi-Tech 1432 at +41142227xx. Two different calls to this receiver left the modem in a state where it held carrier but did not respond to DTR. Manual intervention was required to hangup the line. This is a serious problem that alone makes the modem unacceptable for unmonitored operation.

The Pitney-Bowes 9300 at +141696989xx and the Supra at +150866368xx failed because there was no response to the post-page message. The Telebit modems at +151974615xx and +161723336xx, and the Multi-Tech 1432 at +41142227xx failed because the modem locked up during Phase B/C; for example:

4.3.8 Zero One Networking ZyXEL 1496E

The Panasonic Panafax PD-4200 at +121282556xx required an extra call because the response to the EOP was apparently not received. Unfortunately the modem does not indicate this directly but instead reports:

Three tries were required to transmit the third page to the NEC BitFax at +141596576xx. The first try failed with the above problem, while the second try returned:

The OmniFax G66 at +141623971xx failed with problems similar to the above two jobs. The first two pages went through without a problem, but the third page was unsuccessful with either no hangup code or with hangup code 50.

The Supra modem at +150866368xx required two calls because the first failed with a ``Unspecified Transmit Phase B error'' (hangup code 20). Once again a better hangup code would help to understand what actually took place.

The Ricoh 9200 at +161945506xx failed after three attempts to send the second page. As above, a generic error code (50) was returned to the host.

The results are indicative of a modem that has problems with line noise. This is surprising since the modem is reputed to do well as data modem.

4.3.9 US Robotics Sportster

The Supra modem at +121522136xx took a little extra time because of some handshaking messages had to be retransmitted. The NEC BitFax at +141596576xx failed when three consecutive attempts to send the first page were unsuccessful because no response was received to the post-page message. Communication with the Supra modem at +150866368xx failed because the modems were unable to establish training--specifically the sender never received a CFR/FTT response to training. The Telebit modem at +151974615xx failed when no response was received to the first post-page message; subsequent calls failed when no carrier was established. The Intel modem at +181830557xx required three tries to successfully transfer the first page, after which the remainder of the document went through without a problem. The Multi-Tech modem at +4991318587xx required extra time because the second page had to be retransmitted.

5. A Simple Metric

The reliability of a modem, Rm, is given by Equation 5.1:

    Rm = # pages delivered / total # pages				(5.1)

    Rm = 1

The performance of a modem, Pm, is given Equation 5.2: .EQ I (5.2)

    Pm = Sum {r:Receivers} Sum {p:Phase A-Phase D} T(r,p)		(5.2)

The above equations measure how well a modem does under error-free conditions. When a modem does not transmit a facsimile on the first call it should be penalized. Also, when a modem sends multiple copies of a page it should be penalized. Thus we define an error metric for a modem as Em:

    Em = Wep (# extra pages transmitted / maximum # extra pages) x	(5.3)
	 Wec (# extra calls / (.67 x maximum # extra calls))

Given the above definitions, we define a quality rating Qm by:

    Qm = Wr x R m + Wp x (Tbase / Pm) + Em				(5.4)

For a modem that transmits all the test data without extraneous phone calls or page retransmissions the metric reduces to:

    Qm = Wr x 1 + Wp x (Tbase / Pm) - 0
       = Wr     + Wp x (Tbase / Pm)

So long as a modem is able to transmit all the test pages its rating is controlled by its performance metric Pm and any error costs. A failed call incurs cost through an extra phone call and possibly an extra page transmission. This cost appears in the error term Em. Modems are usually double-penalized for pages that are never successfully transmitted because the failed page appears both in Rm and indirectly in Em as extra calls and (probably) extraneous transmitted pages.

Table 16 shows the parameters for Qm derived from the test data. The performance metric Pm was calculated as described above. Because there were relatively few modems that supported V.29 but not V.17, the results for the Everex 24/96D were not used in calculating ``fill in'' entries for failed phases (since the Everex times were significantly skewed from the norm).

To complete the calculations for the Qm we need to select a baseline time and values for the different weights. Choosing weights requires selecting ranges for the different factors that reflect desired characteristics of a ``good modem''. Our criteria weights reliability significantly higher than performance, with extra calls more important than extraneous pages. In addition we want quality metrics for a good modem to be around 100. A possible set of weights that meets our criteria are: Wr of 80, Tbase of 2:23:11, Wp of 16, Wep of 8, and Wec of 20. [The value of Tbase is based on an analysis of the T.4-encoded data, the average phase times, and the receiver capabilities. It represents the best expected time that a sender can do.] Using these weights we define a FaxStone to be:

    FaxStone[m] = Qm[Wr=80, Wp=16, Tbase=2:23:11, Wep=8, Wec=20]	(5.5)
	        = 80 x Rm + 16 (2:23:11 / Pm)
			  + [-8 (extra pages/132) - 20 (extra calls/132)]

This set of parameters has many good intuitive properties. The reliability term is in the range [0,80] while the performance term is in the approximate range [0,16]. This weights reliability over performance by a factor of five-as desired. The error metric is in the range [-28,0] with the majority of the error metrics in the range [-5,0]. The overall range for the quality rating is large enough to distinguish between modems but not so wide as to make comparisons difficult. Differences in modem reliability are distinguishable and it is possible to identify raw performance by looking solely at the Pm term. Clearly other weights may be selected in order to emphasize other performance characteristics of the modems.

Table 17 shows the FaxStones calculated from our test data. The columns marked R, P, and E are the weighted reliability, performance, and error terms, respectively.

6. Conclusions

The Everex 24/96D proved to be very reliable also, though it was unable to connect to several of the more difficult receivers. Its performance lagged significantly behind all the other modems. This is due to it being based on old hardware technology.

The remaining modems all rated fairly close together: Digicom Scout+, ZyXEL 1496E, USR Sportster, SupraFAXModem v.32bis, Telebit T3000, and Twincom 14.4/DF. The Scout+ had a significant problem establishing carrier with a variety of receivers. The ZyXEL 1496E was particularly disappointing because of the relatively low cost of the modem and its excellent reputation as a data modem. The test results however indicate that the firmware has problems and the modem may be susceptible to line noise during facsimile operation. The USR Sportster modem had the highest performance rating of any modem but rated very unreliable in testing. Also, as reported earlier, the Sportster had a serious firmware problem that makes unattended use uncertain. The Telebit modem had at least one firmware problem that when corrected will significantly raise the modem's FaxStone rating. This modem also appears to be one of the quickest modems and also appears to handle poor line conditions well. The Twincom 14.4/DF rated last in the tests; it also exhibited serious firmware problems.

Table 17 includes price estimates in US dollars for the modems that were tested. Given these prices, the best buy is the AT&T DataPort 14.4/FAX. The one complaint that can be made about the modem is that it does not support V.17. Since Telebit offers fax upgrades to existing T3000 and WorldBlazer modems for less than US$200 this may also be an excellent route to take for people that already own a Telebit modem, once the Telebit firmware problems are resolved. The Multi-Tech modem looks to be an excellent modem for people that require a Class 2 modem. The only complaint one can make about the MT1432BA is that it does not support 2D-encoded data (though this should be trivial to add to the firmware).

Future Work

Another worthwhile area of work is to get a better understanding of the value of the optional T.30 Error Correction Mode (ECM). ECM could only be used to communicate with the facsimile machines in this experiment, but it is potentially a worthwhile mechanism for improving the reliability of facsimile communication when poor line conditions are encountered.

Finally, the set of receivers used in this experiment were all located in highly industrialized areas of the world. It is important to recognize that other parts of the world may not have such high quality phone systems. In such areas devices such as echo-suppressors may have more of an effect on communication. The test results reported here are likely not to reflect how well the modems work under such conditions.

Acknowledgements

The following people volunteered their time and equipment; their help is greatly appreciated.

Adrian Collins	    Bill Morrow		Brian Katzung	    C. Harald Koch
Carsten Koch	    Charlie Slater	Dan Busarow	    David Vrona
Dirk Husemann	    Heiner Meuser	Ian Darwin	    James Stansell
Jim Patterson	    Jochen Ruhland	John R. Koehring    John Schwegler
John Sellens	    John T. Kohl	Joseph E. Sacco	    L. Todd Masco
Larry Williamson    Lyndon Nerenberg	Martin Lichtin	    Paul Lupa
Peter Shipley	    Rickard Schoultz	Robert Weiner	    Vance Shipley

Certain vendors were kind enough to provide me with modems for use in testing; their assistance was critical: Digicom Systems (Wolfgang Henke), Multi-Tech (Pete Hanlon), Twincom (John O'Connell). and US Robotics (Pete Frankiewicz). Thanks also to Nick Doshi at Zero One Networking and Eric Ziegast of UUNET for providing firmware updates for existing modems.

Availability of Materials

References

[McConnell]

Kenneth R. McConnell, Dennis Bodson, and Richard Schaphorst, FAX: Digital Facsimile Technology and Applications; Artech House, 1992.

[CCITT T.30]

Procedures for Document Facsimile Transmission in the General Switched Telephone Network. CCITT Recommendation T.30

[CCITT T.4]

Standardization of Group 3 Facsimile Apparatus for Document Transmission.

[Class1]

EIA/TIA-578. Asynchronous Facsimile DCE Control Standard (Service Class 1). November 1990.

[SP-2388-A]

Asynchronous Facsimile DCE Control Standard (Service Class 2). October 30, 1991.

Appendix A - Detailed Group 3 Protocol Transfer Times

Appendix B - Test Facsimile