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March 3, 1997

Torture test sheds light on fax/modems and some of the troubles they have

Fax/modems have made life easier for many Windows users who must frequently zap documents to far-flung offices around the world. But the fax standard isn't perfect. Problems have revealed themselves to Windows users who inexplicably can't send certain faxes to certain recipients.

The makers of Symantec's WinFax Pro (formerly Delrina WinFax Pro, before Delrina became Symantec's Toronto operation) have isolated many of the anomalies in fax peripherals and tried to cure them.

In WinFax Pro 8.0, which is due out soon, Symantec has developed what might be called "adaptive" faxing.

WinFax Pro 8.0 starts out a fax transmission by assuming that the receiving fax is telling the truth when it identifies the features it supports. (This is during the initial "handshaking" period when fax machines establish mutual ground rules with each other.)

But many fax machines are "lying." They claim to support advanced speeds and protocols but then fail to do so given real-world conditions.

Symantec's answer is to "trust but verify." When a feature has been agreed to, but the machine on the other end shows signs of losing it, WinFax Pro 8.0 changes the way it sends data in an effort to at least get the fax to go through.

Even this kind of adaptation, which is more than simply "falling back" to a slower speed, doesn't solve all of the mysterious fax gremlins.

Symantec has also found that the international standards for Class 1 and Class 2 fax/modems actually allow some bit patterns to be transmitted that can hang up a telephone connection when sending normal documents containing graphics.

Symantec says this occurs given any of the following conditions.

  • The sending PC's serial port is configured to allow 19.2Kbps.

  • The sending application is using two-dimensional compression, which is a way for fax machines to speed up transmission times.

  • The sending and receiving ends are communicating at 14.4Kbps, a faster speed than the 9,600bps rate common to older fax machines.

    When sending a page that contains large areas of the color gray, for example, the above specifications produce bit patterns that cause a "data underrun" condition (not enough data) in the sending modem.

    This can cause the telephone connection to hang up, as though the fax has ended.

    Setting the sending PC's port speed to 38.4Kbps can avoid the problem, Symantec says, but some modems won't fax at port speeds other than 19.2Kbps.

    Even if a sending modem does work in this configuration, the receiving modem is then likely to experience a "data overrun" (too much data) and abort the transfer anyway.

    Symantec gave me two "fax torture test" files. They contained presentation output with gray backgrounds. The files crashed in the middle on some receiving hardware and produced dropouts (white bands across the document) on other machines. Testing with the files Symantec had provided, the modem would just hang up -- giving the appearance that the fax was going through -- but the receiving machine actually got nothing.

    If you have WinFax Pro 7.0 or 7.5 (both of which implement 2-D compression), try these two files yourself and see if they reproduce problems you've been having sending faxes. Download LIV0303.ZIP, unzip it, and send either file as a fax attachment. WinFax Pro 8.0 is programmed to deal with the Class 1/Class 2 anomalies and sends the documents reliably.

    In a statement, Stacey Breyfogel, Microsoft program manager, said the Redmond, Wash., company "is aware that this behavior is a possibility in the Windows 95 fax application."

    "However, our customers did not raise it as an issue during the beta program and have not raised it presently through tech support," Breyfogel said.

    Stay tuned.

    Brian Livingston is the co-author of Windows 95 Secrets Gold and four other Windows books (IDG Books). Send tips to or fax: (206) 282-1248.

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    Copyright © 1997 by InfoWorld Publishing Company


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