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Why T.38 is not the solution


As VoIP gets more and more popular, it’s users are struggling how to deal with their existing needs for fax services. Some may just connect their fax machine to an analog telephone adapter (aka ATA) and send their fax transmissions via their ITSP. While some may experience random success with this setup, most will experience an unacceptable high failure rate. The T.38 protocol was thought to be THE solution for fax over VoIP as T.38 uses data packets instead of analog modulation but it has its share of issues as well.

So what is the problem?

The actual problem lies with the fax transmission itself, fax has been designed as a real-time process. The sending party continuously sends information, which the receiving party is re-assembling into a picture, if there is the slightest interruption (even temporary) of this information stream, the transmission will fail. Such interruptions may be due to packet loss or jitter, while a few of these remain undetected during a voice call, any data lost will affect the fax transmission.

So what is the solution?

Well, it doesn’t make much sense to print a document, then have the fax machine digitise the information and convert it into analog modulation, the ATA then converts the analog signal back into data packets and forwards them to the ITSP and at the end of the line the reverse is done.

Did you notice the process??

  • Digital source is printed producing an analog document.
  • The analog document is scanned into a digital image.
  • The digital image is converted into an analog modulation.
  • The analog modulation is converted into digital T.38 packets.

On the receiving end, the process may be similar:

  • The ITSP delivers T.38 to the ATA.
  • The ATA converts T.38 into analog modulation.
  • The fax machine assembles the analog modulation into a digital image.
  • The fax machine prints the digital image.

So before the documents leaves your house the document was converted to analog twice and back into digital twice. In the process shown above there are 7 digital-to-analog and analog-to-digital conversions. I don’t even consider the amount of information lost during that process.

Why not send the document over a purely digital path, e.g. email? Yes, sometimes you have to sign a document, so there are digital signatures or you may print, sign, scan and email the document (dramatically reducing the number of A/D and D/A conversions).


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