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Faxing over VoIP - A clash of technologies?

Posted by Gregg Haughton on Oct 6, 2011 12:37:00 PM

One of the most common problems we deal with when we install a VoIP phone system, or when a client changes to VoIP network services, is problems sending faxes over the new system / services. We often wrestle with communication errors, incomplete transmissions, or are unable to fax.

The first fax machine was invented in 1843 by Scottish mechanic and inventor Alexander Bain and was closely related to the telegraph. Although the technology has certainly evolved, the way a fax is sent has basically remained the same. Is it any wonder there are problems when we try to use 19th century technology on 21st century networks?

A modern fax machine sends images by optically scanning a document and converting the image into a binary code; a combination of 1's and 0's which represent white and black (and shades of gray). The binary code is then translated into sound using a fax modem. That's the squeal you hear when the fax "talks" to another machine. A high frequency squeal may signal 1 and a lower squeal 0. A fax modem uses 16 different frequency bands within the 3000 hertz available on a standard analog phone line to be able to transmit at a top speed of 14,400 baud. If there is noise on the line, the machine will slow down to a lower baud rate. There is also error correction to help overcome noise on the line. It all works very well, and folks have depended on fax machines since the early 80's for inexpensive, reliable, nearly instant document delivery.

What happen to cause the problems we now see so frequently? Well, blame it on the internet or at least internet protocol (IP). Engineers looking for ways to leverage existing broadband data networks developed VoIP (voice over internet protocol) to allow voice calls to be made over data networks. To be most efficient, they developed compression technologies to allow many simultaneous voice calls to go over broadband networks. Voice traffic is turned into data packets using a codec (also called an IP gateway), and routed with all the other data traffic traffic across broadband networks. Therein lies the problem. Data from the same source may take many different routes to its final destination, where it is then reassembled, uncompressed, and decoded back to voice traffic. To the human ear, it all sounds fine. But to a fax modem, it sounds like gibberish.

So what's a fax user to do? There are several approaches:

1) Keepyour fax machine on standard analog lines. These are the lines that the machines were designed to run over. Be careful that the analog line is not just an analog channel off IP services. Many phone companies are pushing to deliver all their services over IP because it's cheaper for them. In many cases, analog lines are only available from the local exchange carrier like ATT or Verizon. Put your fax machine on an analog line and you will have few problems.

2) Don't use compression -the codecs on the edge routers can be programmed to run specific telephone numbers uncompressed. Have your carrier run the fax numbers at G.711 rather than G.729. You will have to tell them what your fax number(s) are and that you want them configured for G.711. This will work for most standard fax machines although we have seen situations where the programming "slips off".

3) Use E-fax and Scanners in place of fax machines. You can have your fax number forwarded to an e-fax service where the fax is changed to an e-mail with a PDF attachment. For outbound documents, scan them, and send them as an e-mail with a PDF attachment. This avoids the problem by eliminating the fax modem.

Faxing will not likely go away any time soon. It's cheap, easy, secure and usually reliable. If you are having problems, or are changing services or systems and want to make sure you can still fax reliably, follow these guidelines.

If you know someone who might find this useful, please refer this blog to them.

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Topics: Faxing with a VoIP Phone System, Faxing and VoIP

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