Fax Over VOIP?

Can I Send a Fax With VOIP?

Can you send a fax over a phone line that uses VOIP (Voice-over-IP)? The question is important to people who have VOIP phone service through their ISP, and for international faxers who pay dearly for traditional phone service to other countries. Many have switched to VoIP for voice calls, and would like to use VoIP for faxing too. (See my companion article What is Internet Telephony? for more info on VOIP.)

There are some hurdles to overcome when faxing over a VOIP line, but it can be done. Faxing is like voice phoning in that both work best with an old-fashioned analog signal. The smooth, continuous variance (modulation) of an analog electrical signal yields the highest fidelity sound because sound is itself an analog signal. VoIP uses a choppy, on/off digital signal, so it inevitably loses some of the information that is input by audible fax tones or human voices. How much information is lost partially determines whether you can understand the person who is speaking on a VoIP call, and whether your fax will go through.

Factors That Influence Success or Failure of VOIP Faxing

The best tip I can offer is to reduce your fax transmission speed to 9600 bps to maximize VoIP fax success rates. The faster one tries to send a fax, the more likely it is to fail on a VoIP connection. That's because more fax data is jammed into each transmission second while the digital on/off signaling rate remains constant. More data gets lost as faxing speed increases, until data loss exceeds the intelligible threshold and the receiving fax device aborts the transmission, effectively saying, "I don't get it, try again."

Also, turn off Error Correction mode on both fax sending and receiving machines, if it is on. VoIP has a fairy high signal noise level due to jittery instabilities in the connection. This causes many false positives for "errors" when Error Correction is enabled, which can abort a transmission unnecessarily.

Use a VoIP service provider that supports the T.38 protocol; just ask or check the service's detailed specifications online. T.38 was developed specifically to improve the success rate and quality of faxes transmitted over VoIP. Note that it just improves, it doesn't perfect.

The stability of your Internet service is critical. If your ISP has frequent, brief interruptions of service you may not notice them too much when emailing or Web browsing. But a fraction of a second of downtime can kill a fax transmission over VoIP. Monitor your ISP's reliability using Task Manager's Network Connection Monitor or another monitoring tool. If it's jittery and faxes keep failing, you may need to switch to a more reliable ISP. MyVoIPSpeed is a free online testing service designed specifically for VoIP service monitoring.

Don't even bother trying to fax over VOIP on a wireless connection of any kind; WiFi, cellular, satellite, etc. Wireless connnections have so much latency (lag) in them that virtually every fax machine will assume the sending device hung up on it and abort reception.

Transmitting fax over VoIP can be a trial-and-error matter. It will never go as smoothly and reliably as it does over landlines. If you send or receive faxes internationally and expensively quite often, then fax over VoIP may be worth the effort. But for occasional or cheap domestic faxing, the money you save may not be worth the aggravation you receive. Check out my Free Internet Faxing article to learn how to send a free fax online, without a fax machine.

Got comments or questions about faxing over VOIP? Post your thoughts below...