Fax Over VOIP?

Category: Fax

I'm thinking of switching to VOIP phone service with Vonage or my cable provider. But I'm not sure if I can send a fax over a VOIP connection. Does faxing with VOIP work well?

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.
Faxing Over VOIP

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...

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Most recent comments on "Fax Over VOIP?"

Posted by:

Norby Fleisig
30 Sep 2009

I have used Vonage (with a separate fax number, though that is not necessary) for three years now, with no problems after the initial setup tweaks. Most fax problems I've had (about 10 total bounced faxes) were traceable to my multi-function printer - not the really best idea if you do a lot of faxing anyway. If you have Vonage service, an additional dedicated fax is only about $10/month. The short answer is "Yes, it works fine."

Posted by:

01 Oct 2009

We have Comcast ISP in Houston and Vonage VOIP. FAXing has not been a problem. We have Vonage set at high bandwidth. When it is set at a lower (medium) bandwidth, FAXing was inconsistent.

Posted by:

Kenneth Bush
01 Oct 2009

I've used fax over VOIP for several years with a few different ISP's. My current one is a DSL connection with an advertised speed of 512K; it's the fastest available from any provider. I reduce my transmission speed to 9600 bps (overseas mode) and it's also recommended to turn off error correction. Occasionally I'll get an error message about line quality, but waiting a few hours can make a difference. It also helps if one has no other internet activity during the fax transmission.

Posted by:

01 Oct 2009

I've had so many problems here in Germany that I gave away my fax machine and started using an on-line faxing service. Actually 2 one for Germany and one for the US. I only send a couple of faxes a month so I use a pay-as-you-go service. So if all else fails you can try one of those too. Dr Bob has a few recommendations on this page you can check out.

Posted by:

01 Oct 2009

I have used Vonage with either Comcast or Verizon DSL for several years and have only had a few instances in which the connection wasn't good. I recommend faxing with VOIP.

Posted by:

Stu Berg
05 Oct 2009

I've used the free http://www.box.net on-line file sharing service for years primarily because it also offers free outbound faxing to anywhere in the WORLD for free. I just upload my document file to my account there and Fax it. Did I also mention that it's FREE! :-)

Posted by:

13 Oct 2009

I have noticed a trend in companies switching over to FoIP services due to the fact that it eliminates hardware dependency and simplifies disaster recovery (through virtualization), reduces long distance costs and need to maintain fax ports on a PBX system, among others. But as you mention, there are cons as well (e.g. possible loss of information). It depends on the reliability and stability of the setup.

There is actually a free webinar on this topic (Thursday, November 12, 2009): Reduce Costs and Improve Efficiencies with Virtual Fax over IP.


Posted by:

08 Oct 2010

I have tested this and have found that ECM mode should be left on. ECM is an added layer of error detection that only makes faxing more robust over VoIP. What happens without ECM is that the sending machine will indicate successful, but pages might be blank, missing, or cut off in the middle at the receiving end and you won't know about it unless you verbally confirm over the phone. With ECM, a success means all pages were received 100% error free. If there are errors received, then ECM will retransmit those errored sections of the page called "octets" until all octets are received error free. So it will just take longer to fax over a less than perfect internet connection.

As for 9600 vs 14400 - some machines don't let you control the baud rate independently of ECM, so I would rather not do away with ECM mode, so I would just leave it at 14400. Secondly, 9600 uses and older modulation method that is less robust over noisy lines. So you might gain some reliability, but then you loose some again because of the older modulation type. 14400 uses TCM modulation. TCM is used right up until V.34 33600 modems. Then V.90 and V.92 use PCM to squeeze more than 33600 out of a phone line. Givin that even 9600 vs 14400 isn't a big jump in baud rate anyway, I'd not mess with the settings of the fax machine and let everything go at the standard 14400 + ECM mode.

Also I want to point out that internet connections experiance "jitter", where packets are received at slightly different times. Normally this doesn't bother VoIP calls a whole lot because ATA's have a jitter buffer to compensate. But when your internet connection gets bad, you'll start to hear small audio drop outs and glitches when the jitter buffer can no longer compensate. Those drop outs is what affects fax and modem signals over VoIP. With VoIP as long as you're not using a compressed codec, VoIP uses the same exact codec as the PSTN in which your old POTS line was connected. Even through you have an "analog" line, once it reached the central office, everything's converted to digital from there. The codec is G.711 or uLAW. The only major difference of the PSTN and VoIP is the PSTN doesn't have network jitter as the internet does. So over VoIP, when the jitter buffer can't compensate, the audio drop outs cause fax octets to be received in error. However, the good audio in between the drop outs will be more than capable of handling 14400 baud fax. So those errored octets will be retransmitted again if need to be, making ECM mode quite favorable over VoIP. Also, VoIP calls for which you dial 10 digit phone numbers are converted to PSTN at the provider's proxy. The audio loss of the internet jitter is still there though.

Posted by:

01 Jun 2012

I have been arguing with vondage; the $12 extra to use a fax at least twice a month is a waste of my time and effort and money. I have no trouble sending faxes but to be told that I must first contact the other end to tell them to ring me so that I can disconnect my telephone & plug in my fax printer is the most ludicrous comment I've ever heard from any firm. I solved the problem a different way, I live in an enclosed village and when I send the fax the office picks up my answer. Not as good as picking it up on my own machine but an million percent better than anything that Vondage has to offer.

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