Re: [hylafax-users] Unreliable fax services via VoIP connection

[Lee Howard <faxguy@xxxxxxxxxxxxxxxx>]

Lucas Barbuto wrote:

> Our provider suggests that we should provision analogue PSTN lines for 
> faxing because fax via VoIP is never safe and rarely works well in 
> their experience, but others in the business aren't satisfied with 
> this answer.
>
> Does anyone on the list have experience in this area? Should I 
> definitely be looking at the VoIP link as the most likely culprit? Or 
> somewhere else? 


Think about this...

Modems take a stream of data and turn it into a stream of modulated 
audio.  The other endpoint is supposed to take that audio and demodulate 
it in order to reproduce the data.

Now add VoIP to that equation, in-between the endpoints.  Remember that 
VoIP operates on UDP/IP networks... meaning there is no paricular 
guarantee that the packets/frames will be received in the order in which 
they were sent and, because it is UDP, packets/frames may even be 
dropped (lost) entirely.

Okay, now pretend you're the modem on the receiving-end of a stream of 
modulated audio that has been passed over a lossy VoIP transport... and 
imagine that you run into portions of audio that are missing or that 
contain audio that was synthesized by your PBX or ATA.  How are you 
going to properly demodulate that audio when it is, in a literal sense, 
corrupted?  The answer is that you can't.

Some modems will work better than others in the VoIP environment, 
although "better" doesn't get you very far in this case... the best 
modem cannot compensate for corrupted audio.  Some PBX/ATA will provide 
for better VoIP-faxing than others, but the problem remains that it is 
the transport that is the fault, not the particular equipment.

*IF* (and that's a big if) you and your VoIP provider are somehow able 
to mitigate (er, eliminate) the lossy nature of the VoIP audio channels 
then you can reliably use VoIP for fax.  Most VoIP providers are not 
going to be able to do this, and since your VoIP provider has 
recommended that you move your faxing to analog lines, I would suggest 
that you follow their recommendation.

Lee.

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