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

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.


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