Re: [hylafax-users] Failure to receive silence (synchronization failure)

[Lee Howard <faxguy@xxxxxxxxxxxxxxxx>]

Alex R [MaxoTel] wrote:
> I read that document the last time someone told me about it.
>
> But that doesn't help me in understanding what is causing this specific
> problem...

Well, you need to ask more specific questions then in order to get a 
more specific answer.

Very specifically the problem with faxing over VoIP is that VoIP uses 
UDP as a medium (as compared to TCP) and thus it's lossy.  The audio is 
modulated data.  If you remove chunks of audio then you remove chunks of 
data.  Thus, the "listener" doesn't consistently get all of the data 
being communicated, and furthermore the "choppiness" of the data causes 
the two endpoints to routinely become out-of-sync.

The degree of problem varies directly with the degree of lossy-ness of 
the UDP medium at the time of the call.  Some devices may tolerate some 
things more than others... and some fax applications may tolerate things 
better than others (i.e. HylaFAX+ is about as tolerant as I can make it 
of VoIP trouble), but in the end unless you have some way to control and 
mitigate the problems inherent with the UDP medium, then you'll likely 
have inconsistent and unreliable results.

However, what I think that you want me to do is point out a problem for 
you to fix.  And, unfortunately, the problem cannot really be fixed.  
This is why T.38 came along as an attempt to fix it... and even then 
T.38 still is not a perfect solution (it still uses UDP).

Thanks,

Lee.


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