[Asterisk-Users] T.38 Faxing -> at astricon ?

[Lee Howard]

Roger Schreiter wrote:

> I think, you agree, that VoIP is somewhat similar to ISDN, as it
> transports analog audio data in a digitally coded way.
>
> Noone doubts, that ISDN is suitable to transport analog fax.
> Finally the PSTN is 99,9% digital (ISDN/SS7), even if some
> subscriber lines are still analog.
> (Ok, ISDN is a managed network, and thus very high quality.)


The fact that ISDN and VoIP both use digitial audio is irrelevant.  The 
reason why your average VoIP connection cannot be used for faxing is not 
because of the digitized nature of the audio, but rather the medium over 
which it is being transported.

The PSTN virtually guarantees timely and orderly delivery of every 
single audio sample.  An IP network cannot do this, and so as the amount 
of "jitter" on the IP network varies its usability for fax will also 
vary.  Modulated data cannot tolerate receiver-end misordering of audio 
samples, synthesizing of missing audio samples, or dropping audio 
samples.  If any of this happens then data corruption will certainly 
occur.  And, depending on the frequency and timing of that data 
corruption, and the resiliancy of the fax endpoints, that data 
corruption can (and is well-known to) cause fax failures.

Lee.