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.
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