|faxguy at howardsilvan...
|Posted: Sat Jan 21, 2017 2:04 pm Post subject: [asterisk-biz] Affects of jitter and packet loss in fax tran
|On 01/21/2017 02:07 AM, Tahir Almas wrote:
What feedback are you expecting to get?
Essentially your investigation has revealed that packet-loss/jitter are
problematic for fax. This is not a revelation to the industry at large...
Your investigation reveals that some minimal level of packet-loss/jitter
is generally tolerable by fax. Again, this is not a revelation to the
There are tolerances in ITU T.30 (fax protocol) and especially so with
ITU T.30 Annex A (error correction mode for Phase C) which enable fax
receivers (and senders) to cope with a nominal amount of corruption to
the audio stream. Furthermore, ITU T.38 permits packet "redundancy"
(which causes packets to be sent in duplicate, triplicate,
quadruplicate, etc. in hopes that at least one of those packets makes it
through). However, if the audio stream quality is poor enough then the
tolerances in T.30, ECM, and even T.38 redundancy will be exhausted, and
faxes will fail much more regularly in those conditions.
A lot of it also has to do with the coincidental timings of the audio
corruption and the equipment involved. If one 20ms moment of jitter
occurs at the very outset of a long image page of fax (Phase C) when ECM
is not being used then chances are very good that the fax will fail.
The receiver cannot reliably anticipate how long it must wait for the
end-of-page signal (Phase D). Even so, some fax receivers simply are
not programmed well-enough to tolerate early Phase C jitter - even with
ECM when it's relatively simple to calculate how long it must wait for
The solution - whether T.38 is being used or G.711 - is to eliminate
packet-loss/jitter. If that cannot be done reliably then the faxing
must be done via proxy somehow (through a fax service provider).
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