[VoiceOps] VoIP T38 Fax reliability

Gabriel Kuri gkuri at ieee.org
Wed Jan 6 19:15:18 EST 2010

It seems to be hit and miss, certainly not 100%, but better than
trying to use either PCMU or PCMA over the commodity Internet for
FAXing. PCMU/PCMA might work over a local private LAN with QoS enabled
and the jitter down to almost nothing. The biggest issue I've seen
with T38 is the ATA's inability to properly negotiate a Super-G3 fax
down to G3 or slower speeds. In our experience, you need to disable
the Super-G3 feature of the FAX machine to slow it down to either 14.4
or 9600 and disable ECM. Many of the newer FAX machines have a "VoIP
Compatibility" mode which essentially slows it down to 9600 and
disables ECM.

We predominantly use Cisco/Linksys/Sipura ATAs, but would like to hear
if anyone has better experiences with other ATAs, particularly in
business environments where there's potentially a lot of FAXing.


On Wed, Jan 6, 2010 at 6:00 PM, Ujjval Karihaloo
<ujjval at simplesignal.com> wrote:
> I know this a thorn in everyones flesh, I would like to know of experiences
> with T38 Voip Faxing. We have had good and bad. Those that don’t work just
> don’t and they don’t even want to troubleshoot and just switch over to an
> Analog line for Fax. We mostly use AudioCodes and Sipura T38 ATAs.
> Any of you folks do T38 and whats the experience like.
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