[cisco-voip] Question regarding Rightfax Configuration and "Transmission Error"

[Frank Arrasmith]

Calling all Rightfax gurus,
   I have the following question regarding the Rightfax configuration and
transmission errors.

Background:
My enterprise has CUCM 10.5 with a pretty dialed in Fax/T38 setup over SIP
and MGCP gateways.  For the most part, faxing is pretty solid inbound and
outbound to and from PSTN GW's, CUBEs, ,analog VG's, and regular fax
machines(we have a lot).  We have a SIP trunk connected to Rightfax 10.5
server, which is managed by another group where we have limited
access/experience with Rightfax configuration settings.  It took us awhile
to get the Rightfax servers with the correct t38 setup because they had
only run traditional CAS T1 prior to us, so it was new to everyone, but it
is up and stable except for the following issue..

Symptom :
The problem we see with Rightfax is with Transmission errors.  It starts
with our internal customers reporting that they do not receive a fax even
when the sender receives confirmation.  Upon further review we see that the
suspect call is listed as a "Transmission Error" in Rightfax, so the fax
never gets delivered to the customers account/mailbox even though the call
completes.

Analysis:
Since we are running T38, we can packet capture at the server, and  we see
normal fax protocol exchange, except for the suspect calls where we see
"RTN" Messages.  My understanding of the T.30 protocol is that when a RTN
message is delivered to the sender, that is an indication for the sender to
slow down and resend the last page. We actually see in the messages where
the RTN message gets sent, and the sender complies with the notice, and
sends again at 12000, then call completes as normal with an EOP message and
a DCN message.  In these cases,the Rightfax team actually looks at the fax
image and may see a bit of blurriness, but perfectly legible text ,even tho
its still marked as transmission failure. We have asked them if there is a
setting that can be tuned where the RTN does not cause the service to mark
the transmission as failure. To this they reply, "It was working fine
before when we were on CAS, so it must be on your end."

I understand where there are cases where the RTN message is sent because
the call quality is actually terrible, but in those cases, there is usually
several RTN messages and the sender will drop down to 4800 or below , and
then usually give up and the call will fail.  This type of failure is rare
(unless we have a major outage) and in this case the sending fax sees that
it failed and will proceed with its normal retries.

Question:

Is this RTN to transmission failure hard coded, or is this a configurable
setting?  If this were a regular fax machine, i think this would be a non
issue as the receiver would see the crappy page as well as the good copy of
that was sent again in their bundle of received pages.

Any insight is greatly appreciated and for anyone just getting into FAX
over IP with Cisco, I highly recommend the following book and Cisco Live
presentations from these guys from Cisco TAC.

Fax, Modem, and Text for IP Telephony [Book]
<https://www.google.com/aclk?sa=l&ai=DChcSEwiXgOn0wY7TAhXJCioKHRVCANoYABAMGgJ0bQ&sig=AOD64_1ddnEIE3WNy8NH5g7yJ-WnrcChJA&ctype=5&q=&ved=0ahUKEwjei-P0wY7TAhUnj1QKHejtCy0QwzwIEg&adurl=>
from Textbooks.com
by David Hanes, Gonzalo Salgueiro






Thanks,
 Frank
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