[VoiceOps] Problems with T.38 fax with Grandstream GXW-4248

Mark Lindsey lindsey at e-c-group.com
Fri Sep 30 19:54:10 EDT 2016

I've also had mixed luck with t.38, but the t.30 fax standard allows only 3
errors; after the third error is detected, the fax is to be cancelled.

But working to make t.38 work is worth the effort. The networks have to be
phenomenally clean to have a reliable faxing service that works all the

If you're doing 10 pages at 28800 bps, that'll take 350 seconds, and
tolerate no more than 0.017% packet loss. That's an unusually clean

If the fax is slower, e.g. 9600 bps, the fax is longer (600 seconds at 9600
bps), so the exposure to errors is greater and the network reliability
requirement is higher -- <=0.01% loss allowed.

This assumes errors are randomly distributed; if they come in clusters the
requirements are much stricter.

Mark R Lindsey
mark at ecg.co

On Sep 30, 2016 at 18:18, Greg Lipschitz <Greg at thesummitgroup.com.au> wrote:

Have you tried the opposite, disable T.38 and use G711a/u for the payload.

We have had far greater success with the G711a/u RTP. T.38 still seems to
be very hit and miss cross devices be it ATA or even a real fax machine.


Greg Lipschitz  |  Founder & CEO  |  The Summit Group
*Greg at thesummitgroup.com.au* <Greg at thesummitgroup.com.au>
*thesummitgroup.com.au* <http://thesummitgroup.com.au>
*1300 049 749* <1300%20049%20749>
*Level 1, 39 Railway Road, Blackburn VIC 3130*
[image: The Summit Group] <http://thesummitgroup.com.au>
*From:* VoiceOps <voiceops-bounces at voiceops.org> on behalf of Nelson Hicks <
nelsonh at socket.net>
*Sent:* Saturday, October 1, 2016 4:47:30 AM
*To:* voiceops at voiceops.org
*Subject:* [VoiceOps] Problems with T.38 fax with Grandstream GXW-4248

We have a customer with multiple Grandstream GXW-4248 voice gateways
that wants to use T.38 for sending and receiving faxes. They are running
the most recent release ( We have tested this successfully
with other devices, including a Grandstream HT-702 and a Grandstream
GXW-4004, but when configuring the Grandstream GXW-4248 the same way,
the audio from the Grandstream to the fax machine cuts out at the
renegotiation to T.38 and does not resume. When looking at a packet
capture of the fax call, the Re-INVITE appears successful, but the first
and only UDPTL packet sent by the Grandstream is reported as malformed
by Wireshark:

ITU-T Recommendation T.38
     [Stream setup by SDP (frame 510)]
         [Stream frame: 510]
         [Stream Method: SDP]
         seq-number: 32768
             type-of-msg: t30-indicator (0)
                 something unknown here [too long
                     [Expert Info (Warn/Undecoded): something unknown
here [too long integer(per_normally_small_nonnegative_whole_number)]]
                         [something unknown here [too long
                         [Severity level: Warn]
                         [Group: Undecoded]
[Malformed Packet: T.38]
     [Expert Info (Error/Malformed): Malformed Packet (Exception occurred)]
         [Malformed Packet (Exception occurred)]
         [Severity level: Error]
         [Group: Malformed]

Does anyone have any experience with trying to use T.38 with a
Grandstream GXW-4248?


Nelson Hicks
Network Operations
(573) 817-0000 ext. 210
nelsonh at socket.net

VoiceOps mailing list
VoiceOps at voiceops.org
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