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

Greg Lipschitz Greg at thesummitgroup.com.au
Fri Sep 30 18:18:50 EDT 2016

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
1300 049 749
Level 1, 39 Railway Road, Blackburn VIC 3130
The Summit Group
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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