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

Nelson Hicks nelsonh at socket.net
Fri Sep 30 14:47:30 EDT 2016

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

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