[asterisk-users] T.38 on a MAX/Lucent/Ascend TNT

[Ben Winslow]

Hello folks,

I've been trying to get T.38 over SIP working with calls terminated by a 
MAX/Lucent/Ascent TNT.  As far as I can tell, SIP and T.38 are actually 
working perfectly; however, I can't get the TNT to properly terminate a 
FAX call.  Does anyone have a working configuration for SIP and T.38 for 
calls from a TNT or APX?

Here's a brief description/diagram of my test setup:

Laptop --RS232--> Modem --POTS--> Channel bank (ADIT 600) --CAS T1--> 
TNT --Ethernet/SIP--> Asterisk --SIP--> t38modem.

The TNT is running TAOS 11.0.4 with both the SIP and realtime fax 
features licensed.  I've tried using several modems, and with each I can 
establish a data call without any problems (although at a maximum of 
31200bps/3429 baud using a 33.6 modem, for reasons I haven't dug into 
yet.)  Whenever I try to send a fax from the laptop, however, the call 
always seems to fail in the first HDLC phase (phase B) with either a 
timeout or error 23 (COMREC invalid command received.)  The modem is 
connected directly to the channel bank and the channel bank is connected 
directly to the TNT in an attempt to reduce the number of variables.

With my current configuration, the call to Asterisk will come up as a 
voice call, then be dumped into t38modem when Asterisk receives the 
1100Hz CNG tone from the sending modem.  At that point, the call is 
dumped into t38modem which re-invites the TNT with the T.38 options, and 
the TNT usually sends a T.38 t30-ind of 0x00 (no-signal) or 0x3a (???), 
although I do occasionally see more promising messages like 
v29-7200-training or t30-data/hdlc-fcs-ok.  Shortly thereafter, the 
dialing modem will give up and terminate the call.

If I try dumping the same call into iaxmodem instead of t38modem, the 
call actually progresses further -- the real modem receives and decodes 
the HDLC CSI/DIS from iaxmodem, but the high-speed trainup always fails 
for calls coming from the TNT.

Does anyone have any advice or suggestions?  Has anyone actually made 
T.38 work with one of these devices running ANY TAOS version?

Thanks,
-- 
Ben Winslow <winslowb at pa.net>