Look for an intermediate firewall between your workstation and the unit with a short TCP session timeout setting.
I think you're on to something, Jay. If I start at the workstation mentioned above, but SSH into an Asterisk server on the same subnet as the TA908e and THEN SSH into the TA908e the connection doesn't drop. Something with my (intranet) router doesn't agree with Adtran's SSH server.
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I will jump in here because I had the same problem and could not find an answer that worked. Setting the line-timeout parameter worked fine for any value under 10 minutes. Above that it simply did not work and the session was terminated.
It turns out that if you have the firewall enabled there is another parameter that terminates sessions at 10 minutes. It is called 'ip policy-timeout'. What you need to do is set it to whatever you want (in addition to setting the line-timeout value).
ip policy-timeout [tcp | udp] <port> <timeout>
So it would look like:
ip policy-timeout tcp 22 28800
Its in seconds so 28800 secs = 8 hours.
Hope this helps somebody else.
Thanks Ricvil, I'll try that.