Inactive Telnet Session Timeout

A longstanding problem I have is that when I telnet from home to work, any
session inactive for about 15 minutes is disconnected.  My home PC's are
behind a router/firewall.  The problem disappears if I set the PC I telnet
from to a fixed IP address, and designate it as the DMZ.  Of course that is
not so good because it leaves the machine exposed.  I believe the AS/400 TCP
and Telnet timeout parameters are adequately defined.  I have tried the
Mochasoft 5250 telnet client as well as Win2K telnet from the command
prompt, with identical results.  Since the problem dissapears when running
in DMZ mode, it seems likely that more than the telnet connection on port 23
is at work here.  The AS/400 must be trying to communicate to my PC using
some port(s) in addition to port 23, and my router is blocking it.  I could
buy a telnet client with a keep-alive feature, but I'd rather not.  So, if
anyone sees a problem with my AS/400 settings, knows the ports used to
maange the session timeout conversation, knows of a cheap telnet client with
a keep-alive feature, or some other solution, please advise.  -Dave K.
                      Change TCP/IP Attributes (CHGTCPA)
TCP keep alive . . . . . . . . .   120           1-40320, *SAME, *DFT
(that's 120 minutes)
                       Change TELNET Attributes (CHGTELNA)
 Session keep alive timeout . . .   0             0-2147483647, *SAME,
*CALC...
(that's "never check for a timeout condition", which I will set to a
reasonable value after all this is resolved")