ssh connections now freezing, if left idle for a few minutes

[M. Fioretti]

Greetings and... an admittedly very broad questions, maybe not
even related to Fedora except for "what are the best tools on
Fedora to diagnose this?"

I have this fedora desktop on which I usually keep open shells
connected to several servers, via ssh (on different ports too).

Until 3/4 days ago, I could leave all of them "hanging" for
DAYS, without typing anything at those prompts, and they'd
remain alive and ready to accept commands. Now, all of them
freeze if I leave them idle for more than a few minutes.
Every other connection (email, web, etc...) has no problem.

I have also noticed that if I rsync via ssh several GBs of stuff in
the same moments, from/to the same servers, with the same
accounts, same ports... etc, THOSE operations work just fine,
pretty quick too.

In other words, it's all and only the plain AND unattended
ssh connections that systematically freeze.

I ran yum update, and that did not change anything. At this
point, it is likely that either my ADSL provider and/or its
modem are doing something funny, so the question is:

how could I discover what happened, and where?

Thanks,
Marco