PuTTY is able to keep the SSH connection for sometime even if the network cable is unplugged! I've heard about this long time before, but never had a chance to test it, until today.
There's some work on the ventilating fan in the lab today, while we were already in work. So I had to disconnect the network cable and move my laptop around. After about 5 minutes I reconnected the cable. The connection to Athena dialup server was broken, but all the three other connections to our lab's servers managed to survive, and even the Emacs (through X-Win32) window was still able to work.
Why would Athena disconnected while the others didn't? I guess maybe our servers have a longer timeout threshold (or sth. similar), or maybe Athena uses a different (more restrictive, and perhaps, secure) mechanism to detect network connections.