Modem constantly send/receives?

The moment I connect to my ISP my iBook's internal modem begins sending
and receiving, whether or not I have launched any app. It never stops.
It's always communicating. According to the Send & Receive activity
bars of the Internal Modem window the modem is sometimes communicating
at a low or intermediate rate, sometimes at the max.

The Pricess Listing window shows the highest % CPU apps to be
Internet Connect (10 - 15%)
Window Manager (0 - 3%)
ATSServer(0 - 9%)
SystemUIServer (0 - 3%)
and others
out of 37 processes running. The rankings vary wildly.

(As I was typing this note (Thoth 1.7.2) a mysterious new process
showed up in 4th place and went away before I could interrogate it. Its
name was an unintelligible string and its user was neither root nor me
but "???". Others with different names and "???" User are popping up
and disappearing within the ranking too. Sometimes there are 1, 2 or 3,
most of the time none. Their names are never repeated. They do affect
the count of active processes. Bulletin: one just showed up at the top
of the % CPU ranking with 6884%! What the heck?)

My system and my apps are set up to check manually for updates. I do
this every week or so, but this modem activity is perpetual.

I use no AV protection but the Firewall is up.

The system: iBook 600MHz, 14" dual-USB, 640MB, 20GB; OS X 10.2.8

Should I suspect that my system has been compromised?