RADIUS, how long connection last

Hello,

I wrote a program which authenticate users sending requests
to Free RADIUS server. This area of using RADIUS is
obvious for me.

But I know that RADIUS is used to log modem connection
time, too.
How does RADIUS get to know that somebody has disconnected?
Does device send information about disconnection to a RADIUS
server or it is made in different way?
How to configure RADIUS server, what parameters to use for RADIUS
client to use RADIUS to log connection time (or time of connection
and time of disconnection)?

Thanks in advance,
Vicky.