ISDN ("Integrated Services Digital Networks", called RNIS in France)
is a special type of fully digital telephone service; it's mostly
used to connect to your Internet service provider (with SLIP or
PPP). The main advantage is that the speed is higher than ordinary
modem/telephone connections, and that you can have voice
conversations while downloading stuff. It only works if your
computer is equipped with an ISDN card and both you and your service
provider purchased an ISDN line from the phone company. For
details, read <http://www.alumni.caltech.edu/~dank/isdn/> on the WWW.

Select this option if you want your kernel to support ISDN.