Hi Kalman --
ISDN and ATM are BIG topics - so it's hard to know how to respond
without knowing what you are looking for in some detail. Basically,
ISDN - a service for homes and businesses, about 128 kbps, and
available, if at all, from your local telephone company. Since
there are varieties of standards, you might want to talk to your
local phone company first.
A good web source for ISDN informatin is Yahoo -> Computers ->
Telecommunications, or ...
ATM - a service for large businesses and networking companies,
usually starting at several Mbps and going up to about 155 mbps.
ATM is an "infrastructure" service, and you can get tons of
The Yahoo Computers/Telecommunications index (as above) also has
some pointers to ATM information.
Good luck !
FYI. There is an "intermediate level" telecommunication service
called "Frame Relay" that you might want to check into, depending
on your interests and needs. At the very high end, you would want
to also check into something called SONET.