ISDN should have been implemented decades ago. Businesses were clamoring for
it because it is a digital service and more than twice as fast as a 56K modem,
but the phone companies were barely aware they had such a thing, and certainly
had no idea how to get it installed for you. Finally they got serious and
started rolling it out - just in time to be steamrolled by DSL.
So why should anyone be interested in ISDN now? Because you can't get DSL in a lot of places, and won't be able to for some time. For DSL, even at its slowest speed, you must be within 3 miles of a telco central office and the wiring to that office must be relatively new.
ISDN is normally a "dial-up" service, charged by the minute while you are connected. This is quite unlike DSL which is always connected, so ISDN can be much more expensive if you use it much. Connection speed, however, is nearly instantaneous. There is no tone dialing and long, noisy modem negotiation.
An alternative is IDSL, which uses ISDN technology for an "always on" service to places where other forms of DSL can't be had. It operates at ISDN speeds (128k max).
ISDN does have some problems if you are running a Microsoft based network and have an ISDN router on the network. Design problems with MS networking cause the ISDN modem to be dialed rather often for no reason.
ISDN requires good phone lines and is not available everywhere. It varies widely in cost, both for installation and per/minute of use. The attachment device, properly called an ISDN terminal device, is not a modem, but it is commonly called an ISDN modem.
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