It supplies two independent 64-Kbps channels that can be used for voice or data. ISDN is widely used for corporate remote access, but it can also be used as an Internet connection. Most major ISPs support dial-up ISDN at no additional charge. Your phone company provides the line and phone service, but you need to provide your own phone equipment and ISP account. If you want to connect your LAN to the Internet via ISDN, you'll need a router.
ISDN pricing varies widely from market to market, even within the same state.
On balance, ISDN is a good deal for users who want instant connections but can't get ADSL or cable service and don't want to pay high satellite rates. If you can't get any of these fast services or if they cost more than your budget allows, then it makes sense to upgrade to a V.90 modem. But before you do, make sure your phone line can get a modem connection faster than 33.3 Kbps. V.90 modems are available in external and internal models. If you already own a 56K unit that conforms to the old x2 or K56flex, you can upgrade it to V.90 via a download from your vendor's Web site.