ISDN - (Integrated Services Digital Network) - (1) ISDNs transfer information digitally through phone lines instead of the usual comparatively inefficient analog phone line transmission.  Special equipment is needed to be able to have an ISDN.  Some users (particularly in the past) have dial up ISDN where, other than the cost of installation, you only pay for when you're actually on the network (Internet). This is instead of paying a monthly rental fee. DSL has largely superceeded ISDN.

(2) A series of CCITT Recommendations related to the transmission of voices and data down the same line.