There are two points of view into the ISDN world. The most common viewpoint is that of the end user, who wants to get a digital connection into the telephone network from home, whose performance would be better than a 20th century analog 56K modem connection. Discussion on the merits of various ISDN modems, carriers' offerings and tariffs (features, pricing) are from this perspective.
Since the principal consumer application is for Internet access, ISDN was mostly superseded by DSL in the early 21st century. Inexpensive ADSL service offers speeds up to 384 kbit/s, while more expensive versions are improving in speed all the time. As of fall 2005, standard ADSL speeds are in millions of bits per second.
There is a second viewpoint: that of the telephone industry, where ISDN is a core technology. A telephone network can be thought of as a collection of wires strung between switching systems. The common electrical specification for the signals on these wires is T1 or E1. Between telephone company switches, the signaling is performed via SS7. Normally, a PBX is connected via a T1 with robbed bit signaling to indicate on-hook or off-hook conditions and MF and DTMF tones to encode the destination number. ISDN is much better because messages can be sent much more quickly than by trying to encode numbers as long (100 ms per digit) tone sequences. This results in faster call setup times. Also, a greater number of features are available and fraud is reduced.
ISDN is also used as a smart-network technology intended to add new services to the public switched telephone network (PSTN) by giving users direct access to end-to-end circuit-switched digital services and as a backup or failsafe circuit solution for critical use data circuits.
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