电信全网介绍—NISDN

The traditional network layout has one network for telephony, a second for telex, a third for packet-switched data traffic, and so on. In other words, basically one network for each service category. This is no longer an ideal situation from a network operator's point of view, considering today's cost of network management.
The transition from analog to digital technology has posed another challenge to operators. In the 1970s, a common objective was the creation of an integrated digital network (IDN), that is, a network with integrated digital switching and digital transmission.
In this context, it was quite natural to design an integrated services digital network (ISDN) with highly advanced signalling to meet different service and bandwidth requirements in one network. The first ISDN recommendations were worked out from 1980 to 1984.
ISDN is fundamentally a circuit-switched network. In the 1990s, circuit switching began to face competition from the cell-switched ATM technology with a broadband ISDN (B-ISDN). Accordingly, the first ISDN concept is called N-ISDN, for narrowband.