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Integrated Services Digital Network (ISDN)
Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network.
ISDN is an extension to the standard public-switched telephone network that allows for telephone company customers to transmit digital data directly over the telephone wires without the use of a modem. Your Macintosh connects to an ISDN line through the use of a terminal adapter, or TA. While it's easiest to understand a TA by thinking of it as an "ISDN modem" this is technically incorrect -- by definition, a modem modulates and demodulates signals from digital to analog and back to digital to allow transmissions of digital signals through the analog telephone network.
An ISDN Terminal Adapter places digital computer signals onto the phone company's digital network. (It also does some signal filtering and conditioning, but that's a relatively minor point.) The telephone companies have made ISDN service available in many areas, although it's far from universal. It requires special switching and signal handling devices at the central office although, contrary to some previously published information, it does not require fiber-optic cabling or anything other than ordinary telephone wire.
Standard ISDN service, known as PRI (Primary Rate Interface) consists of two line (Bearer, or B-Channels), each capable of transfer rates of up to 64Kbps. Many ISDN TA's allow the two b-channels to be multiplexed together to form a single 128KBps transfer stream using protocols such as Bonding mode 1, or Mutlilink PPP.
It is important to understand that despite the theoretical 128K speed of a bonded ISDN connection, your Macintosh can only communicate with the TA as fast as the ports available to the Mac. For example, if you connect a TA to the Macintosh via the Modem port, older Macs cannot reliably reach speeds much beyond 38,400 bps.