ISDN digital subscriber line
ISDN Digital Subscriber Line (IDSL) uses ISDN-based digital subscriber line technology to provide a data communication channel across existing copper telephone lines at a rate of 144 kbit/s, slightly higher than a bonded dual channel ISDN connection at 128 kbit/s. The digital transmission bypasses the telephone company's central office equipment that handles analogue signals.
Source: Wikipedia — ISDN digital subscriber line (CC BY-SA 4.0)