This means you can extend the data capacity of your line by a factor of 40 or more using modern digital modulation techniques. Using technology developed in the 1980s, ISDN is the first service to exploit the bandwidth potential of twisted-pair loops with complex digital signal processing. By sending 80,000 pulses per second, ISDN can stuff a 160-Kbps signal through loops up to 18,000 feet long. ISDN also employs complex circuits called echo-canceler hybrids that allow signals to be sent simultaneously in both directions on one pair of wires. About 75 percent of loops in the US are shorter than 18,000 feet.
Longer loops require special engineering, even for regular phone service.
ADSL provides the telcos with high bandwidth without necessitating a costly reengineering of the local loops. If you tried to talk over a line carrying a baseband signal, you would hear an unacceptable level of noise. Transmissions at frequencies above the band used for voice are called passband signals. Telephone companies developed ADSL in the late 1980s to provide video dial-tone services over the telephone loops. ADSL provided as much capacity as possible for the transmission of digital television signals from the network to end users and reserved less bandwidth for transmission from end users to the network.