Initially, it was believed that ordinary phone lines could only be used at modest speeds, usually less than 9600 bits per second. In the 1950s, ordinary twisted-pair telephone cable often carried 4 MHz television signals between studios, suggesting that such lin…

Telephones are connected to the telephone exchange via a local loop, which is a physical pair of wires. The local loop was originally intended mostly for the transmission of speech, encompassing an audio frequency range of 300 to 3400 hertz (…