Twisted pairs of copper telephone wire, commonly referred to as twisted pairs, have been in existence since the late nineteenth century when Alexander Graham Bell first invented them. In fact, most of the transmission lines in a typical telephone network, commonly referred to as the telephone loop plant, are twisted pairs. Although much of the loop plant has been or is currently being upgraded with optical fiber, it is expected that twisted pair technology will remain in place for many years due to the high cost of upgrading and due to the ability to achieve high transmission rates over twisted pairs.
U.S. Pat. No. 3,989,902 shows a regulator which can be used in combination with equipment connected to a telephone type line. The regulator senses the amount of direct current being passed by the line and adjusts its internal AC impedance in inverse relationship thereto. This will shunt alternating current signals more severely when connected to short line loops having relatively small insertion loss, protecting a central office from excessive signal amplitudes.
U.S. Pat. No. 6,477,212 shows an apparatus for reducing local area and broad area interference in a twisted pair transmission system. The apparatus includes a detection device, such as an antenna, for example, for detecting electromagnetic interference coupled into a twisted pair line, a sampling/scaling device which samples and scales the detected signal, and a combiner device which combines a signal correction component with the signal received over the twisted pair by a differential receiver. The sampling/scaling device includes an analog-to-digital converter (ADC) which converts the detected analog signal into a digital signal and a digital signal processor, which receives the digital signal from the ADC and processes the signal to generate a correction signal. The correction signal is then subtracted from the signal received by the differential receiver.