Single-Ended FDR to Locate and Specifically Identify DSL Loop Impairments

Widespread deployment of Digital Subscriber Line (DSL) Internet service is impeded by faults in the telephone lines and, for high-speed video services, even a minor discontinuity is important. Identification of faults can help service providers "pre- qualify" lines for DSL service and fault monitoring is essential to maintain an established service. Some faults affect many customers and should be repaired immediately while others, affecting only one customer, degrade slowly and repairs can be scheduled later. Thus, repair can be more effectively scheduled if the type of fault is known. With sufficiently sensitive test equipment, the entire length of the telephone line could be tested from the switching center which avoids costly dispatch of technicians and facilitates automated monitoring. Our method uses frequency domain reflectometry (FDR) and measurements show extended range with enhanced sensitivity and resolution when compared to the well- known time domain reflectometer (TDR). The FDR method easily detects a change of wire gauge at 1200 meters while a commercial TDR instrument is barely able to do so. To remotely identify the fault type, we introduce a new measurement of reflection phase angle: water in the cable (~160deg), bad splice (~10deg), gauge changes (~135deg), bridged taps (180deg), and open circuits (0deg).