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Chapter 11. Fault Locating Description If a copper T1 span line has failed, it can be very time-consuming to isolate the fault. Traditional fault location is a technique for determining precisely which line repeater or cable section has failed within one span line. Fault location utilizes one loaded VF pair within the T1 cable (T1 pairs are always unloaded). When central office alarms indicate that a span line has failed, normal traffic is blocked or removed from it at the office repeater. A special T1 test set injects so-called fault locating codes into the span line. These fault codes contain bipolar violations that are repeated at a discrete audio frequency. This is most easily thought of as sending T1 frequency modulated with an audio tone. At the first repeater housing an audio tone filter with frequency "A" is present. There is a special output from each regenerator that is fed to the audio tone filter. If that regenerator has a good signal containing "A" tone, the tone will pass down through the filter to the loaded pair named Fault Locate. If tone "A" successfully passes back to the central office and can be read on a meter, then assume that regenerator #1 is OK. If tone "B" is sent from the central office, it may be intended for the regenerator in the second repeater housing. If a cable fault has stopped the signal, then the "B" tone will not get as far as the second repeater (in fact, nothing will if the actual fault is between #1 and #2). The "B" tone will not pass through the filter and the "B" tone will not be measured back at the central office. The technician can make the assumption that some type of cable problem has occurred between repeaters #1 and #2, possibly including the line repeater plug-in at #2. System Layout Fault locating schemes can be developed with a dozen or more audio frequencies to cover very long span lines of many repeaters. Filter frequencies are usually assigned in alphabetical order along the span line. The newest fault locate test sets are automated to send all of the frequencies in sequence and monitor the tone receive level for each one. It decides which repeater is likely to have the fault. Newer Techniques After traditional fault locating was established, some newer types of T1 line repeaters were invented for the purpose of eliminating the fault locating filters and the associated cable pair. These non-traditional repeaters had a programmable looping feature which works fine in principle. However, one big headache occurs after a lightning hit to the cable. Each of these looping repeaters takes a period to reset itself after the preceding repeater in the series string. For a long repeatered line, this can be a sizable time delay (minutes).
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