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 | Digital Subscriber Line (DSL) Testing |
6. Bridge Tap Detection
A bridge tap is any unterminated portion of a loop not in the direct talking path between the CO or DLC and the customer's premise equipment (telephone or modem). A bridge tap may be a used cable pair connected at an intermediate point or an extension beyond the customer (see Figure 5). For example, a drop wire that provided a second line in a home could be disconnected at the customers' NID (easier than climbing the pole), and when that pair is reassigned to service for another dwelling, the drop length to the original residence represents a bridge tap. As the OSP evolved, cable splices and cable-pair swaps, combined with poor documentation, have made locating and removing bridge taps a time-consuming and costly challenge. The IEEE reported that in only five percent of loops with bridge taps would the bridge tap adversely impact xDSL transmission.2
Telcordia reports that a significant portion of the loop population (56 percent) has bridge taps.3 The high percentage of loops with bridge taps alone is not reason for concern; the length of a bridge tap is the primary factor that determines if xDSL transmission will be impaired—not simply the presence of a bridge tap. The mean bridge-tap length is only 1,300 feet, as reported by Telcordia, which will not significantly impair xDSL performance. The length of bridge taps impacts the performance of each xDSL differently.3 The HDSL, MDSL, and ISDN transmissions will typically tolerate 6,000 feet of bridge tap. In the case of ADSL, the presence of bridge taps will not prevent ADSL services over copper line, but they can reduce the bandwidth and speed at which the xDSL services operate. With the current trend towards offering lower-speed ADSL/RADSL–based services in the range of 384/384 kbps to 1,544/384 kbps, the effect of bridge taps has been reduced.
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