Bridge tap

Bridged tap or bridge tap is a long-used method of cabling for telephone lines. One cable pair (of wires) will "appear" in several different terminal locations (poles or pedestals). This allows the telephone company to use or "assign" that pair to any subscriber near those terminal locations. Once that customer disconnects, that pair becomes usable at any of the terminals. In the days of party lines, 2, 4, 6, or 8 customers were commonly connected on the same pair which appeared at several different locations.

There are no active components in a bridge tap, just a “T” (or branch) in the cable. This T causes an unwanted Signal reflection (echo) as the signal travels along the unused branch.

Digital subscriber lines (DSL) can be affected by bridged tap, depending on where the tap is bridged. The farther from the customer's location, the better. DSL signals find an impedance discontinuity at the unterminated end, which reflects back through the cable pair, much like a tennis ball against a brick wall. The echo signal is now out of phase and mixed with the original, creating, among other impairments, attenuation distortion. The modem receives both signals, gets confused and "takes errors" or cannot sync. If the bridged tap is long, the signal bounces back only in very attenuated form. Therefore, the modem will ignore the weaker signal and show no problem. Most subscriber loop pairs in the world have bridged taps, so it definitely isn't always a DSL killer. One method of fixing this problem is to ask the tech to "cut dead ahead" your line. This removes the extra cable past your house and can improve DSL performance and stability.

A bridge tap can also be referred to as a 'multiple' or a telephone pair 'in multiple'. [Newton's Telecom Dictionary 23rd Edition 2003, Newton, Harry; p110] Newton's Telecom Dictionary, 23rd Edition; 2003; Newton, Harry; p110

ee also

* Telephone

References