Bridge tap
From Wikipedia, the free encyclopedia
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.
Digital Subscriber Line can be affected by bridged tap, depending on where the tap is bridged. The farther away from the customer's location, the better. DSL signals find an impedance discontinuity at the unterminead end, and reflect 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 and gets confused. This is when you "take errors" or cannot sync. If the bridged tap is long, by the time the signal bounces back, it returns 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.
