Q: Bridge Tap?
A: 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.
DSL can be affected by bridged tap. It depends on where the bridged tap is located ... the farther away from the customer's location, the better.
DSL signal reflects back through the cable pair from the end of a bridged tap, much like a tennis ball against a brick wall. The deflected signal is now out of phase and mixed with the original. 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, the original signal is far ahead and more powerful. Therefore, the modem will ignore the weaker signal and shows no problems.
Almost every cable pair in the world has bridged tap on it, so it definitely isn't always a DSL killer.
A bridge tap is a technique for telephone installs that taps you into a line that runs past your house, rather than terminating at your house. The line doesn't go to anyone elses premise, it just ends, maybe a mile up the road. This is useful should people move out, demolish a house or whatever, the phone company can move the tap up or down to service whomever it wishes. Unfortunately, you have a mile of unterminated phone line running up the road ready to pick up any interference, like a huge TV antenna. Because it isn't terminated, signals bounce up and down the line and can reinforce or cancel the signals your equipment is trying to send to your ISP. This can and does play havoc with 56k modems. Telcos will not pay much attention to customer complaints about bridge taps since they do not guarantee data beyond 2400 baud, however, a DSL line is supposed to be a new clean install, and so you should not get any bridge tap surprises.. but if your provider mentions this, now you know what they are talking about.