Bridge tap

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A bridge tap is typically a wire which hangs from a pair of lines in the Public Switched Telephone Network. This line is usually an old tap line to a business or home. Bridge taps create a reflection point for high frequency waves on the line creating problems for DSL use on that line.

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. 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.