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
pedestal
A pedestal (from French ''piédestal'', Italian ''piedistallo'' 'foot of a stall') or plinth is a support at the bottom of a statue, vase, column, or certain altars. Smaller pedestals, especially if round in shape, may be called socles. In ...
s). 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 users were commonly connected on the same pair which appeared at several different locations.
A bridge tap has no
hybrid coil
A hybrid coil (or bridge transformer, or sometimes hybrid) is a transformer that has three windings, and which is designed to be configured as a circuit having four ports that are conjugate in pairs.
A signal arriving at one port is divided e ...
or other
impedance matching
In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize si ...
components, just a “T” (or branch) in the cable. Thus the bridge presents an
impedance mismatch
In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize sign ...
. The unused branch of the T is usually left with no device connected to its end, thus has no
electrical termination
In electronics, electrical termination is the practice of ending a transmission line with a device that matches the characteristic impedance of the line. Termination prevents signals from reflecting off the end of the transmission line. Reflect ...
. Both the tap and its unterminated branch cause unwanted
signal reflection
In telecommunications, signal reflection occurs when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber. Some of the signal power may be reflected back to its origin rather than being carried all the ...
s, also called echoes.
Digital subscriber line
Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric dig ...
s (DSL) can be affected by a bridged tap, depending on where the tap is bridged. DSL signals reflect from the discontinuities, sending the signal back through the cable pair, much like a tennis ball against a brick wall. The echoed signal is now out of phase and mixed with the original, creating, among other impairments,
attenuation distortion
Attenuation distortion is the distortion of an analog signal that occurs during transmission when the transmission medium does not have a flat frequency response across the bandwidth of the medium or the frequency spectrum of the signal.Rowe, St ...
. 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.
Line in bridge tap technology can be much less performing caused to the high attenuation added and to the length of the cable itself.
Bridged tap technology does not add latency. But it affects the performance of the line in general, it affects the bandwidth a lot.
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]
References
External links
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*{{cite web, url=https://www.infobloom.com/what-is-a-bridge-tap.htm, title= Bridged tap definition with photo
Local loop
Signal cables