Triple-Play Service Deployment

Chapter 4: Troubleshooting the Copper Plant for IP Services
TDR Response (V)
7.3
3.6
0
-3.6
-7.3
Figure 4.12 TDR trace of a bridged tap
0 600 1200 1800 2400 3000 3600
Distance (Feet)
xDSL modems can tolerate some level of noise and attenuation,
thus some bridged taps are acceptable for most types of xDSL
service. Certain bridged taps may render the loop unacceptable.
For ADSL service, the length of all bridged taps on the span should
total less than 2,500 feet with no single tap exceeding 2,000 feet.
Taps of 300 to 800 feet in length have a very large impact. For
instance, several short taps (under 100 feet) might be acceptable,
but one long tap may be unacceptable. However there should be
no bridged taps close (within 1,000 feet) to the modem. The closer
the bridged tap is to the modem, the higher the energy level of the
reflected signals. If the tap is very close to one modem, the
reflection may contain more energy than the signal from the other
modem. At this point, the modem close to the tap may be unable
to distinguish between data and unwanted reflections.
The expanded spectrum of VDSL2 makes it more susceptible to
shorter bridged taps in the range of 20 to 100 feet.These short taps
may be enough to cause a substantial rate reduction. Typically,
much more stringent requirements, including in some cases
removal of all bridged taps, are required for VDSL2 than for ADSL.
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