Triple-Play Service Deployment

102
TDR capabilities
A time domain reflectometer (TDR) is used to identify faults, such
as bridged taps, wet sections, load coils, shorts, opens, and splices,
and their location on copper cabling. Functioning much like sonar
on a submarine, the TDR sends an electrical pulse down a copper
pair and analyzes the energy that is reflected back. Reflection
occurs when the launch pulse detects any changes in the
impedance characteristics on the copper pair. The shape of the
reflection identifies the nature of the fault. Any impairment that
lowers the impedance creates a downward bump in the trace
while an impairment that increases the impedance creates an
upward bump.
Figure 4.9 TDR without TVG and TDR with TVG
Chapter 4: Troubleshooting the Copper Plant for IP Services
The open end of a pair or a hard short will show the largest
reflections on a trace. Opens,load coils,the end of a bridged tap,and
wet sections, produce an upward trace. Shorts and the first portion
of a bridged tap signature produce a downward trace. Splices
typically produce a combination signature: upward followed
immediately by a downward trace. Because the launch pulse
attenuates as it travels down the pair, the further an event is from
the test set, the weaker the reflection. Because cable gauge affects
attenuation, a TDR’s range is reduced on smaller wire gauges.