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Loading coils are still used in long cables.
A common application of loading coils is to improve the voice-frequency amplitude response characteristics
of the twisted balanced pairs in a telephone cable.
Loading coils inserted periodically in series with a pair of wires reduce the attenuation at the higher voice
frequencies up to the cutoff frequency of the low-pass filter formed by the inductance of the coils.
With loading coils of exactly the right inductance, neither capacitance nor inductance dominate: the response
is flat, waveforms are undistorted and the characteristic impedance is resistive up to the cutoff frequency.
Loading coils prevent xDSL frequencies, which are higher than voice frequencies, from passing. This is a
common fault for new areas getting DSL or TV, and loading coils are normally only used in rural applications.
DSLAM integration
In a multiple dwelling unit (MDU), you may encounter a rack-mounted DSLAM. If so, it should be treated in
the same way as a DSLAM at a SAC cabinet, with the feeder coming in and distribution going to individual
customers.
Figure 40: Multiple Dwelling Unit
Vaults
A vault is a concrete enclosure that is normally underground and in the basement of central offices. This is
where the outside cables are connected to the inside cables on the frame.
Page 48 of 130
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