Triple-Play Service Deployment

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Chapter 4: Troubleshooting the Copper Plant for IP Services
Balance
Good balance is critical for a pair to be able to cancel noise or
interference coupled in from sources inside or outside the cable
binder. Balance measurements qualify the overall health of the
pair. Theoretically, the tip and ring conductors should have the
same electrical characteristics. The more alike (balanced
electrically) the conductors are, the better the pair is at rejecting
noise, both internal and external to the cable. A balance
measurement quantifies the amount of similarity. If a pair is out of
balance, it will not reject noise as well, resulting in reduced xDSL
data rates or even loss of synch.
The major characteristics of a pair are resistance, capacitance, and
inductance. A well-balanced pair has these electrical characteristics
closely matched on tip and ring which means the same
amount of leakage to ground is present. No series resistive faults,
and no half taps, loads or other factors that would make one of the
main characteristics of tip different from ring should be present.
When the tip and ring conductors are very close to the same
electrical balance, the circuit can reject interference.The reason for
this is that any noise coupled into the pair will be coupled in
equally on each wire, thus cancelling each other. The noise will
then be “rejected.” On a pair with poor balance, the noise is
coupled in on one wire stronger than the other thereby
establishing a differential signal across the tip and ring. This
unwanted signal appears as noise damaging the DSL signal at the
receiver. The more noise, the lower the quality of the DSL signal.