Triple-Play Service Deployment

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Chapter 5: Troubleshooting the Premises Wiring
The noise immunity test simplifies the interpretation of results by
allowing Pass/Fail thresholds to be set. These thresholds ensure
that technicians make consistent and clear decisions. Pass/Fail
indicator thresholds are programmable by the end user and can
be fixed for field technicians. Programmability is necessary
because end users may be deploying different equipment and
services over time with different signal types and protocols. Certain
protocols and services are more tolerant of noise than others.
Modern home coaxial networks built with good quality cable,
components, and workmanship typically will have minimum 50 dB
of shielding effectiveness in the FM band by design. Substantially
less than this is indicative of a significantly damaged cable run,
splitter, or connector or poor workmanship. Damaged cable and
connectors are not likely to improve over time and are more likely
to degrade. CableLabs’ formal studies on home networking
shielding in real networks indicate 95% of homes have greater
than 36 dB of shielding effectiveness.
The ingress resistance/noise immunity test has been in practice for
more than one year at cable operators. Noise immunity tests can
be performed either as an absolute test where the absolute signal
levels on the coax are examined, or as a calibrated test where a
comparative/relative shielding effective number is calculated from
the difference between the local off-air strength and the signal
strength on the closed coaxial network. Typical off-air signal
strength is between -10 and +10 dBmV. On average, operators
have converged on 30 dB of shielding effectiveness as a decision
threshold for replacement of connectors, splitters, or coaxial runs.
For an absolute reading JDSU recommends a -30dBmV threshold.
Any signal greater than -30dBmV should be considered a failure
and warrants corrective action for proper network operation.
Calibrated tests should be performed in areas where FM signal
strength is very high (over +20dBmV) to avoid replacing cable that