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Technical Training Session
August 20th
, 2014
Return Path Impairments
Presented by Bruce Smylie
Arcom Digital
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Topics1) Noise Impairments
a) Noise Funnelingb) Ingress vs. Impulse Noise
2) Common Path DistortionCPD
3) Impact on performance
a) CNR
b) MER
c) Service calls, line calls, repeat calls, overall node performance
3) Remediation
a) Traditional
b) Preventive
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Noise Funneling
Simply put, noise funneling is the summation
of all unwanted noise into the return path
Includes noise generated within the
subscribers home, noise generated by active
devices (Gaussian noise), noise generated as a
result of poor or broken shielding allowing
unwanted signals to ingress into the cableplant, noise generated when working on plant
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Noise Funneling
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Impulse Noise
Impulse noise is defined as noise caused by
electromagnetic interference into a cable
networks characterized by pulses with a
duration typically under 10 uS. (1)
Due to the Funnel Effect, a single Impulse
Noise source can impact an entire node,
sometimes causing laser clipping. Fortunately,Impulse Noise is short duration.
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Impulse Noise
Impulse noise is difficult to locate due to itsintermittent nature much like the signals from
ham radio operators and CB radios and is
usually shorter duration. Typically, this type ofnoise comes from motors with brushes.
Induction type motors are usually very quiet in
nature. Brush type motors are the most
common on hair dryers, vacuum cleaners,
electric mixers, electric drills and other small
AC power tools.
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Impulse Noise
Some larger motors are capacitor start,
induction run motors which may generate a
fairly severe noise spike when starting.
Bottom line, Impulse Noise source can be
elusive and many times appears to fix itself.
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Impulse Noise
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Ingress Noise
Ingress noise is defined as noise caused by
electromagnetic interference into cable
networks. Its power decreases with increasing
frequency. It is permanently present but
slowly varying in intensity as a function of
time. (1)
Common sources of ingress noise include CB,short wave radio, amateur radio.
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Ingress Noise
In the return frequencies between 5 and 47
MHz, there are amateur radio frequencies in
use at 7.0 to 7.3 MHz, 10.1 to 10.15 MHz, 14.0
to 14.35 MHz, 18.068 to 18.168 MHz, 21.0 to
21.45 MHz, 24.89 to 24.99 MHz, and 28.0 to
29.7
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Ingress Noise
Amateurs may operate with up to 1500 watts
peak envelope power (PEP) on all these
frequencies except 10.1 - 10.15 MHz, where
power is limited to 200 watts PEP.
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Ingress Noise
Loose connectors or line connectors that have
been improperly installed cause most of the
difficulties you will encounter in the outside
plant. Similarly, at the drop, you can expect
problems related to F-connectors that are
loose and/or corroded.
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Ingress Noise
Single-shielded drop cable can be another
source of ingress. In the case of some older
plant, there may be enough degradation in
the drop cable to cause not only problemswith the reverse path, but also for
downstream digital services.
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Source of Ingress Points
Customer Equipment
Loose connectors
Damaged cable Craftsmanship
Improperly disconnecting customer (i.e.
customer has dish or other service providerthat is using existing inside wiring and signals
feedback into cable network)
Many, many more
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Ingress Noise
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Ingress Noise
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Ingress NoiseModem causing CNR alarm
Corrected by using RSA
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Ingress Noise
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Ingress and Impulse Noise
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Sources of Ingress
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Common Path Distortion
An inter-modulation distortion created by
nonlinearities found at metallic junctions.
Downstream signals encounter a defect that is
acting as a diode
In the NTSC analog world, these distortions
are observed at every 6 MHz intervals
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Corona effect
Air ionizations, Plasma effect
Magnetic materialsHysteresis
Core saturation
Poor Contacts Loose, corroded
Non linear behavior
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CPD is a non-linear distortion
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CPD
Occurs on both downstream and upstream
portion of the cable network
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CPD
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CPD
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CPD
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CPD
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CPD
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Sources of CPD
Every connector interface in coaxial plant is potential source:
Terminators
Splitters
Amplifiers Junctions
Splices
Taps
Filters
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Sources of CPD
Bimetallic Corrosion- Electrochemical corrosion between
dissimilar metals
Single Metal Corrosion - Corrosion from air and moisture
in unprotected metals
Crevice Corrosion - Connectors
Stress Corrosion - Stress corrosion at bend, shear, and
clamp points
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Sources of CPD
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Sources of CPD
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Sources of CPD
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Sources of CPD
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Sources of CPD
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Sources of CPD
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Sources of CPD
Defect causing both CPD and Ingress
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SNR Terminology
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Impact on performance
CNR reduction caused by raised noised floor
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Impact on performance
CNR Reduction
MER Impact
Slow modem traffic
Noisy telephony
Loss of data traffic (outage)
Increased truck rolls
Increase in repeat calls
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Traditional Remediation
Traditional methods include one tech at
headend talking with one tech in field as pads
are pulled or power disrupted
This is not only intrusive but many times only
temporarily eliminates CPD or Ingress source
Wait until problem is customer affecting
Pull pad or power , breaking diode effect
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Preventative Approach
Know the overall health of your network
Prioritize work load by knowing which areas
have what type of problem
Minimize system downtime by utilizing non
intrusive techniques
Utilize tools and materials that minimize
subscriber induced ingress
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Overall Network Health
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Overall Network Health
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Overall Network Health
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Q&AReference Material
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