return path impairments in hfc networks

7/25/2019 Return Path Impairments in HFC networks

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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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Q&AReference Material

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