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ISE Columnist Don McCarty, OSP Expert

Transporting 3 Totally Different Circuits on the Same Copper Cable Pair

June 1, 2016
DSL and IPTV were introduced by handing the technicians a test set and watching for pass/fail like we did in the POTS environment. Sixteen years after the advent of DSL, […]

DSL and IPTV were introduced by handing the technicians a test set and watching for pass/fail like we did in the POTS environment. Sixteen years after the advent of DSL, problems in provisioning and maintaining these circuits still persist that were caused by this practice.

The biggest myth is: if the circuit is properly designed and is free of DC type faults with good bonding and grounding and is within the reach for that particular POTS, DSL, or IPTV service, then that individual service should function properly. There is a bit more to it.

I have preached this constantly: Many customers are utilizing 1 cable pair for 2 or 3 separate services. These 3 services, POTS, DSL, and IPTV over the copper infrastructure, are drastically different and should not be compared to one another.

There are issues that affect only POTS and only DSL and only IPTV — and these issues don’t affect the other 2 services. Then there are issues that affect all 3 services. Moving the circuit to another cable pair may fix 1 of the services, and problems with that cable pair could bring down another service.

For example, a Triple Play customer loses dial tone because one side of the cable pair is open in the distribution plant. That circuit is moved to another cable pair in a different binder next to a T1 circuit. The customer gets dial tone, but his Internet and IPTV are affected because T1 disturbs the bandwidth service.

For this reason, the best "fix" is to carefully diagnose the root cause of a failure and then repair the cable pair, rather than moving the circuits to another cable pair. To identify and rectify the root cause, the field technician must be properly trained and have adequate test equipment. Moving to another pair is a last resort.

By properly training the technicians on IP-based services and streamlining your process, existing subscribers will receive reliable service as the network migrates. Then by properly training the technicians on DSL and Copper, and streamlining your process, your existing customer base will remain intact during the migration to fiber optics.

And finally by properly training the technicians on fiber optic handling and testing, and streamlining your process, this technology will be as reliable as it was designed to be.

When any or all of the services fail, it is best to remove the equipment from both ends and test the cable pair. Following are the issues that must be addressed.

DC Faults
DC type faults such as unwanted crossed battery from other cable pairs, shorted or grounded cable pairs, open cable pairs, series resistance, and split cable pairs, if solid enough will affect both POTS and bandwidth circuits.

When any of the above faults are identified a trained field technician with quality hand-held test equipment that has a resistance bridge, an open meter, and a time domain reflectometer, will locate the root cause or causes of any DC type faults.

Interferers such as AM radio, ham radio, and short wave radio, affect bandwidth services but have little effect on POTS service unless the POTS customer is located under the antenna causing the interference.

If interferers are affecting bandwidth, then the bits per bin graph on the test set will show any bins that are compromised including the frequency of any interferer, and then the spectrum analyzer will show the frequency and the amplitude of the interferer.

Disturbers are other circuits in the same binder group that is providing POTS, DSL, and IPTV. Two good examples of interferers are T1 and HDSL which use more power than DSL or IPTV. POTS circuits are not affected whereas DSL is progressively affected and IPTV is severely affected.

Again the bits-per-bin graph on the test set will show any bins that are compromised, including the frequency of any disturber, and then the spectrum analyzer will show the frequency and the amplitude of the disturber. The only solution is to move either the disturber or the DSL circuit to another binder group.

Bonding and Grounding Issues
Bonding and grounding has an effect on all 3 services but for different reasons. All cable shields should be bonded from the central office or remote to mitigate all frequencies above 1kHz which reduces the amplitude of interferers from affecting DSL and IPTV.

This process also moves low frequency harmonics to the cable shield. By periodically attaching the cable shield to the distribution power neutral low frequency odd triple harmonics are transferred from the cable shield to the distribution power neutral. Then those unwanted harmonics are carried back to the distribution power substation by the distribution neutral. This reduces power influence on the POTS circuit which is a huge issue for POTS service. Those low-frequency harmonics of 60 Hz have little or no adverse effect on DSL and IPTV.

Impulse Noise
Impulse noise affects all 3 services, but in different ways. An example of impulse noise is electric fence noise. The POTS service customer hears the popping, the DSL service has constant modem retraining, and the IPTV service has a myriad of problems including pixelation, macroblocking, and set the top box dropping, etc.

All quality hand-held test sets can identify impulse noise but not its source. Most impulse noise is power related. The divide-and-conquer method of opening the circuit up and testing both ways does not work when trying to locate the root cause of impulse noise. Following the amplitude to its highest will get you in the vicinity, keeping in mind that the root cause can be from any other DSL circuit on the DSLAM or blade.

A well-trained field technician armed with quality test equipment can resolve most of the Triple Play issues, given time.

Signing off
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About the Author

Don McCarty

Don McCarty is the OSP EXPERT columnist for ISE magazine, discussing the issues around provisioning, testing, and maintaining copper for all services from POTs to IPTV. Don is also president of and the lead trainer for McCarty Products, a technical training and products company training field technicians, cable maintenance, installation repair, and Central Office technicians and managers. For more information, email [email protected] or visit