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ISE Columnist Don McCarty, OSP Expert

The Value of Longitudinal Balance in DSL Circuit Evaluation

May 1, 2016
As we are writing this column, both Vernon May (a bandwidth expert) and I are training together on a combination DSL – cable fault locating course in Tennessee. We are […]

As we are writing this column, both Vernon May (a bandwidth expert) and I are training together on a combination DSL – cable fault locating course in Tennessee. We are teaming up on this month’s column to discuss the longitudinal balance test. It is one of many discussions that we get into on providing quality bandwidth.

I have been preaching for years that if you are going to provide bandwidth over the copper infrastructure then the copper infrastructure must be stellar, and the longitudinal balance test properly applied and interpreted — and this will indicate the health of any copper cable pair. We have had a difference of opinion but we’ve worked it out in this column. Send us your thoughts.

Vernon Says:
Unless I’ve spoken with you in the last few months, you might have heard me say that I don’t use longitudinal balance for DSL circuit evaluation. My logic was simple: if capacitance balance passes, longitudinal balance will also. Despite asking several experts in the industry, nobody ever offered a logical argument against my conclusion.

Frankly, many DSL evaluation and troubleshooting strategies include the "DSL must need a better cable pair than POTS" mindset. For example, if POTS is good at >10 meg Ohms TR, TG and RG, DSL must need a cable pair with resistance >150 Meg ohms. There was never any field study that provided convincing evidence to support this, but it seemed logical and was almost universally adopted. I had put longitudinal balance into this category.

Being a long-time McCarty disciple, Don and I share many experiences and beliefs in common, but we also have our differences. Since our collaboration, Don has proven to me that longitudinal balance test is a valuable test for DSL circuit evaluation.

Let’s assume that the cable pair has no resistance faults, which is easy to determine with a simple volt/ohm meter. Let’s also assume that the Capacitive balance is >98% (passing results for almost everybody). It is still possible to have a series resistance issue that can impact DSL performance.

Series resistance is a relatively simple condition to detect, but we do have to short and ground the cable pair at the far end. Longitudinal balance will detect this condition and most of the time can do so without having to go to the far end. That has value.

Also, if we are looking for a good pair for testing or subscriber service, longitudinal balance is a good initial evaluation test. Warning: I am not suggesting that longitudinal balance is a replacement for your complete battery of tests with pass/fail thresholds. I am saying that if a pair fails the longitudinal balance test, you will find some kind of fault that will fail the complete battery of tests, so knowing this should save time, and you move on to the next pair.

My previous impression of the value of longitudinal balance adds to the long list of my mistaken impressions over the years. We all must continue to learn in this business. It is possible to teach an old dog new tricks, but sometimes it takes an older dog to do it.

Don’s Take:
When I am qualifying pairs for bandwidth or testing pairs when there is a customer having DSL problems, the first test I run other than an AC voltage test for safety is the longitudinal balance test. If the pair fails the longitudinal balance test I look for crossed battery from other circuits, a short or ground on the pair, a capacitive unbalance, a split cable pair, or series resistance.

If the pair passes the longitudinal balance test there is nothing that I can do to make that pair better. I can then shift to other DSL failure problems such as bonding and grounding issues, interference issues such as AM radio interferers or disturbers such as T! circuits or HDSL circuits in the same binder group.

Field technicians are inadvertently dispatched on DSL issues that have not been the cable pair, so when that quality field technician states that the pair is free of DC type faults and passes the longitudinal balance, remember that that field tech cannot do anything to make that cable pair better.

Signing off
I am thoroughly enjoying bouncing my theories and thoughts off Vernon May who is undoubtedly one of the brightest in this industry. If there are topics you’d like us to take on and voice our shared or differing opinions, send us your ideas. You can email me at [email protected] and you can reach Vernon at [email protected].

Also, be sure you have ISE EXPO 2016 (formerly OSP EXPO) on your calendar. Vernon and I have a killer seminar!

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