OSP Magazine, Mar '10 (Noise Mitigation in Bandwidth Bus.)

[ksloan]

Noise Mitigation in the Bandwidth Business
11 Tips From the Trade
by: Don McCarty
OSP Magazine

My last few columns have touched on AC influence on customer service because it’s a subject that keeps people awake at night, and solutions aren’t well understood. Little attention has been paid to unacceptable power influence on copper cable pairs over the last decade by reactive telcos who often chose to ignore the problem unless it was so severe that the customer couldn’t talk over the hum on the line. If your company plans on being in the bandwidth business using paired copper conductors in that first mile, everybody in the company had better get involved with all aspects of unwanted AC influence.

AC influence has always been perceived as a narrowband Plain Old Telephone Service (POTS) problem stemming from harmonics of 60Hz distribution power affecting voice grade frequencies from 0 to 4kHz. Those good folks in the bandwidth business ask, “Why should we be concerned with unwanted harmonics from 60Hz to 3kHz? We start our bandwidth services at 25kHz.”

If you are going to be in the bandwidth business and use any copper conductors in your plant you’d better be concerned and take a proactive stance with all departments involved to reduce AC exposure at all frequencies. Don’t wait until you receive a customer complaint and then dump the problem onto the cable maintenance technician’s shoulders.

Not only do you need to be concerned about AC interferers coming from associated distribution power AC exposure that generates harmonics up into the megahertz range, but also about AC influence coming from AM radio, Ham radio, Shortwave radio, and disturbers from other circuits within the cable sheath.

Over many years of dealing with noise, I’ve picked up some tips that may help you:

1. It is a given that the longer the circuit the more AC exposure is present and the higher the unwanted influence is going to be. There are a myriad of unwanted AC signals present on any copper circuit but we get involved only when there is enough amplitude to disrupt service.

2. Fiield technicians never eliminate the “noise” across the pair created by these unwanted AC signals. With good bonding and grounding techniques some unwanted AC signals are reduced to a point to where they do not disrupt service. Other unwanted AC signals are associated with the distribution power and the root cause identified and rectified.

3. From an engineering standpoint, cable design and cable placement is all important. If an aerial distribution power lead is on the north side of the road in a rural area, place the telephone cable on the north side of the road. Placing the telephone on the other side of the road does not decrease the interaction with the power system and makes it harder to tie the cable shield to the power company neutral four times a mile.

4. In buried subdivisions place telephone pedestals next to power pedestals and require grounding from the power to the telephone cable. Make sure that the drop is placed within 20 feet of the power panel for convenient grounding of the Network Interface to the power ground electrode.

5. From a construction aspect, continuous bonding from the CO or remote to the customer’s interface is required. All bond connections and ground connections should be torqued to 30 to 40 inch-pounds. Can wrenches and ratchet wrenches are unacceptable because the connection is either under-torqued or over-torqued, creating series resistance.

6. Don’t let your central office (CO) and remote people off the hook. Equipment in COs and remotes also generate unwanted AC signals that disrupt service. DC rectifiers can be big hitters.

7. Installation and repair technicians can contribute significantly to reducing AC influence. The National Electric Code requires that the Network Interface Ground be tied to the Power ground electrode. The ground resistance should measure from 0 to 25 ohms. This simple step will drastically reduce AC interference and decrease the AC voltage tip and ring to ground.

8. REMEMBER: EVERY UNUSED (LEFT IN) AERIAL DROP WIRE ATTACHED TO A SPAN CLAMP TIED TO A TELEPHONE CABLE PAIR IS AN AM RADIO ANTENNA AND MUST BE DISCONNECTED FROM THE CABLE PAIR TO REDUCE AM RADIO INERFERENCE AND REDUCE IMPULSE NOISE.

9. After every install or repair with dial tone present at the network interface the field technician should measure and record Loop Current, Station Ground Resistance, Circuit Loss, Circuit Noise, Power Influence, and Longitudinal balance. If the customer has bandwidth services the Achieved Rate, the Maximum Achievable Rate, Forward Error Correction, and Header Error Control information should also be recorded.

10. It is up to the management team to provide a database so this information can be used at a later date, especially the Circuit Noise, Power Influence, and Longitudinal Balance when doing Noise Mitigation. The database would show when the problem first occurred and how many customers and cables are affected. This process is an incredible time saver.

11. All field technicians should inspect every terminal or pedestal that they access. Any bonding issues should be addressed and if that field technician is unable to correct the problem(s) an unacceptable field condition report should be submitted.

When cable maintenance is given the task of reducing AC interference, the first step is to make sure that all cable problems have been rectified before jumping into the process of reducing AC influence. Sections of air-core cable with water present must be replaced, and any splice or encapsulation with water present should be repaired.

Then, and only then, the process of noise mitigation can begin. Remember it is a convoluted process and the root causes come from many sources. All departments must be proactively involved in reducing AC interference. Don’t dump it on the cable maintenance technician.