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Tracking Down Line Interference


Tracking Down Line Interference

This happened to me on Wednesday night.  I thought I'd share this story because it illustrates both the sensitivity of U-Verse VDSL to sources of interference and also illustrates an ad-hoc troubleshooting procedure that may help find these things.


I'm watching TV on Wednesday night, and about 8:30 I get a picture freeze.  Not entirely uncommon, I see it a few times a month.  Usually comes back within 10-15 seconds.  Not so in this case.  I wait 2-3 minutes, picture is still frozen.  So I think there might be some sort of outage.  I go to the computer and try to bring this forum up ... no Internet either. Smiley Surprised


So I browse the forum on my smartphone.  No mention of any outage.  So I bring up my 2Wire RG's web interface.  No VDSL connection ... go look at the RG, blinking red broadband light.  Uh oh.  Not good.


I watch the RG for a few more minutes, and the line is going up and down.  It'll go blinking green, then solid green, then get a solid service light and be up for 10-15 seconds.  But then it loses it again and goes back to a blinking red broadband light.


I log into the RG and look at the C53 page for DSL diagnostics ( ), there's a huge number of retrains listed there.  Normally, my line connects with a Max Rate of about 49,000 kbps.  The first retrain initially showed a max rate of 43,000, and each subsequent retrain is going down.  I have about 10 retrains listed, and the max rate is down to 37,000 on the most recent one.


I look at the bitloading during a brief period where the line was up and there's a large notch/hole in the graph in the 100 kHz - 300 kHz range, which is way down low in the spectrum.  This is some sort of interference, and because the max rate is going down with each retrain, that means the interference is getting worse.


So now I have to try to track down this interference, because at this point I have no TV and no Internet.  I go outside and look around and see if there's anyone doing anything odd, like running a generator or something.  Don't see anything.  I figure the interference must be coming from somewhere inside my house but I have no idea where.


The first thing I try is to see if there's power line interference that's causing it.  I have the RG on a UPS, so I unplug the UPS from the wall and let the RG run on battery power.  This isolates the RG from any power problems.  No dice - line keeps going down, and at this point, it really isn't coming up.  The broadband light is cycling between flashing red and flashing green, but I'm not even briefly getting the service light anymore.


So I think maybe the interference could be coming through Ethernet connections.  I disconnect the Ethernet connections to the RG, no change.  Only thing left is the VDSL line itself, and I have a home run on that directly to the NID.  The only other thing in that path is the alarm system, I even go so far as to shut down the alarm system and remove power from it just to test, but it didn't make any difference.


So I figure there must be some other component in the house that's doing it.  Fine -- go outside to my circuit breaker box and start turning stuff off.  A/C, Furnace, Water Heater, Dryer, Washer, Dishwasher/Garbage Disposer, all major stuff.  I leave only 4 breakers turned on, they're the ones for outlets & lights.


Go back inside -- no change.  Line is still down.


OK, go back outside, shut off the last 4 breakers.  Now I have no power in the house, but the RG is still up on the UPS.


Line comes up in about 20 seconds and stays up.


So I think Wow, I have it isolated.  Turn all breakers back on except the 4 for lighting and outlets.  Go in and check the RG, line is still up.  So I start going back outside and turning the last 4 breakers on one at a time and checking the RG between each one.  Living room - no problem.  Garage/bathrooms - no problem.  Bedrooms 2 & 3 - no problem.  Turn on the master bedroom - line goes down. Smiley Surprised  Turn the breaker back off, line comes back up.


OK, so now I have to divide up the master bedroom.  I figure the likely source is the TV/STB/Slingbox/Popcorn Hour area, so I unplug the whole power strip there and turn the breaker back on.  Line stays up.  Cool, so at this point it's isolated in that mini-entertainment center.


I unplug everything from that power strip and plug the strip by itself back in.  I wait 3-5 minutes just to make sure, but the line stays up.  I then start plugging devices back in one at a time and waiting 3-5 minutes between each one to see what happens.


Line goes down about 2 minutes after I plug one of the Slingboxes in.


I go to unplug it and the power supply brick of the Slingbox is way hotter than normal.  I figure the power supply has gone south.  Line comes back up by the way.


So I go check the RG's parameters -- totally normal. 49,000 kbps max rate, smooth bitloading graph.  I have found the culprit.



This troubleshooting procedure took me about 2 hours to go through.  The amazing thing is what this illustrates:


• The Slingbox has absolutely no physical connection to the RG or any of its wiring.  It's on a different power circuit, it isolated from the rest of the network by a switch, and any other cables only connect to the TV.  The interference here was completely via RF, and was being received by the telephone wiring acting as an antenna.


• VDSL and the 2Wire RGs are highly sensitive to interference from a variety of sources, including sources that are too weak to affect anything else.  But even these weak sources are enough to cause VDSL to fail.


• Troubleshooting this type of interference is time-consuming and beyond the scope of AT&T premises technicians.  Interference of this type will require the homeowner to troubleshoot.



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Message 1 of 58

Re: Tracking Down Line Interference

Thanks, I downloaded a fresh version - rather than using a USB key to copy from my desktop machine. All went as expected.


Relative to my dsl retrains.... things have degraded considerably from my previous bitloading graph. It seems to be related to incoming line or connector, as after I put the cable back into the plastic mounting box, things got worse.


I was suspicious of the data cable AT&T supplied. So I made my own and the bitloading graph on my previous posts were from using that cable. If I re-plug in the AT&T (12 ft) cable, things get worse than they are now.


Technician is coming out tomorrow. Could I have a bad Universal Station protector? or poor grounding on the telephone line (from pole to house) in the NID?


Here is my current status:







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Message 31 of 58

Re: Tracking Down Line Interference

Hi -


This observation has nothing to do with our previous interactions. But I have noticed an interesting anomally on both your pgm screens and the RG screens - relative to MAC addresses of the TV set top boxes. i. e. they differ by a value of +1. You will see the difference in the below screen shots. I'm just curious why this is and how it works.  You'll notice the MAC adrr that ends in :04 on the Channel/Streams screen, now ends in :05 on the COAX/HPNA screen. The set top box displays :5 as it's MAC adddress - very curious??? 










Message 32 of 58

Re: Tracking Down Line Interference

Yes, the U-Verse boxes have two physical network interfaces, one is the Ethernet interface and the other is the coax/HPNA interface.


The MAC address of the coax/HPNA interface is reported in the Coax/HPNA tab, while the MAC address of the Ethernet interface is the one reported in the Channels/Streams tab.


In many STBs, the two MAC addresses are nearly identical except for the final octet, but in others they are not.



As far as your line goes, you have bad interference on the line as well as at least one, possibly more than one bridge tap.  The technician needs to have the I&R group come remove all the bridge taps and condition the line.


Message 33 of 58

Re: Tracking Down Line Interference

Interesting on the MAC addresses, makes sense since both the RG and STB have coax and RJ45 connectability.


On my line problems - argh... what is a bridge tap and why would I have more than one? (is that a 3M scotchlok?)


Thank you sir!

Message 34 of 58

Re: Tracking Down Line Interference

wow, nice explanation on the troubleshooting of your interference problems. will need to remember this. of course, next time the problem will be entirely different ....

thank you SJ



Message 35 of 58

Re: Tracking Down Line Interference

A bridge tap is a extra length of wire attached to your pair somewhere in the middle of the run that is not terminated anywhere.  Bridge taps interfere with xDSL services and have to be removed for proper operation.


Bridge taps are usually left over from a different customer who used to use that pair.


Message 36 of 58

Re: Tracking Down Line Interference

Thank you again. AT&T tech came out this morning and determined this... they installed a new line on the poles this past week and had me "dual connected", not the correct term for this, but this is done so people don't lose phone service when they install a new main line. Good for POTS,  but plays havoc with Uverse lines - ARGH! He corrected the dual connections and things look good. I've noticed I have the fastest max rate I've ever seen, also the largest value for noise margin - YEA! I included screen shots below for reference.



Message 37 of 58

Re: Tracking Down Line Interference

OOPS.. here is the stats page.



Message 38 of 58

Re: Tracking Down Line Interference

I just got done with my fifth visit from a technician.  We were getting internet and phone, but the router stopped sending a signal for TV to the receivers at night.  I was just about to fire ATT when i discovered a shorted outlet in our house (which is ungrounded as well).  Turns out that the short was knocking out my TV signal.  It would only happen at night when we turned the lights out....meaning more current was going through the electrical short.  When ATT finally sent someone with brains, we discussed the issue and we simultaneously discoved that the coax was grounded.  This is a no-no as any house electrica  interferance will backfeed up the ground wire.  I fixed the outlet short and also removing the yellow ground wire where the 2 wire connects to coax has fixed the problem.  Uverse needs clean power to work right and if you are in old home take note.



Message 39 of 58

Re: Tracking Down Line Interference

Good job on your part.  Glad you got an educated tech and the problem is now resolved.  Enjoy.

Message 40 of 58

Re: Tracking Down Line Interference



That bitloading graph and stats look MUCH better.  This looks to be properly working service, and you should have few if any problems now.


Message 41 of 58

Re: Tracking Down Line Interference

Hi - interesting observation/solution relative to having the coax grounded. When I had both Comcast and Direct TV, they grounded the coax splitter. I have grounded the AT&T coax splitter as well.


My ground wire is connected from the coax spiltter to the cold water pipe.


It was my understanding that having it grounded would pass the interference TO ground, not the opposite.


I welcome further information/discussion on this.

Message 42 of 58

Re: Tracking Down Line Interference

I know this is an old post but I'm having a similar issue with "dirty electricity" line interference in the house and maybe someone can point me in the right direction. 


I noticed while watching live or recorded tv from my wireless receiver the show freezes intermittentely. I moved the WAP within 10ft. of the TV with no success. I began to notice that while my TV is freezing my furnace is running too.


So where do I start? What should I be checking for first?


I've purchased some ferrite magnets for the WAP power cord hoping I can reduce some noise but I haven't tried it yet. Anyone else have suggestions?



Message 43 of 58

Re: Tracking Down Line Interference

Kramer_1984 - Is your furnace an oil burner?? Do you have it serviced annually??

It has a couple motors where the brushes could be noisy. What I am curious about is the igniter. I can see where that could be quite noisy (electrically speaking). But, I believe it should only operate for a short period of time. Not continuously like the motors. Perhaps a control is sticking.

A previous post suggested looking for electrical noise with an am radio. Maybe try that near the furnace. Also, some folks had power bricks go bad. Check those out with the am radio.
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Message 44 of 58

Re: Tracking Down Line Interference

I'll give the AM Radio test a chance tonight and will also try to relocate the gateway power supply to a different outlet using a long extention cord. Also, the furnace is not an oil burner type, it's a 2 yr. old high efficiency Lennox gas furnace.


I'll post my findings.



Message 45 of 58
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