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.
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 ( http://192.168.1.254/xslt?PAGE=C_5_3 ), 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. 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.