February 4, 2009

Fixing DSL lost sync problem

Filed under: Hardware,Misc — Nate Lawson @ 11:46 am

I have had an annoying problem for almost a year. Whenever someone picks up our phone, the DSL modem would lose sync for a minute. Usually that was enough for some connections to time out. Since we don’t use the home phone much, I put up with this longer than I should have.

I called AT&T to have them check out the line. It passed their automated line test. Before this, I had carefully narrowed down the problem. I unplugged all phones from their jacks and made sure each had a proper DSL filter on them. I checked the alarm system. I tried with a different phone to be sure it wasn’t that. I moved the DSL modem to another jack. No difference. Picking up the phone or going back on hook would cause the modem to lose sync. At all other times, it was fine.

The tech came out and did some line quality tests. We disconnected the internal wiring and plugged the DSL modem directly into the external wiring. The problem still happened. He called for some assistance but his support was baffled too. He finally apologized and said maybe the modem was bad.

Last night, I tried with a different modem and had the same issue. I did some more looking and found a bit of information on this. Back in the old days, Pac Bell would install an MTU (maintenance test unit) or “half ringer”. This device allowed them to do a line test without the customer being involved. However, the voltage change of going on-hook causes it to “bounce” the line. Before DSL, this didn’t matter because no one was on the line to hear the bounce. DSL is like an always-on modem connection so any noise or interruption will cause it to restart the sync cycle and you lose your Internet for a minute.

I dug into my telco box (NID) this morning and found this was the problem. To prevent others from wasting hours arguing with phone support that there really is a line problem, here’s how to diagnose this yourself. I’ll use my box as an example, but keep in mind these devices come in various shapes.

Telco box (NID) from the outside

Telco box (NID) from the outside

First, find your telco box. This is where wires enter from the street and connections are made to your inside wiring. There’s a screw on the right that allows you to open the cover.

Inside the telco box

Inside the telco box

Once you open the cover, you’ll see two sections. The inside wiring is on the right and is accessible by opening each terminal cover. The telco side uses a special screw so it’s harder for you to open. In most cases, you won’t need to open that side anyway. As you can see, only the top two terminals of my box are in use for inside wiring. The others are still available. If removing an MTU, you only need to do it from lines that are actually used. I found that every single one of these terminals had an MTU behind it!

Inside AT&T's side of the point of demarcation

Inside AT&T's side of the point of demarcation

Just to be thorough, I checked inside AT&T’s side of the terminals. Indeed there is no MTU here, just some wiring posts.

Finding the MTU

Finding an MTU

The MTU is the little black circuit board here, behind the terminals. It is wired in series with the inside wiring so I can’t just cut it out. Some people cut it out and then use gel-filled wire nuts to splice the wires. I chose an easier and less clean route of stripping the wires and attaching them directly to the screws on the right side.

The finished wiring job

The finished wiring job

I repeated this for both terminals that were in use. I didn’t bother with the others for now. Finally, I put everything back together and tested for dial tone. DSL was working and the problem was gone!

Here are some other links to info about this problem and pictures of other MTU devices.

All in all, this wasted about 6 hours of my time troubleshooting, calling AT&T, explaining it to the tech, etc. Too bad I can’t bill them for my time. I hope this article will save your time and that the telcos will educate their support staff more on this very common problem.


  1. Haha. I had this exact same problem and like you put off figuring it out because I did not want to deal with AT&T. After a year or so we got rid of our land line and went to all cell phones so the problem never happens anymore. I should check my box and see if any MTUs are installed. On a separate note I recently spent an hour and half on the phone with AT&T tech support to fix an issue on their end where I knew exactly what was wrong in advance. It took the first hour and 15 minutes to get past Tier 1 in India until I got to Tier 2 in Texas who simply believed me and fixed the issue right away. And that is why I avoid calling like the plague.

    Comment by Eric Collins — February 4, 2009 @ 2:45 pm

  2. I too have had problems. I was told in AT&T’s DSL training, that DSL will not work adequately if you are not within 13,000 feet of the office.

    Most likely you are not in adequate distance from the telco office.

    AT&T technicians should check this out before they hook up service.

    Comment by Sandy — September 10, 2009 @ 9:09 am

    • Nope, I’m less than 3000 feet from the CO. Please re-read the article and see that removing the MTU fixed the problem completely. Perhaps you can tell AT&T that their DSL training doesn’t cover this common issue?

      Comment by Nate Lawson — September 14, 2009 @ 5:09 pm

  3. I did a google search, “why does my dsl drop when phone rings”. This page came up near the top. I followed your steps, and in about 20 minutes I had fixed a problem I’d had for YEARS.
    Thanks so much!!!!!

    Comment by greg — March 18, 2010 @ 5:31 pm

    • Thanks, greg. I’m glad to hear it helped someone. That makes it worth the time it took to write up.

      Comment by Nate Lawson — March 19, 2010 @ 11:57 am

  4. Thank you very much! I’ve been trying to solve this problem with various filters and splitters for the last month. I cut out the MTU’s and everything now works like it should. Wish I’d found this site sooner… Cheers!

    Comment by Solved — October 24, 2010 @ 11:15 am

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