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My Internet connection routinely becomes slow. It still works, but it feels "clogged", and even the maximum speed it intermittently reaches is only 1/10th of the normal maximum speed. speedtest.net actually reports lower download speeds than upload speeds, when it should be the opposite. Verizon tells me to unplug the DSL modem for 30 seconds and plug it back in, and my connection is restored to normal. Chat support said "There is some connection issue from the central office" and couldn't give me any more details.

So what does unplugging the modem actually do? Why does it fix the problem? Is there some other way to trigger the same effect? It seems to me that something must be poorly designed if it requires this, either the modem itself or something at the central office. What kind of network problems might cause issues like this?

(My modem, specifically, is a Westell Wirespeed 2100 (B90-210015-04), which the Internet says is "simply a bridge modem. It does not have a web interface, routing functions, or built in PPPoE. That means you will have to use a PPPoE client from somewhere else.")

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3 Answers 3

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What is happening here, is that your modem is slowly desynchronizing from the ISP. This desynchronization causes packets you are sending and receiving to become corrupt, requiring them to be resent, and thus, lowering your speed.

To fix this issue easier, I suggest a simple solution. Get one of those outlet timers that people use for when they go on vacation and want to have lights turned on. Then set it to power the router from, say, 4:16am to 4:15am, so that it is powercycled during a time you're not going to be using it in the first place.

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That's a damn good idea. I already have the router set to disconnect and reconnect every night, and I was imagining writing some stupid script to check if the speed has decreased and somehow reset the modem, but your idea is infinitely smarter. :D Do you have a reference for the desynchronization and corruption? I'm not sure what that really means. –  endolith Sep 3 '10 at 23:25
Nothing I can link to. One of the previous VPs at my job used to make telco equipment, and explained it to me. Essentially, a DSL moden has several "channels", each with a clock rate. These clocks occasionally desynch and, thus, disconnect from that channel. Less channels = less bandwith. Powercycling the modem makes it flush and re-negotiate all the channels, putting them back in synch. –  Ryan Gooler Sep 4 '10 at 4:22
Seems like a well-designed system would do that automatically. en.wikipedia.org/wiki/Digital_Subscriber_Line#Basic_technology says "Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether they are usable." Oh well. I just picked up a digital timer that can be programmed to the minute. :) In this case, channels are just frequency bands, like analog TV channels, higher than audio. Some telephone wires can handle more bandwidth/channels than others, and noise of channels may change over time. –  endolith Sep 4 '10 at 19:51
And since the higher frequencies are used for downstream en.wikipedia.org/wiki/File:ADSL_frequency_plan.svg, I think it makes sense that my downstream connection is the one that drops, while upstream is largely unaffected. It's conceivable that something is intermittently cutting out high frequencies, and the modem is dropping those channels. –  endolith Sep 5 '10 at 16:06

Unplugging the modem forces a refresh from the main server, basically. There's more to it than that, but you are resetting the modem, which causes it to update it's settings from the ISP. The network issues are all going to be on the ISP end, and in that case there is very little you can do as the end user to fix it.

ISP support people are notorious for the modem reset, as well as resetting routers. Also, don't tell them you have a router if you can avoid it (if you do) since they will unconditionally blame the router for the issue. The classic line is "Our signal looks good to the modem, so it must be an issue on your end."

The short answer is, it's an ISP issue and the fix is to get a new ISP. You may have an issue with the actual modem, but it will be hard to convince them to replace it for you.

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I believe that I also resets the memory of the DSL Modem, and since most ISP ship out cheap modems, the memory gets used up pretty quick. –  KronoS Sep 3 '10 at 15:25
@KronoS - Good point! –  JNK Sep 3 '10 at 15:42
Yeah, I'm asking about the "more to it than that" part. –  endolith Sep 3 '10 at 16:05

From Tefus FAQ, section "Some network basics about internet service" :

if Telus has oversubscribed your trunk cable (has too many customers with ADSL on the set of cables going from your place to your ADSL port), then there will be interference between your set of telephone wires carrying your ADSL signal and the signal on the telephone wires of the person next to you in the cable bundle. When this happens, BOTH your ADSL modem and the other ADSL modem slow down until the bit-error-rate decays to an acceptable level. This is completely automatic - you have NO control over this. Now, extend this over multiple connections. As the load on the system imposed by users increases (time-of-day) the amount of interference increases. ALL the modems then start to throttle - quite correctly - so that everyone gets at least as much of the pie as they can get without robbing their neighbour.

Another thing - the higher the maximum ADSL transfer rate, the LOWER the number of ADSL pairs that are allowed in a cable bundle because the higher-frequency-signals used to achieve the higher ADSL transfer rate spread further from your pair than the lower-frequency-signals used for a lower ADSL transfer rate.

So, the result of the above is that if Telus offers high-speed ADSL to everyone - and a cable that has been working fine with an ADSL-pair-density originally set for 1.5Mb/s is suddenly faced with a lot of those people upgrading to 3MB or 6Mb/s, guess what happens? The higher interference caused by the higher-transfer-rate signals causes EVERYONE to slow down, because there are more EXISTING ADSL pairs in that cable than the enhanced or extreme-speed-service can support. And it only gets worse for every customer Telus upgrades from standard to enhanced or extreme service.

Now, please note that this trunk-cable-interference occurs on the Telus side of the demarc. You have NO control over this - trunk-cable ADSL density is a function of decisions made by Telus sales. IMO, from the results being shown, both enhanced and extreme service is massively oversubscribed - and a whole bunch of enhanced and extreme users should be downgraded back to 1.5Mb/s to allow the system to recover stability at that rate.

So it seems like your ISP simply has too many high-speed subscribers on your cable bundle. My guess is that by resetting the router you are restarting the line at its normal speed, then again sliding down the hill as the interference from the other lines starts biting in.

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I don't think that's related. This is an issue where the speed drops to less than 1% of the normal speed. 25 kbit/s instead of 3000 kbit/s. And it's restored to 3000 after a reset. –  endolith Sep 3 '10 at 16:11
@endolith: If the above explanation holds, then your ISP has made some BIG cabling mistakes in your area. Or maybe your router is dying : Try another one and see. –  harrymc Sep 3 '10 at 16:32
Router or modem? Rebooting the router doesn't fix the problem. Unplugging the modem does. –  endolith Sep 3 '10 at 17:50
Also, it's worked fine for years, with this happening only occasionally. Lately it happens all the time. I wonder if it's because I started running an Ubuntu LiveCD on one of my computers. –  endolith Sep 3 '10 at 17:52
@endolith: Computer equipment doesn't improve with age. However, I still think the problem is with Verizon cabling. Otherwise how would they know how to fix it, even temporarily? –  harrymc Sep 3 '10 at 18:26

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