I am a business-class CenturyLink customer. I had a new bonded-pair ADSL2+ circuit installed at my new office in downtown Boise, Idaho and within a couple days we started to notice some odd behavior. The modem is a C2000T upgraded to the latest firmware (CTH003-4.12.0.100).
Aside from phones and tablets, there are three computers on this network:
- Alice's laptop: a Thinkpad running Ubuntu Linux 14.04
- Bob's laptop: a Macbook Pro 15" late-2013, running OS X 10.9.4.
- Charlie's desktop: a generic Windows 8 PC
We are able to consistently reproduce a problematic condition when Bob, from his Macbook, starts a sustained upstream transfer (eg, uploading a file via FTP that takes more than a couple seconds).
The symptom is that, for all devices on the network, the internet connection is effectively down. I'll show between 85% and 100% packet loss during these periods. This is also true for Bob, even though his upload continues.
When I swapped the C2000T out for an old C2000A I had sitting at my previous office, the problem went away.
Additional information and troubleshooting information:
- When Bob stops his upload, our ability to access the internet will recover, but it will take between 5 and 30 seconds -- as if the modem is freeing up resources that had been consumed by the saturating upload.
- Problem cannot be reproduced using any other device -- only Bob's Macbook. Although, we have not tried with any other Macbooks, due to lack of availability.
- Problem is reproducible no matter what software or protocol Bob uses. He could be uploading via FTP with Transmit, or FTP via command-line, or even just uploading a small file via SFTP from his IDE. It doesn't matter.
- Problem is reproducible even if Bob is not connected directly to the C2000T's wireless network. We tested this by hanging a little D-Link wireless router off one of the C2000T's ethernet ports and connected Bob to the D-Link's wifi network instead.
- We had a CenturyLink tech swap our C2000T for another C2000T and the problem persists.
- We did NOT try to reproduce the problem on the firmware that the C2000T ships with. We have only tested with the most recent firmware.
- I monitored the "Modem resources" tab of the router's web control panel. The CPU utilization seems to hover between 15% and 25% during normal usage AND while anybody besides Bob is uploading. When Bob uploads we'll see it spike up to near 40%. The other modem resources values remain steady whether he's uploading or not.
- A CenturyLink tech ran a line test between the demarcation and the central office and it was clean.
- A CenturyLink call-center tech claimed to see a large number of "blocks uncorrected" coming across during a Bob upload, but subsequently neither the in-person tech nor another call-center tech were able to see this same thing, so the first call-center tech could have been wrong.
- Curiously, during one troubleshooting session with a CenturyLink call-center tech, we found that if the tech was remotely pinging our modem from his tools, we could avoid the problem. As long as he was actively pinging the modem, Bob could upload and the connection would stay usable for everyone. When the pinging stopped, the problem resumed as before -- connection unusable until Bob stopped the upload.
Our "solution", for now, is that we're borrowing an old C2000A from our old office, and we have a CenturyLink tech (who I know from around town) looking around for a C2000A he can salvage from their parts supply garage. He mentioned that they've had to get new shipments of the C2000A recently, because people are reporting problems with the C2000T, but he said this is a new one for him, mostly he's seeing problems with the C2000T losing PPP authentication.
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