What is wrong with my cable internet?

October 23, 2017 8:27 AM

I'm having a problem with my cable internet that I'm hoping that you can help my diagnose. In short, every time I try to run a backup (large, hundreds of gigabytes), my entire network connection grinds to a halt and I have to reboot my cable modem. How can I troubleshoot this problem?

I'll try to be as detailed as I can, but am happy to respond to further requests for information.

I have Comcast and a Arris SURFboard SB6121 cable modem that I bought ~6 years ago (so old that it was made by Motorola when I bought it). It is hooked up to a relatively recently-bought ASUS router running DD-WRT. There is a second router (also running DD-WRT) acting as an access point that's connected to the first over Powerline ethernet adapters.

When I try to backup (to Backblaze), it will sometimes run for a while (or sometimes just a few minutes) before it completely bogs down the network; I can't connect any device to any outside internet site. I am moving quite a lot of data to Backblaze, but it's not moving it at tremendous speed (it appears it's only uploading at the rate of 1 or 2 Mb/s). I have successfully uploaded large amounts of data over this connection before (the recent shuttering of Crashplan is why I'm now backing up to BB), so this behavior is new.

Possibly relevant: the "upstream" light on my cable modem doesn't appear to light up. Or rather, it does during a cable modem reboot, but not after the cable modem has connected to the internet.

I can still fully connect to my wireless network when this happens; it just doesn't connect me to the internet. I can't reach the status page for my cable modem when it gets in this state, either.

My working theory is that it's the cable modem; the uplink light turning off and the failure mode being during upload just seems like too big of a coincidence. Is there a way I can test this (other than replacing the cable modem)? Is there something else that could be causing it?

posted by Betelgeuse to Computers & Internet (8 answers total)

Best answer: I also have Comcast, and for the past year my internet speeds would grind to a crawl intermittently, with increasing frequency over time. Rebooting the modem would sometimes but not always fix the issue. Comcast's first-line technical support told me different things every time I called them, and none of them fixed the problem in the long term.

Then, just a few weeks ago, a second-level support tech told me I should try replacing my modem. My old modem was a six-year-old SB6120 (slightly different model but same generation as yours). I replaced it with a Netgear CM500 (based on this Wirecutter review) and everything has been working great since then. (The new modem also supports higher max speeds than the old one, though this only matters if your upstream connection supports them too.)

I can't promise this will fix your problem, but I hope it helps.
posted by mbrubeck at 8:45 AM on October 23, 2017 [2 favorites]

Best answer: I also recommend the Netgear modem. I have never had it just flake out on me. If my network connection is down, it's because there is a Comcast outage in my area.
posted by kindall at 8:51 AM on October 23, 2017

Best answer: Yeah, your modem is outdated. I can almost guarantee that the problem will go away the second you plug that new modem in....
posted by kuanes at 9:14 AM on October 23, 2017

Best answer: Replacing the modem is a good starting point, but as someone who's bought three modems and two wireless routers to fix a problem with my connection (my connection drops due to upstream line noise on cable company's end) I can tell you no fix is guaranteed until it actually works.

Some other things to explore:

What up/down bandwidth are you paying for?

What do you get on a speed test on Comcast, a third-party speed test site, and on Backblaze's own speed test? I'd speed limit Backblaze well within your limit.

Have you fully updated the firmware on your router to the newest DD-WRT?

Does the problem occur if the DD-WRT repeater is removed from the equation? (i.e. is it a repeater issue or an overall wireless/connection issue?

Does the problem occur if the backblaze computer is hooked up via ethernet? (i.e. is it a connection issue or a wireless issue?)

Run Google's Namebench DNS benchmark software and set your DNS servers on the router to their recommendations.

Check the log of your cable modem for unusual events, including T3/T4 timeouts.
posted by bluecore at 10:03 AM on October 23, 2017 [1 favorite]

Best answer: Nthing Replace the modem and see if that fixes the problem.

Do you have automatic throttling turned on in Backblaze?
posted by gregr at 10:08 AM on October 23, 2017

Best answer: If at all possible, it would be helpful to connect your computer directly to the cable modem and try running the backup, so that you can at least pin down if the router or home network is at fault.

I'd also check on Backblaze automatic throttling, and a set of comparison speed tests.

And then I'd probably blame your modem - but only after those steps.

(Our network crawls when there's rain in the area. I'm like 75% convinced about this, and I suspect that there's water leaking into Time Warner's - excuse me, Spectrum's - crappy cables somewhere. But I can't explain how water leaks could happen, or why they'd make an intermittent difference to my network speed without killing it outright, or how to convince Spectrum that it's their problem. One of these days I'll actually do a rigorous test.)
posted by RedOrGreen at 1:13 PM on October 23, 2017

Response by poster: For anyone who needs closure it was, indeed, the cable modem. I replaced my cable modem with my spiffy new Netgear CM500 and, not only did the modem not flake out when put under load, but the upload speeds were much faster.
posted by Betelgeuse at 6:18 AM on October 27, 2017 [1 favorite]

Yay!
posted by kindall at 10:46 AM on October 27, 2017