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Cable Internet Issues -- bad cable modem?
June 27, 2014 8:17 AM   Subscribe

I've been having a lot of problems with my Comcast cable internet (both performance, and with their service, but this question focuses on performance) over the past few weeks. Is this really caused by a bad cable modem? Many details inside.

Downloads are bouncing between very slow (~1.5Mbps per comcast's speedtest site) and very very slow (small bits of text will load on pages, and that's it). Interestingly, upload speed on their test site was ~5Mbps. Following 7 support calls, 1 chat, 1 hangup on me by management, 0 responses on twitter (other than from AT&T) and 2 no-shows by comcast for scheduled tech appointments, someone finally showed up. He looked at the modem, listened to me explain the problem, and immediately said that the modem is bad.

I currently have a Motorola SB6141; I purchased it March 31, 2013. My question is: before I go out and buy a new modem, is this logical? Does it make sense that a bad modem will cause bad download speeds, but not upload? Do these things really die within 14 months? Wouldn't it show up as bad in their diagnostic tests (that all show the modem as working perfectly, according to multiple support people)? Any other suggestions for me to try, before replacing this stupid thing?

They refused to plug in a test modem to see if that would get different results, unless I agreed to rent one from them.

Thanks for your help.
posted by inigo2 to Technology (22 answers total) 1 user marked this as a favorite
 
Datapoint..I'm in Boston and having the same Comcast slow speed/relatively new modem problem.
posted by kinetic at 8:30 AM on June 27


I have Comcast and I had internet problems that were magically cured by a new modem. In my case, I had an old Motorola Surfboard that was not DOCSIS 3.0 (which yours is, so YMMV), but it was basically the same symptoms you had. Slow loading and stuttering connections. I bought a new modem, got it hooked up, and I was off and running. It's been close to two years since with no problems at all.
posted by AgentRocket at 8:31 AM on June 27


It's not implausible. The modem contains circuitry both to drive a signal onto the cable, and to receive a signal from it; if a fault has developed on the receive side, it could easily be the cause of what you're seeing.

Hardware faults do occur pretty much at random. They're rare, because modern hardware is pretty good, but every now and then it will be your day to be unlucky.

One thing I'd personally check before replacing the modem: if it has a wall-wart power supply, try it with another wart that has the same output voltage, the same or greater output current rating, and a compatible plug. If the power supply is failing in a way that makes it electrically noisy, I wouldn't be surprised to find that affecting receive performance more than transmit performance.
posted by flabdablet at 8:38 AM on June 27


Did you try to contact @ComcastWill on twitter?

My experience is that you can get good service from Comcast if you press your way past the first layers of contact.
posted by mullacc at 9:05 AM on June 27


One thing I'd personally check before replacing the modem: if it has a wall-wart power supply, try it with another wart that has the same output voltage, the same or greater output current rating, and a compatible plug.

Good suggestion, thanks. Sadly, just tried it and no change.

Did you try to contact @ComcastWill on twitter?

On Twitter I have tried contacting:
@comcast
@comcastcares (who I believe is the same person as @ComcastWill)
@XFINITY
@ComcastMike
@ComcastMelissa
@ComcastMark
@ComcastLarry
@ComcastKim
@ComcastOrlando
@comcastbusiness

And the one time I got past the first layer of contact on the phone, the guy hung up on me.
posted by inigo2 at 9:10 AM on June 27


Check the modem's status page at http://192.168.100.1/

If your downstream SNR reading is below about 35dB, you will have serious problems. It should be closer to 40dB. If your power level is below about -10dBmV, that indicates a problem as well. On the upstream side, the power level should be below 50dBmV. 50 is pretty marginal and will likely cause trouble.

If all those parameters are normal, the problem is most likely congestion and not a modem problem. However, a DOCSIS 3.0 modem may in fact help because it bonds multiple downstream channels, which generally will get you more consistent speeds even on tiers that don't allow you to use the extra bandwidth.
posted by wierdo at 9:11 AM on June 27 [3 favorites]


Oh, oops, the 6141 is a DOCSIS 3 modem, so it's most likely congestion unless the modem is reporting bad levels. Do you also have a cable box? If so, are you experiencing any pixelization or other video problems?

FWIW, cable modems rarely go bad in my experience. I have had squirrels chew up my cable lines more often than I've had the modem go bad. Seriously. (3-0 in favor of the squirrels so far)
posted by wierdo at 9:15 AM on June 27


I had a cable modem that would occasionally go down for several minutes but showed otherwise fine signal levels. Replacing it fixed the problem. These things do "go bad" (probably due to crappy capacitors in the power supply).
posted by ryanrs at 9:16 AM on June 27


Downstream Signal to Noise Ratio shows 39dB for Bonding Channel Value 1, and 38 for 2-8.
Power Levels are (all in dBmV) 8, 7, 7, 7, 7, 6, 6, 6 for Channels 1-8 respectively.

Upstream power is 50, 50 and 49 dBmV for channels 11, 10, and 12, respectively. So -- does this imply that there's an issue with the modem?

I do have a cable box (that I think Comcast has finally convinced me to cancel, but that's neither here nor there). TV (including HD) is fine.
posted by inigo2 at 9:22 AM on June 27


Those are marginal signal levels. You should have much better downstream SNR for that received power level. I'm at about -2dBmV downstream power with the same SNR. Also, upstream power of fully 50dBmV is higher than it should be, but not quite out of spec.

If you look at the log page, does it appear to be losing sync periodically? (It should say something about ranging every time it has to resync)

FWIW, you could buy a modem from Best Buy and return it within 30 days if it doesn't fix the problem.
posted by wierdo at 9:40 AM on June 27


Have you recently moved the modem to another outlet or plugged another device into your coaxial network? If so, you might disconnect the new device or move the modem back and see if that solves your problem. I am not an expert in this area, but I have been on a few troubleshooting calls with cable techs and seen problems caused by too much load on the network. Given what @wierdo said SNR and power levels, sounds like you are somewhat close to the edge there.

If your home network hasn't changed -- do you live in a condo, apartment complex, or duplex where someone else might have put too much load on your tap?

Another alternative would be to buy a new modem from a place that accepts returns, see if it solves your problem, and if it doesn't return it.
posted by elmay at 9:41 AM on June 27


I'm in an apartment, and nothing has changed (haven't moved any of the modem, router, computers), haven't added/removed anything using power from those outlets (or any other outlets in my apt).

I guess the buy-and-return route (spending $20 more than I would on Amazon) is what I'm leaning towards. Comcast is the absolute worst.
posted by inigo2 at 9:53 AM on June 27


If you look at the log page, does it appear to be losing sync periodically? (It should say something about ranging every time it has to resync)

Some messages, but nothing in the past hour. Seems like these three repeat (newest message at the top):
MDD Recovery following MDD Loss;....
Lost MDD Timeout;...
DS Partial Service Fallback..
posted by inigo2 at 9:58 AM on June 27


If a new modem doesn't help, you can try contacting the Comcast Customer Connect Team on the broadbandreports forum.
posted by LuckySeven~ at 10:28 AM on June 27


I'm loathe to suggest this because your upstream power is already so high, but since your downstream is hot, if you have a 2 way splitter on hand (or 50 feet of RG6 cable) you could try plugging it in ahead of the cable modem to see if getting the downstream power below +5 helps. Might save a trip to the store, anyway.
posted by wierdo at 10:29 AM on June 27


Well, I couldn't find a splitter in my magic-bag-of-random-crap, so I did some quick googling and found that the Zoom 5341J (a) had good reviews, (b) was in stock at my mostly nearby Best Buy, and (c) was more or less the same price there as Amazon. So, I grabbed it, just finished hooking it up, and things look a lot better.

I appreciate all the tips/things to look at here, definitely educated me on cable modems a bit. (I still hate Comcast, though.)
posted by inigo2 at 1:07 PM on June 27


My Comcast guy warned me when he was installing my cable internet that sometimes you can be on a crowded wireless channel so you should switch the channel your internet connection is on. He recommended an app called Wifi Analyzer to see if my connection is on a crowded channel. If so, switching may fix my internet speed, he said. To be honest, I haven't run into this problem though. Anytime my internet doesn't work, I simply turn off my modem and turn it back on and it fixes itself.

If you don't go wireless and are plugged in directly, then I don't know. But if you are wireless, plug your computer into the modem directly with an ethernet cable and see if that fixes it. That would narrow things down to your wifi then.
posted by AppleTurnover at 8:14 PM on June 27


This unstable connection is such a common problem that it has resulted in a new word entering the lexicon. You've been comcasted.
posted by Yowser at 2:11 AM on June 28


The Moto SB6141 is a great modem with a fairly bullet-proof reputation, and I'd be surprised that it was the problem. Do you have a router, too? If so, have you tried hooking your modem straight to the computer to see if that improves anything?

When Comcast forced me to "upgrade" my tv service to digital, their tech did a test on my feed and discovered that it was pretty weak throughout the house, which affected both the tv and internet service. He installed an amplifier at the main line into the house. You might be having the same issue.
posted by Thorzdad at 6:22 AM on June 28


sometimes you can be on a crowded wireless channel

It was wired.

The Moto SB6141 is a great modem with a fairly bullet-proof reputation, and I'd be surprised that it was the problem. Do you have a router, too? If so, have you tried hooking your modem straight to the computer to see if that improves anything?

I tried wired direct to the modem, wired through the router, multiple ethernet cables in both of those scenarios, and wireless. None of them had different results.

My google searches came up with a decent number of people with issues with the sb6141 (though obviously it's the people with issues that comment online).

Replacing the modem has worked thus far, anyway
posted by inigo2 at 11:03 AM on June 28


Just for kicks, can you post the signal levels you get with the new modem?
posted by wierdo at 12:47 PM on June 28


Hopefully this shows up legibly; if not, I'll follow up with a screencap.
Startup Procedure
Procedure	Status	Comment
Acquire Downstream Channel	657000000 Hz	Locked
Connectivity State	OK	Operational
Boot State	 	 
Configuration File	OK	
Security	Enabled	BPI+

Downstream Bonded Channels
Channel	Modulation	Frequency	Power	SNR	Correctables	Uncorrectables
1	QAM256	657000000 Hz	 3.9 dBmV	40.9 dB	52	0
2	QAM256	615000000 Hz	 6.5 dBmV	42.0 dB	1	0
3	QAM256	621000000 Hz	 6.0 dBmV	41.8 dB	3	0
4	QAM256	627000000 Hz	 6.1 dBmV	41.9 dB	2	0
5	QAM256	633000000 Hz	 5.5 dBmV	41.7 dB	10	0
6	QAM256	639000000 Hz	 5.8 dBmV	41.9 dB	20	0
7	QAM256	645000000 Hz	 5.0 dBmV	41.5 dB	18	0
8	QAM256	651000000 Hz	 4.7 dBmV	41.4 dB	30	0

Total Correctables	Total Uncorrectables
136	0


Upstream Bonded Channels
Channel	US Channel Type	Symbol Rate	Frequency	Power
1	ATDMA	5120 Ksym/sec	36000000 Hz	50.8 dBmV
2	ATDMA	5120 Ksym/sec	29400000 Hz	50.5 dBmV
3	ATDMA	5120 Ksym/sec	23000000 Hz	49.5 dBmV
4	Unknown	0 Ksym/sec	0 Hz	 0.0 dBmV

posted by inigo2 at 1:28 PM on June 28 [1 favorite]


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