Question Netgear CM500 starting losing internet

[Thump553]

Basic facts: Xfinity 200Mbps service, using Netgear CM500 modem bought new and installed Sept 2020 with little to no problems until recently.

About 2-3 weeks ago lost internet. Disconnected modem power for a minute, reconnected, still offline. Contacted Xfinity online (phone browser) their service sent something over the line that got the modem working again.

Same problem, same solution again two nights ago.

Same problem 5am today. This time xFinity computer said couldn't reach my modem, check power and cable connections. Unfastened everything and literally after almost two hours the internet came back on.

Any idea what's going on? Is my modem going bad? Any suggestions for a replacement? This is an enormous PIA because our cellphone connection without WiFi is painfully slow (seems slower than dialup) in this area.

Every time Xfinity says no problems in the area.

PS: I am a neophyte/idiot regarding networks (that probably is obvious). Thanks for any help.

 

[mindless1]

I'd first try a different power adapter. Looks like it uses 12V, 1.5A (or higher), standard barrel plug type.

Replacement modems, do you want to go cheap to only support 200Mb, or spend more for a little futureproofing of potentially higher speed plan than 200Mb?

It's only Docsis 3.0, 16x4 channel and you can pick up one of those in a major brand (check what xfinity supports) cheap. IDK your location but what it looks like is that xfinity supports using a docsis 3.0, but on their support page, they aren't listing those, not even the model you have. They do list the SB8200 below.

Arris SB6190-RB, Docsis 3.0 refurb $26

Amazon.com

(may work but may not be on their support page)

Amazon warehouse Arris SB8200 Docsis 3.1 $54:

Amazon.com

("Ships from Sold by Amazon Warehouse", not 3rd party)

HOWEVER, once upon a time ago, I started getting dropped connections every fall when the night temperature fell within a certain range. It was an old cable run to the premises and at just a certain temperature range it would drop out. I think the connector at the street box was flaky. Worked fine again once it got colder outside towards winter, and worked fine all the time with a cable line amp on the line, and then a few years later, the line got worse and was replaced, then worked fine w/o the cable line amp.

I'm suggesting it would be useful to log your modem's signal strength when it stops working and if it's near (negative) -15dB or lower downstream, you probably have an intermittent cable line problem. You should be able to see the signal strength by logging in with your browser pointed to something like http://192.168.100.1/ or whatever your modem's IP addr. happens to be. The netgear page below suggests a minium signal strength of -7dB but for years I had no trouble at around -10dB to -12dB... that low due to # of splitters I needed to get everything hooked up including CATV.

How do I view the downstream and upstream power levels of my cable modem or modem router?

Downstream and upstream power levels are important when determining if the cable line that is connected to your cable modem or modem router is receiving sufficient power. Good Downstream power levels are within -7 to +7 dBmV. Good Upstream power levels are within 38-48 dBmV. To view the upstream...

kb.netgear.com

 

[Thump553]

I'd first try a different power adapter. Looks like it uses 12V, 1.5A (or higher), standard barrel plug type.

Replacement modems, do you want to go cheap to only support 200Mb, or spend more for a little futureproofing of potentially higher speed plan than 200Mb?

It's only Docsis 3.0, 16x4 channel and you can pick up one of those in a major brand (check what xfinity supports) cheap. IDK your location but what it looks like is that xfinity supports using a docsis 3.0, but on their support page, they aren't listing those, not even the model you have. They do list the SB8200 below.

Arris SB6190-RB, Docsis 3.0 refurb $26

Amazon.com

(may work but may not be on their support page)

Amazon warehouse Arris SB8200 Docsis 3.1 $54:

Amazon.com

("Ships from Sold by Amazon Warehouse", not 3rd party)

HOWEVER, once upon a time ago, I started getting dropped connections every fall when the night temperature fell within a certain range. It was an old cable run to the premises and at just a certain temperature range it would drop out. I think the connector at the street box was flaky. Worked fine again once it got colder outside towards winter, and worked fine all the time with a cable line amp on the line, and then a few years later, the line got worse and was replaced, then worked fine w/o the cable line amp.

I'm suggesting it would be useful to log your modem's signal strength when it stops working and if it's near (negative) -15dB or lower, you probably have an intermittent cable line problem. You should be able to see the signal strength by logging in with your browser pointed to something like http://192.168.100.1/ or whatever your modem's IP addr. happens to be.

Thanks for the insight,

Futureproofing is pretty useless for me. Although I live about twenty miles from the center of Hartford (state capitol of CT) we didn't even have basic cable available until the early 90's and even still 200Mbps is faster available here. That's fast enough for me. Frontier's been talking about bringing fiber in to the area, but they have been singing that tune for a couple of years. They did run a new line on the telephone poles nearby a few months ago but my street has buried lines and nothing done there.

Your description of cable line problems describes our house to a T. We have high ledge and rocky soil in the yard and already had the connection from the street to the yard replaced maybe twenty years ago. Our street has buried cable and power lines and nearly all the Xfinity street boxes and/or box covers are broken and have been for a long time.

My modem was on Xfinity's approved list when I bought it in 2020 but like you say it seems not to be on the list now.

Not having much luck figuring out what my modem's IP address is-the link you posted asks for a user name and password, which I don't know/recall. Will try again later.

BTW after I posted Xfinity called (all the way from India) and IF I understood him correctly they were having intermittent problems at their end when they were trying to reset my modem. Also will check my junk drawer/junk box for another power adapter.

 

[mindless1]

The link had the correct IP # for your modem since you were presented with it's login page.

Unless you have changed the username and password (which it is advisable to do!) the default was user name admin, password is password.

There might be a little hole for a reset switch somewhere on the modem, and a power on sequence where you can press it to reset it, putting the username and password back at the default listed above. If present this should be in the user manual, as should the default username and password if the above are incorrect. If you haven't configured anything custom/important on the modem, there is no harm in resetting it.

 

[Thump553]

I was able to get into the event log (hint-I said I was an idiot hint2 I didn't need to do a factory reset). I got the event log-it only covered the last ten minutes or so. My wife said we lost internet about 1AM and I started trying to fix it about 5AM. Here's what the log says but I'm guessing it doesn't say anything about the cause of the problem:

______________________
Time Priority Description
2023-10-9, 07:09:58 Warning (5) MDD message timeout;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:90:f0:06:02:00;CM-QOS=1.1;CM-VER=3.0;
2023-10-9, 07:05:14 Critical (3) Started Unicast Maintenance Ranging - No Response received - T3 time-out;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:90:f0:06:02:00;CM-QOS=1.1;CM-VER=3.0;
2023-10-9, 07:04:58 Notice (6) Honoring MDD; IP provisioning mode = IPv6
2023-10-9, 07:04:51 Critical (3) SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:90:f0:06:02:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:04:50 Critical (3) DHCP failed - DHCP Solicit sent, No DHCP Advertise received;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:90:f0:06:02:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:04:23 Critical (3) Started Unicast Maintenance Ranging - No Response received - T3 time-out;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:90:f0:06:02:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:04:10 Notice (6) Honoring MDD; IP provisioning mode = IPv6
2023-10-9, 07:04:07 Critical (3) Started Unicast Maintenance Ranging - No Response received - T3 time-out;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:90:f0:06:02:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:03:56 Critical (3) SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:03:55 Critical (3) SYNC Timing Synchronization failure - Failed to acquire FEC framing;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:03:52 Critical (3) SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:03:52 Critical (3) SYNC Timing Synchronization failure - Failed to acquire FEC framing;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:03:52 Critical (3) No UCDs Received - Timeout;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:03:42 Warning (5) Lost MDD Timeout;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:03:35 Critical (3) SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:03:34 Critical (3) No UCDs Received - Timeout;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:03:24 Warning (5) Lost MDD Timeout;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:03:17 Critical (3) SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:03:16 Critical (3) No UCDs Received - Timeout;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:03:06 Warning (5) Lost MDD Timeout;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:02:59 Critical (3) SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:02:58 Critical (3) No UCDs Received - Timeout;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:02:48 Warning (5) Lost MDD Timeout;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:02:41 Critical (3) SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:02:40 Critical (3) No UCDs Received - Timeout;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:02:30 Warning (5) Lost MDD Timeout;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:02:23 Critical (3) SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:02:22 Critical (3) No UCDs Received - Timeout;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:02:12 Warning (5) Lost MDD Timeout;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:02:05 Critical (3) SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:02:04 Critical (3) No UCDs Received - Timeout;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:01:54 Warning (5) Lost MDD Timeout;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:01:47 Critical (3) SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:01:46 Critical (3) No UCDs Received - Timeout;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:01:36 Warning (5) Lost MDD Timeout;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:01:29 Critical (3) SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:01:28 Critical (3) No UCDs Received - Timeout;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:01:18 Warning (5) Lost MDD Timeout;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:01:11 Critical (3) SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;
2023-10-9, 07:01:10 Critical (3) No UCDs Received - Timeout;;CM-MAC=9c:c9:eb:4c:6e:60;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.0;CM-VER=3.0;

 

[mindless1]

It shows that at least during that time period, the modem was stable enough to make the log, leading me to continue to wonder if it is not the modem but rather the cable connection itself.

Best to look at downstream signal strength, next time the connection is lost. What is the signal strength right now with it working?

 

[Thump553]

If I'm reading in the correct place there the downstream power ranges from 6.6 to 7.7 dBmV for sixteen channels, and upstream from 42.9 to 46dBmV for four channels.

No problems since it came back on yesterday morning.

 

[mindless1]

The power levels look good, now a waiting game to see what they are like when it drops out.