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Modem is constantly dropping upstream connection

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Modem is constantly dropping upstream connection

About 2 weeks ago our Xfinity cable connection started dropping out randomly. I've also noticed an average of 2-3% packet loss when the connection is stable. Restarting the modem seems to solve the issue temporarily, but it starts happening again after a day at most, with connection drops happening as frequently as every 5 minutes.

 

We're using a Netgear CM500 cable modem verified by Comcast. These are the log messages I see during a span of time when the connection becomes very unstable and then drops out completely: 

 

2019-9-6, 08:29:17 Critical (3) SYNC Timing Synchronization failure - Failed to acquire FEC framing;CM-MAC=b0:b9:8a:79:4b:20;CMTS-MAC=2c:31:24:61:60:ea;CM-QOS=1.1;CM-VER=3.0;
2019-9-6, 08:29:09 Critical (3) Started Unicast Maintenance Ranging - No Response received - T3 time-out;CM-MAC=b0:b9:8a:79:4b:20;CMTS-MAC=2c:31:24:61:60:ea;CM-QOS=1.1;CM-VER=3.0;
2019-9-6, 08:26:56 Warning (5) MDD message timeout;CM-MAC=b0:b9:8a:79:4b:20;CMTS-MAC=2c:31:24:61:60:ea;CM-QOS=1.1;CM-VER=3.0;
2019-9-6, 08:26:37 Critical (3) SYNC Timing Synchronization failure - Failed to receive MAC SYNC frame within time-out period;CM-MAC=b0:b9:8a:79:4b:20;CMTS-MAC=2c:31:24:61:60:ea;CM-QOS=1.1;CM-VER=3.0;
2019-9-6, 08:26:25 Warning (5) MDD message timeout;CM-MAC=b0:b9:8a:79:4b:20;CMTS-MAC=2c:31:24:61:60:ea;CM-QOS=1.1;CM-VER=3.0;
2019-9-6, 08:26:23 Critical (3) SYNC Timing Synchronization failure - Failed to acquire FEC framing;CM-MAC=b0:b9:8a:79:4b:20;CMTS-MAC=2c:31:24:61:60:ea;CM-QOS=1.1;CM-VER=3.0;
2019-9-6, 08:26:09 Warning (5) MDD message timeout;CM-MAC=b0:b9:8a:79:4b:20;CMTS-MAC=2c:31:24:61:60:ea;CM-QOS=1.1;CM-VER=3.0;

 

And here is what the channel bonding looks like at this time. I noticed that usually all downstream channels are locked, in this case only a few are, with very high "Uncorrectables:" 

 

Startup Procedure
Procedure Status Comment
Acquire Downstream Channel 597000000 Hz Locked
Connectivity State OK Operational
Boot State OK Operational
Security Enabled BPI+
IP Provisioning Mode Honor MDD honorMdd(4)
 
Downstream Bonded Channels (Partial Service)
Channel Lock Status Modulation Channel ID Frequency Power SNR Correctables Uncorrectables
1 Not Locked Unknown 13 597000000 Hz 0.0 dBmV 0.0 dB 0 0
2 Locked QAM256 1 525000000 Hz 0.1 dBmV 34.4 dB 58073 113574
3 Locked QAM256 2 531000000 Hz -1.1 dBmV 33.4 dB 92785 70530
4 Locked QAM256 3 537000000 Hz -2.1 dBmV 32.4 dB 97275 86855
5 Locked QAM256 4 543000000 Hz -3.7 dBmV 31 dB 97562 91521
6 Locked QAM256 5 549000000 Hz -5.1 dBmV 29.6 dB 96154 104626
7 Locked QAM256 6 555000000 Hz -7 dBmV 27.8 dB 99220 109609
8 Locked QAM256 7 561000000 Hz -8.6 dBmV 26.4 dB 353811 245679
9 Not Locked Unknown 8 567000000 Hz 0.0 dBmV 0.0 dB 0 0
10 Not Locked Unknown 9 573000000 Hz 0.0 dBmV 0.0 dB 0 0
11 Not Locked Unknown 10 579000000 Hz 0.0 dBmV 0.0 dB 0 0
12 Not Locked Unknown 11 585000000 Hz 0.0 dBmV 0.0 dB 0 0
13 Not Locked Unknown 0 0 Hz 0.0 dBmV 0.0 dB 0 0
14 Not Locked Unknown 0 0 Hz 0.0 dBmV 0.0 dB 0 0
15 Not Locked Unknown 0 0 Hz 0.0 dBmV 0.0 dB 0 0
16 Not Locked Unknown 0 0 Hz 0.0 dBmV 0.0 dB 0 0
 
Upstream Bonded Channels
Channel Lock Status US Channel Type Channel ID Symbol Rate Frequency Power
1 Locked ATDMA 1 5120 Ksym/sec 36500000 Hz 37 dBmV
2 Locked ATDMA 2 5120 Ksym/sec 30100000 Hz 37 dBmV
3 Locked ATDMA 3 5120 Ksym/sec 23700000 Hz 37 dBmV
4 Locked ATDMA 4 5120 Ksym/sec 17300000 Hz 37 dBmV

 

Is this something Comcast can diagnose on their end? I read on another forum that there may be a script sending reset connection requests to the modem continuously. Or it's possible our connection frequencies are getting too far out of bounds due to an infrastructure-related issue in the neighborhood. 

 

Any help would be appreciated! 

Expert

Re: Modem is constantly dropping upstream connection

The downstream power is on the weak side and it may be intermittently fluctuating even lower to out of spec levels. And the SNR's are too low / out of spec. That can cause random disconnects, spontaneous re-booting of the modem, speed, packet loss, and latency problems.

In a self troubleshooting effort to try to obtain better connectivity / more wiggle room, check to see if there are there any excess/unneeded coax cable splitters in the line leading to the modem that can be eliminated/re-configured. Any splitters that remain should be high quality and cable rated for 5-1002 MHz, bi-directional, and no gold colored garbage types like GE, RadioShack, RCA, Philips, Leviton, Magnavox, and Rocketfish from big box stores like Home Depot, Lowes, Target, Wal-Mart etc. Splitters should be swapped with known to be good / new ones to test

If there aren't any unneeded splitters that can be eliminated and if your coax wiring setup can't be reconfigured so that there is a single two way splitter connected directly off of the drop from the street/pole with one port feeding the modem and the other port feeding the rest of the house/equipment with additional splits as needed, and you've checked all the wiring and fittings for integrity and tightness and refresh them by taking them apart then check for and clean off any corrosion / oxidation on the center wire and put them back together again, then perhaps it's best to book a tech visit to investigate and correct.



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New Poster

Re: Modem is constantly dropping upstream connection

The cabling was installed brand new from street to the house a year ago... it's literally one cable that feeds a coax jack that the modem connects to. No splitters whatsoever. So that leads me to believe it's not an issue I can self-diagnose at this point. 

 

Here's another example after it reset itself and all channels came back up. I notice the frequency it selected is different, and power levels look fine this time. But invariably it will go down again before long: 

 

Startup Procedure
Procedure Status Comment
Acquire Downstream Channel 669000000 Hz Locked
Connectivity State OK Operational
Boot State OK Operational
Security Enabled BPI+
IP Provisioning Mode Honor MDD honorMdd(4)
 
Downstream Bonded Channels
Channel Lock Status Modulation Channel ID Frequency Power SNR Correctables Uncorrectables
1 Locked QAM256 25 669000000 Hz 0.1 dBmV 34.1 dB 3322 968
2 Locked QAM256 26 675000000 Hz 0.2 dBmV 33.9 dB 58156 113647
3 Locked QAM256 27 681000000 Hz 0 dBmV 34.3 dB 92846 70612
4 Locked QAM256 28 687000000 Hz 0.3 dBmV 34.5 dB 97345 86990
5 Locked QAM256 29 693000000 Hz 0.2 dBmV 34.5 dB 97607 91599
6 Locked QAM256 30 699000000 Hz 0.3 dBmV 34.6 dB 96200 104702
7 Locked QAM256 31 705000000 Hz 0.2 dBmV 34.7 dB 99270 109686
8 Locked QAM256 32 711000000 Hz 0.5 dBmV 34.9 dB 353854 245758
9 Locked QAM256 33 717000000 Hz 0.5 dBmV 34.8 dB 4799 2254
10 Locked QAM256 34 723000000 Hz 0.7 dBmV 35 dB 114553 176289
11 Locked QAM256 35 729000000 Hz 0.6 dBmV 32.8 dB 349557 310145
12 Locked QAM256 36 735000000 Hz 0.7 dBmV 32.5 dB 245985 134380
13 Locked QAM256 37 741000000 Hz 0.6 dBmV 33 dB 661081 327651
14 Locked QAM256 38 747000000 Hz 0.3 dBmV 34.5 dB 147441 85102
15 Locked QAM256 39 753000000 Hz 0.1 dBmV 34.4 dB 149902 95684
16 Locked QAM256 40 759000000 Hz -0.3 dBmV 34.1 dB 124047 77183
 
Upstream Bonded Channels
Channel Lock Status US Channel Type Channel ID Symbol Rate Frequency Power
1 Locked ATDMA 3 5120 Ksym/sec 23700000 Hz 37.5 dBmV
2 Locked ATDMA 1 5120 Ksym/sec 36500000 Hz 37.5 dBmV
3 Locked ATDMA 2 5120 Ksym/sec 30100000 Hz 37.5 dBmV
4 Locked ATDMA 4 5120 Ksym/sec 17300000 Hz 37.5 dBmV
Expert

Re: Modem is constantly dropping upstream connection

The SNR's are still too low / out of spec. Seems that there is noise leaking into the line(s) somewhere. If there is nothing that you can do it would be best to get a tech out to investigate / correct this.

 

Bear in mind that if the premises facing techs can not find or fix a problem at your home, it is they who are responsible for escalating it to their line / network / maintenance dept. techs. The problem may lie beyond your home in the local neighborhood infrastructure somewhere but it is their S.O.P. to start at the home. Good luck with it !




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Official Employee

Re: Modem is constantly dropping upstream connection

Greetings, gt350cs! I appreciate you taking the time to reach out to us here on the forums. I hope you are having an amazing day. I apologize to hear of the issues you are running into when it comes to your internet. This is certainly frustrating. I would like to have a closer look at this and see what we can find out. Can you please send me a PM with your first and last name so I may further assist you?

 

To send a private message, please click my name "ComcastMichael" then select "Send a Message" on the right side.


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