trouble shooting a cable modem.

[CU]

I have a 6 year old Motorola SB5100 (I think). Lately I have been having internet problems, ie. I cannot access anything outside my network. Normally the modem's webpage works and a restart and/or unplug cable and replug it works. So, I though it was a problem with Comcast to modem communications problem. However last night, the modem's webpage would not load. A restart and an unplugging and replugging the cable would fix it. For an hour or so then the problem would be back again. The webpage not loading makes me think the modem is having a problem/dieing. Any suggestions on how to diagnosis the problem? I plan to call Comcast as this has been happening for a week or so now. I am sure they will say the problem is on my end though. I haven't changed anything with my network in 6 months.

 

[owensdj]

That does sound like the modem itself is dying. You could try switching to OpenDNS servers. Sometimes your ISP's DNS servers have problems. OpenDNS tends to do a better job with theirs.

 

[CU]

Would Comcast's DNS problems be fixed by me restarting my modem? Also every website I try is down when it is down, not just a few?

 

[imagoon]

Look for T1 / T3 retrains. I have seen excessive retrains make a modem give up on life before. When I was on Comcast, lost syncs were a part of life... 2-15 times an hour. Eventually the modem would lock up. 5 different modems... same issue.

 

[hawk82]

Do the LEDs on the front of the modem indicate loss of signal? Post the signal stats from the modem page when your cable modem is connected and working. Post the logs as well.

 

[CU]

Signal:
Frequency 549000000 Hz Locked
Signal to Noise Ratio 37 dB
Power Level 2 dBmv
Channel ID 6
Frequency 29400000 Hz Ranged
Power Level 54 dBmV

Snap shot of logs:
2010-08-04 23:30:36 3-Critical R005.0 Started Unicast Maintenance Ranging - No Response received - T3 time-out
2010-08-04 23:30:32 3-Critical T002.0 SYNC Timing Synchronization failure - Failed to acquire FEC framing
2010-08-04 23:30:25 3-Critical R005.0 Started Unicast Maintenance Ranging - No Response received - T3 time-out
1970-01-01 00:01:11 3-Critical D001.0 DHCP FAILED - Discover sent, no offer received
1970-01-01 00:00:06 3-Critical R002.0 No Ranging Response received - T3 time-out
1970-01-01 00:08:28 4-Error D004.3 ToD request sent- No Response received
1970-01-01 00:08:23 3-Critical D002.0 DHCP FAILED - Request sent, No response
1970-01-01 00:08:16 3-Critical D001.0 DHCP FAILED - Discover sent, no offer received
1970-01-01 00:08:04 3-Critical R002.0 No Ranging Response received - T3 time-out
1970-01-01 00:07:57 3-Critical T001.0 SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing
1970-01-01 00:07:42 3-Critical T002.0 SYNC Timing Synchronization failure - Failed to acquire FEC framing
1970-01-01 00:07:08 3-Critical D001.0 DHCP FAILED - Discover sent, no offer received
1970-01-01 00:04:41 3-Critical R002.0 No Ranging Response received - T3 time-out
1970-01-01 00:04:34 3-Critical D001.0 DHCP FAILED - Discover sent, no offer received
1970-01-01 00:01:05 3-Critical R002.0 No Ranging Response received - T3 time-out
1970-01-01 00:02:13 3-Critical T001.0 SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing
1970-01-01 00:01:58 3-Critical T002.0 SYNC Timing Synchronization failure - Failed to acquire FEC framing
1970-01-01 00:01:54 3-Critical R005.0 Started Unicast Maintenance Ranging - No Response received - T3 time-out
1970-01-01 00:01:15 3-Critical D001.0 DHCP FAILED - Discover sent, no offer received
1970-01-01 00:00:05 3-Critical R002.0 No Ranging Response received - T3 time-out
2010-08-04 21:16:11 3-Critical D007.0 TFTP Failed - OUT OF ORDER packets
1970-01-01 00:05:30 4-Error D004.3 ToD request sent- No Response received
1970-01-01 00:05:02 3-Critical D007.0 TFTP Failed - OUT OF ORDER packets
1970-01-01 00:00:32 4-Error D004.3 ToD request sent- No Response received
1970-01-01 00:00:27 3-Critical D002.0 DHCP FAILED - Request sent, No response
1970-01-01 00:00:15 3-Critical D001.0 DHCP FAILED - Discover sent, no offer received
2010-08-02 00:27:47 3-Critical C206.0 DCC aborted unable to communicate on new upstream channel
2010-08-02 00:27:47 3-Critical R004.0 Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 timeout

 

[CU]

I think the bottom green light (online) is blinking when it is not working. I will check next time it is down.

 

[hawk82]

The upstream power level is getting close to borderline signal issues. Make sure all of your cable connections are tight (finger tight is good). If you have a splitter in front of the cable modem, try disconnecting it temporarily and plug the modem directly into the drop. Then take another look at the signal levels. See if the service stays online.

If that doesn't work, then I'd recommend calling Comcast and see if they can dispatch a tech to look at your service.

http://www.dslreports.com/faq/cabletech/5._Cable_Modems#3412

 

[CU]

No splitters except for the one from Comcast coming from the main line that splits to the cable modem and to line that is used for TV. All connections inside should be finger tight, but I will double check that. Guess I will be calling Comcast, fun. What is a reasonable upstream power level to expect from them?

 

[CU]

What about all the stuff in the logs?

 

[imagoon]

What about all the stuff in the logs?

Those are lost signals / retrains. It means for whatever reason the modem lost the connection to the head end. It is the symptom of a poor connection.

 

[lord_emperor]

Power Level 54 dBmV

You are at the maximum transmit value of your modem, could indicate a dying modem or an issue with Comcast's plant.

Call Comcast and make them fix it.