Bought the CM1200 about a month ago and since it was setup by RCN(isp) I have had it seemingly randomly dropping internet connection. When it drops connection the downstream light will be blinking and I will have to restart the modem to get service back. I had an RCN technician come out to check the power to the modem itself and to the building. He found no issues and said the power was fine in both locations. I have no splitters set up. The disconnect will sometimes happen multiple times a day and sometimes it will be fine for multiple days. Firmware version is V2.02.05 which looks like it is higher than it should be for RCN. Netgear webpage shows V2.02.03 for all other ISPs. Not sure if this could be causing the issue.I have attached a screenshot of what the event log looks like when is disconnects. The critical error being SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing; Any ideas on what could be causing this?

Your modem power levels are all over the place. the downstream channels should be within 3.5dbmv of each other. Yours are way out of this. Did rcn check the line? Did you remove any splitters, amplifiers, or attenuators? check the line for damage, kinks, old/bad/cheap connectors and cables.

Connected directly to the line coming into my unit and power looks a little better again. Still a larger range than 3.5dbmv. Will leave this setup for awhile and see if the issue comes back and will report back either way. Appreciate all the help plemans and FURRYe38.

Please post a copy and paste of the modems connection status page.Have the ISP check the signal and line quality UP to the modem. Be sure the ISP provisions the modem correctly.Be sure there are no coax cable line splitters in the between the modem and ISP service box. Be sure your using good quality RG6 coax cable up to the modem. Be sure to disable LAG on the modem. https://community.netgear.com/t5/Cable-Modems-Routers/General-info-and-Troubleshooting-for-Cable-Modems/m-p/1530376#M12853

Can you get a screen capture of the connection status page when the modem is online and working?

RCN did check the line and they said they didn't see any issues with the power levels, but they seemed to just reference the levels on this page http://ny.speedtest.rcn.net// which just shows one number for each FwdSNR, Channel Pwr, and Transmit Pwr (not sure where it gets the number for these since there are multiple channels. Maybe its just the average). I did remember when internet was initially set up in my apartment the technician opened a panel in my closet that had some cables behind it specifically the line for my unit then a female to female connector that connects that line to the main room wall jack and another unused line that goes to the master bedroom jack. So I went ahead and connected the modem to the mainline coming into the unit (still using the female to female connector since the line in the wall doesn't go far enough). Power levels look marginally better (See attached) still not within 3.5dbmv. Will try connecting without female to female connector as well to see if that changes the power levels at all. Appreciate all the help.

CM1200 Dropping connection intermittently - SYNC Timing Synchronization failure

15 Replies

FURRYe38
Guru - Experienced User
Apr 13, 2021
Please post a copy and paste of the modems connection status page.
Have the ISP check the signal and line quality UP to the modem. Be sure the ISP provisions the modem correctly.
Be sure there are no coax cable line splitters in the between the modem and ISP service box.
Be sure your using good quality RG6 coax cable up to the modem.
Be sure to disable LAG on the modem.
https://community.netgear.com/t5/Cable-Modems-Routers/General-info-and-Troubleshooting-for-Cable-Modems/m-p/1530376#M12853
- markweiz
  Star
  Apr 13, 2021
  I have attached screenshots(can only attach one per post it appears will attach the second screenshot in the following message) of the connection page when the internet was disconnected. The technician that came out tested the signal and line up to the modem and determined it was fine. The ISP has reprovisioned the modem twice with no issues either time. There are no splitters in use and I have tried multiple coax cables to no avail. Ethernet port aggregation is disabled. I did try enabling Enable Extended Upstream Transmit Power to fix the issue, but that did not work.
  - markweiz
    Star
    Apr 13, 2021
    Second screenshot

Forum Discussion

CM1200 Dropping connection intermittently - SYNC Timing Synchronization failure