5 Messages
SYNC Timing Synchronization failure - multiple modems (Motorola MB8611, Netgear C6250)
I believe I'm experiencing a similar issue to what many others have reported on these forums, with a Motorola MB8611 frequently rebooting with SYNC Timing Synchronization failure and No Ranging Response received - T3 time-out errors in the logs. I just got the MB8611 a few days ago to replace our old modem/router combo, which had intermittent issues, but which I'd assumed were more related to the WiFi access point side.
Since installing the MB8611, it seems like I can't get more than ~3h of uptime at best. At worst, the disconnects can be within minutes of each other.
AFAICT, there's one splitter just off the line, and no others between that and the wall connection. Since this is a rental house, I can't comment on the age or quality of the cables. It's maybe 100' from the modem to the drop.
MB8611 event logs, with MAC addresses redacted:
Downstream connection info:
Unfortunately, I forgot to get a screenshot of the upstream channels before swapping routers. However, in my notes I wrote that the upstream channels were between 42.5-43.3dBmV, so I believe that's in spec. Downstream power also appears to be within spec, though on the high end.
When the modem is connected, speeds are great: 1-1.2gbps when connected directly to the modem over ethernet according to fast.com (we have a 1000mbps plan now). Also FWIW, the rest of the networking setup is an Asus RT-AX88U, which has seemed to work fine. I've also had the modem connected directly to my laptop for multiple hours, and observed the same timeout error+restart behavior as when it's connected to the router.
Because the connection is nearly unusable in this state, I have temporarily switched back to the old DOCSIS 3.0 Netgear C6250. We'd used this for a few years and also experienced intermittent issues (though maybe a few times a week, less frequent than with the MB8611)—never diagnosed whether these were gateway issues or issues with the WiFi access point.
Interestingly, looking through logs on the Netgear, I see a few similar SYNC Timing Synchronization failure log messages that happened before the switch:
I haven't seen any in the last ~8h since switching, though, so they're clearly less frequent than with the Motorola.
We have a technician scheduled to come on Sunday. Given how many other issues like this I've seen with this modem (including this one which says "Support has confirmed that this is Motorola software related issue"), I'm very close to just ordering an Arris S33 right now though.
However, seeing the timing sync failure logs on the old modem has made me hesitate, and wonder whether it could in fact be an issue with our connection, or the line. Is this a red herring? Can anyone report on their experience with having this issue with the MB8611 and switching modems?
Related posts:
Accepted Solution
user_4cc66c
5 Messages
2 years ago
tl;dr: The Motorola 8611 seems to be the issue. The Arris S33 works well. I would hope that the 8611 is entirely removed from Xfinity's list of compatible devices, or better, that Xfinity is able to resolve whatever has changed causing the incompatibility, or push Zoom to fix it on their end with a firmware update.
To follow up: the tech came out and was very helpful. He confirmed the power was a bit high, and installed an attenuator (-6 dB I believe). After that, he checked again on his modem/scope and confirmed the signal looked good.
Out of curiosity, he re-provisioned the 8611 for us to see if the attenuator had helped. As before, it only made it 10-15 minutes between restarts. I had to call that night to re-provision the old Netgear again so that we could work remotely the next day.
A few days later, our Arris S33 modem arrived. I installed it 3 days and 12h ago. The uptime currently reads 3 days 12h:42m:22s—zero restarts. We've been very happy with the quality of service with the S33, and we're getting the speeds we pay for.
So we can consider this problem "solved". Except that if I hadn't been within the return window for the 8611, I'd be pretty upset to have to spend another $150 on a new modem while using a device Xfinity used to support just fine, and still lists as "compatible".
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EG
Expert
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110.2K Messages
2 years ago
This may or may not be the root cause of the problem (YMMV) but it should be addressed regardless. And yes, we have been seeing many posts here lately that may indicate a pattern that the 8611 is currently not playing nicely with the Comcast system / firmware.
Yes, the downstream power is too high / out of spec. It may be over-driving the front-end receiver circuit of the modem. That can cause random disconnects, spontaneous re-booting of the modem, speed, packet loss, latency problems, and the un-bonding of channels.
Is there a drop amplifier on the coax cable line leading to the modem ? If so, try removing / bypassing it and see. If not, you can try installing a -6 dB forward path-only attenuator pad such as this one on the coax port on the rear of the modem to knock that power level down and see. You may need to use two of them back to back. 6 dB lower may not be enough. You may need 12 dB of attenuation;
https://www.amazon.com/FPA6-54-Forward-Attenuator-DOCSIS-Internet/dp/B08KTRC3XZ
Good luck with it ! And please post back here with how things turn out, thanks.
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EG
Expert
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110.2K Messages
2 years ago
@user_4cc66c
Thanks for circling back with that update. Hope things hold up for you ! Good luck ! If the problem returns, please feel free to create a new topic here. Now closing your marked "Solved" thread.
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