MB8611 T3 timeouts and timing synchronization failures lead to frequent disconnects

Just want to let you know this is happening to me too.  Everything was fine until I upgraded to the MB8611.

Hi there and welcome to Comcast. Thank you so much for reaching out to us about your service issues. You are in the right place and I am happy to asssit you today. Are you on a wired connection or hardwired connection?

For others folks following, as an update, a tech came out yesterday morning and replaced the length of cable running from my wall to my modem, removed a splitter, and got the upstream power within spec (low 40s). Downstream is in spec but on the high sides 9-13.5 However the past two afternoons I continue to experience drops related to:

"SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing"

It seems to happen more frequently toward the end of my work day. Happened 3-4 times in the past few hours. Very frustrating during work meeting/calls.

I have another tech coming out tomorrow afternoon.

The second tech replaced the rest of the cable run between the house box and the modem and added a forward path attenuator to get downstream power close to ideal (3-5 dbmv, down from 11-13 dmbv). Still had disconnects at the same rate.

Had a third tech visit today - he said the neighbor's rented modem doesn't have any disconnections logged, which eliminates street-side issues, so the only parts left are the line from the street box to the house box, and of course the modem itself. He installed a temporary above-ground run between the street and house junction boxes. I've seen a couple disconnects since he's left, so it's either the modem or the modem firmware Comcast is pushing (which other MB8611 owners are theorizing, as there's an influx of similar posts since the new year on this forum regarding this model).

I have an Arris S33 on order and hopefully that will be the end of this. Unfortunately no option to return the MB8611.

More logs in case any engineers are looking:

Checking in to see if there were any updates or resolution to your issue. I have a similar story here. Issues started intermittently ~6 months ago, right around the time there was a "node upgrade" in my area. I'm finding this is a recurring theme in other forums as I search for resolution.

Similar to others, here are the types of errors I encounter:

CM-STATUS message sent. Event Type Code: 16; Chan ID: 193

CM-STATUS message sent. Event Type Code: 24; Chan ID: 193

Dynamic Range Window violation

Started Unicast Maintenance Ranging - No Response received - T3 time-out

I've been through the standard gauntlet already: two tech visits, new connectors, new line to the house, and am now on my 3rd modem (currently the MB8611). Some of this work seems to have helped the overall signal quality, and the line/signal looks pristine at any given moment in time, but the intermittent drops are still occurring.

I suspect these network upgrades are pushing older infrastructure/equipment beyond their limits, sparking these issues which are difficult for local teams to track down. And/or issues with brand new equipment seems to be a disconnect of some degree between Comcast & the various manufactures, firmware, etc. The fact that there are the same issues with a range of modems suggests the root cause is on Comcast's side though.

I'm stumped on what to try next and am hoping to find somebody who has a success story to share.

I started getting the same issue with a Motorola MB8600. It worked flawlessly for 23 months. Motorola tech support said Comcast has been doing a lot of upgrades and people having problems. right now that modem won't connect at all. I had to dig out an old TP Link 7610 (actually had 2). One of the TP Links worked for a couple days and stopped and the other is working but I am going to shut it off at night.

My home internet was for stable three years. Mid-January, it started to go down with increasing frequency. 

I suspected the modem or the WiFi router. Both were in the garage where exposure to wide temperature variation might have shortened their operating lifetimes.

I replaced the modem and WiFi router with Motorola MB8611, ASUS ZenWifi. Network reliability, however, didn't improve.

The modem's event log showed T3 and T4 timing issues so Xfinity sent a technician who found that a 9.5dB attenuator gave the desired power and SNR. Previously, there was a 5dB attenuator. Home network stability improved but it didn't stay up more than a half day and the modem's event log continued to show T3 and T4 problems.

Trying for a robust solution, I simplified the connection to the modem: two lengths of coaxial cable joined by female-to-female adapters. A 3dB female-to-male attenuator was placed at the modem input.

With three attenuators at hand (3dB, 6dB, 10dB), I planned to try different combinations to find what gave the most network up-time. I also started researching alternatives to cable: DSL, cellular, satellite, mobile hotspot.

So far, the network has been up 24 hours and the modem event log is clean (no T3 or T4 or other errors). It's too early to say if the network is as stable as it once was. I'd be happy if MTBF is over 72 hours.

It was the firmware for the MB8611. Connection has been better after Comcast finally rolled a lot of us MB8611 users back to firmware 19.2.18. Current uptime is 15 days. So I returned the Arris S33, which worked great and would have preferred to keep because of the better management interface, but I didn't have the option of returning the Motorola.

However, I still get T3/T4 timeouts on occasion, though far less frequent. Interestingly, they don't seem to restart the uptime counter like before.

More data for you...

I swapped the (Xfinity recommended!) MB8611 out for an Arris SB33 and my disconnect drama is gone. Every day the MB8611 would disconnect/reset for a few minutes and reacquire communication.  FW is 8611-19.2.18

Typical start of trouble was deep in the physical layer, nothing to do with routers or ethernet cables or other "advice" in the Xfinity troubleshooter:

SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=xx:xx:xx:xx:xx:xx;CMTS-MAC=00:00:00:00:00:00;CM-QOS=1.1;CM-VER=3.1;

The automated Xfinity connection troubleshooter on the web said everything was awesome.

In my case, I suspect something funky with the Moto SB8611 OFDM-PLC channel that can't adapt the QAM channels properly and gives up.  Data to follow...

Downstream

The downstream power levels between the two modems are pretty much the same, maybe a tiny bit hotter for Arris. 

4.2 dBmV average signal for Moto (3.5-5.3 limits).  SNR average 38.8 dB.
4.9 dBmV average signal for Arris (4-6 limits).  SNR average 38.2 dB.

An issue was uncorrectable errors for Moto.  39448 correctable, 121228 *un*correctable in the normal QAM256 channels.  The OFDM PLC channel (sideband used for physical cable parameters) had no uncorrectables, but 11+ million correctables ... which is more or less expected for that kind of channel/modulation.

Arris had 1527 correctable errors and *0* uncorrectable in the QAM256 channels.  61M+ on the PLC channel, all correctable.  The PLC error count was higher than Moto but perhaps since the modem didn't reset, the logs had time to gather far more data.

Upstream

Upstream power for Moto was 43.6 dBmV average.  The symbol rate was 2560 on the top and bottom channels and 5120 on the center 4 channels.

Upstream power for Arris was 43.7 dBmV average.  The symbol rate was 3200 on the top and bottom channels and 6400 on the middle 4 which ought to mean better potential upstream date rates.

Results

speed.cloudflare.com says we get 227 Mbps down with the Arris (I'm on the ~200 Mbps plan) and 13.1 Mbps up.  Unfortunately I didn't measure with the Moto modem before I yanked it.  The Arris hasn't reset or thrown any log entries yet.  The Arris modem likes my local system (Woodinville, WA) much better.