Mysterious & Frustrating Intermittent Packet Loss & Disconnects

I've spent countless hours troubleshooting our Xfinity Internet, including multiple customer calls and a technician visit. We have the 1000Mbps service with unlimited data.

Here are the symptoms:

• When it's good, it's perfect. When it's not good, randomly for several hours at a time:
  • League of Legends is unplayable: lags every few seconds, disconnects every minute or so, or even the Internet would just drop altogether.
  • PingPlotter graphs look like this (this is a UDP ping, not ICMP): (top = to first hop outside the house; bottom = to google.com): https://imgur.com/a/zXE3EOx

I am a software engineer, so it was natural for me to try various setups and process of elimination to see where the problem is. Before I present my findings, a couple of things to note:

• All tests are done via Ethernet using a powerful PC built in 2020.
• Most test scenarios were repeated with a router behind and without a router behind the modem. The router used is Google WiFi (of course, the WiFi part is not used for testing).

Here is a summary of trying different scenarios:

On the cable configuration:

• 4-way 5-1002MHz splitter refers to a splitter previously installed in the house whose history I don't know of.
• Downstairs and upstairs are just two different coax sockets on the wall. The upstairs coax at the TV box initially didn't have a connector, so I used a crimp tool to add a connector.
• 4-way 5-2500MHz splitter refers to a splitter I got from Amazon (yeah, maybe not great) for MoCA setup. The MoCA set up uses a Motorola MoCA adapter; it's not too important for the discussion here.
• "No splitters" means I directly connect my upstairs coax to the incoming cable (via a thing that looks like a MoCA filter or a ground block? Not sure).

On the modem used:

• XB7 is from Comcast. Arris SB8200 is a DOCSIS 3.1 modem; Arris SB6121 is a slow 4x4 DOCSIS 3.0 modem.
• I list "SB8200 (inactive)" as a separate case for clarity: it means that the SB8200 I'm currently using shows in Comcast's systems as "inactive", despite me having activated it via the xfinity.com/activate prompts when I switched from the XB7 to the SB8200 and having received an email that my services are ready.

Further, here are the signal levels as reported by SB8200 right at the cable coming into the house (yeah, I moved my modem and router down there just to test that.)

https://imgur.com/a/JHsCvl0

OK, next, a few more observations:

• The SB8200 has an Event log which shows a series of entries beginning with "MDD message timeout"; these events perfectly align with the packet loss spikes. Also, whenever this happens, the SB8200's downstream light turns from green to blue, and after a few seconds, from blue back to green (the latter event is when the packet loss happens). The manual for SB8200 indicate that blue means DOCSIS 3.1. All lights are normally green (i.e. I'm not getting DOCSIS 3.1?), but this itself is fine because I'm still getting gigabit speeds.
• Sometimes I get channels 3-34 and 48; other times I get channels 17-47 and 48.  Channels 3-34 have much better SNR (because they are lower frequencies, I assume). However the packet loss issue seems to happen in both cases.
• The XB7 does not report any logs; when I try to grab the system logs it shows me nothing. The XB7 also tends to just gives up the connection entirely (goes back to the flashing light cycles). SB8200 does less of that and seems more stable overall, except in December when it was just dropping like, 2% of packets or something.
• Somehow, IPv6 tests perform better than IPv4 tests (but not completely immune from packet loss). No idea why.

Alright, now we can try to logically figure out what's going on here:

• It's not WiFi, because no tests used WiFi.
• It's not any of the Ethernet cables having issues, because the SB8200's errors and light changes correspond perfectly to the packet loss. Also, WiFi devices suffer from spotty Internet as well when it happens.
• It's not any of the coax cables I use to connect from the wall socket to the modem, because I have tried two coax cables, one of which is brand new.
• It's unlikely that the issue was due to the coax wiring in the walls in the house, because I had experienced packet loss both upstairs and downstairs. However I don't say impossible because there was a two month gap between the packet loss events and it's possible they were separate events. Still, this is a recent house (~15 years old) and it's hard to imagine that a wiring issue would cause packet loss attacks to come in waves instead of constantly happening all the time; it also doesn't quite explain why SB6121 never has any issues.
• It's not the modem, because two different modems have issues, one of which is the latest model from Comcast.
• It has something to do with either DOCSIS 3.1, or the higher downstream frequencies because the SB6121, which uses DOCSIS3.0 and only the lower downstream frequencies never suffer from the packet loss issue. Of course, that modem could only get 135Mbps down and 6Mbps up so I don't want to use that just for reliability.
• Some things the technician said:
  • "Your SB8200 shows up as 'disabled' in the systems, so you're gonna have issues". Well... the XB7 also has issues, so I don't think that was relevant. And if the SB8200 would always have issues then I might believe that, but most of the time the SB8200 does perfectly fine except sometimes the spikes happen. If it's an activation issue I'd expect it to either work or not work, not sometimes work and sometimes not.
  • "This connector you put on the cable yourself - the white part is not flush with the connector, so that's probably why you're having packet loss". Yeah I did that connector myself, and I admit I was bad at it, but this technician redid it himself, and right that night I have a packet loss attack again, and with the XB7. So it's not that connector. And even if it's a connector issue, it doesn't explain why these packet loss attacks happen continuously for hours at a time and then never happens for many hours. Shouldn't it happen once in a while all the time?
  • "The signal levels are all green." Dear forum users, what do you guys think? Are my signal levels normal? Does channel 48 (the ODFM channel, I assume) having that many errors look right to you?
  • "This splitter isn't compatible with Comcast." (he was referring to the 5-2500MHz splitter I bought from Amazon) He might be right, but it was working perfectly for quite some days before packet loss attacks came. And also, packet loss still happens without the splitter.

So at this point I have several different hypotheses:

1. Channel 48 (850MHz ODFM channel) has unacceptable SNR and is the root cause of all issues. The fix would be for Comcast to fix that SNR because the cable coming into the house had only 10dB SNR there.
2. Channel 48 is actually fine and all signal levels are fine, but Comcast's networking node at the street is faulty and sometimes does not send MDD messages properly for the higher channels. The fix is still on the Comcast side.
3. Some other highly unlikely issue is the root cause, such as both modems being faulty, both downstairs/upstairs coax wiring being faulty and only in the high frequencies, etc. etc. To eliminate these I'd have to run wired Ethernet tests at the ground block which really isn't convenient especially since it may take a couple of days for the next packet loss attack to happen.

If Comcast and I can't work together to fix this issue, I'd have to sadly go back to 100Mbps plan and use an old modem 😞

What do you guys think?