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24 Messages
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:
| Modem Used | Modem Used | Modem Used | Modem Used | ||
| XB7 | SB8200 | SB8200 (inactive) | SB6121 | ||
Cable configuration | 4-way 5-1002MHz splitter, modem downstairs | Occasional packet loss (always) | Extreme packet loss (December) | Not tested | No issues |
Cable configuration | 4 way 5-1002MHz splitter, modem upstairs | Occasional packet loss (always) | Not tested | Not tested | No issues |
Cable configuration | 4 way 5-2500MHz splitter + MoCA filter, modem upstairs behind MoCA | Severe packet loss and disconnects (February) | Not tested | Severe packet loss (February) | No issues |
Cable configuration | No splitters, modem upstairs | Severe packet loss and disconnects (February) | Not tested | Severe packet loss (February) | No issues |
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.)
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:
- 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.
- 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.
- 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?
Accepted Solution
EG
Expert
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110K Messages
4 years ago
@realyc
You need to get the techs re-involved. There is a severe amount of cable *tilt* with your connection (15 dB). That is the amount of power rolloff / difference between the lowest and the highest downstream channel frequencies. This can not be fixed remotely. It will take * boots on the ground* to fix this.
Bear in mind that if the premises facing techs can not find or fix a problem at your home, it is they who are responsible for escalating it to their line / network / maintenance dept. techs. The problem may lie beyond your home in the local neighborhood infrastructure somewhere but it is their S.O.P. to start at the home.
Good luck !
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Accepted Solution
realyc
Contributor
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24 Messages
4 years ago
Giving an update here. A third tech came today and we spent nearly three hours together on the issue. From what I observed, the tech ran the following tests:
Right after he fixed the house wiring I again observed significant spikes. The modem kept showing MDD message timeouts, but Comcast could not see it on their end. There's definitely some difference in the way the modem reports errors and what is reported back to Comcast. Anyway, the technician was patient and tested the hot drop from the tap, and eventually, he decided that the DOCSIS 3.1 signal was bad. As he described, the 3.1 signal was "all over the place". He called in a maintenance crew, who came pretty soon after, did some work beyond the tap (I *think* they replaced something between the tap and the next thing on the street, but I'm not sure).
Here were the signal levels before the maintenance work:
Here are the signal levels now, after the maintenance work:
There was a huge improvement to the 850MHz (the DOCSIS 3.1 OFDM) channel.
It remains to be seen whether this will finally solve the problem once and for all, but if anything, this is the one big change I was never able to see before. And this fixed the exact thing @EG pointed out at the beginning: cable tilt.
Thanks for everyone who was involved in this. It was a frustrating experience trying to escalate the issue, but it's comforting to know that Comcast has the right people who will solve problems like this. It may just take a while to reach them.
I plan to write another post later to summarize the findings and hope I can help others out there.
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realyc
Contributor
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24 Messages
4 years ago
I'll fix the images later... sorry.
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realyc
Contributor
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24 Messages
4 years ago
OK images fixed.
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CCAbbie
Problem Solver
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1.4K Messages
4 years ago
@realyc Hello, and thank you for taking the time to reach out regarding your connection issue. I appreciate all the research and information you provided and I want to help in any way I can, I'd like to get your account pulled up to see what the signals look like on our end.
Please send me a private message and include your first and last so I can assist you. To send me a Private Message, please click my name “ComcastAbbie” and click “Send a message”
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realyc
Contributor
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24 Messages
4 years ago
Thanks @EG!! That's a very nice catch!! Do you have any tips on how to escalate properly so I can get someone who is knowledgeable enough to take a look? It would be a waste of time if another technician comes on site, looks at his iPad and tells me all the signals are green 🙂
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EG
Expert
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110K Messages
4 years ago
Quite welcome ! The employee @ComcastAbbie that responded to you can get the ball rolling. Did you send the requested private message? If not, please do so.
Good luck with it !
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realyc
Contributor
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24 Messages
4 years ago
Thanks @EG. Yeah I have been in contact with the Comcast agent via private message, but it is going rather slowly and a different person responds every time. I'm not giving up hope yet as we're still working through the basics, but yeah, it's slow.
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realyc
Contributor
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24 Messages
4 years ago
OK, I have some updates. Comcast sent another technician (a very nice guy!) who checked the line at the ground block, two different coax sockets in the house (upstairs and downstairs), and also the tap on the street. If I remember things correctly, his discoveries were:
Note that the tap was very close; there was only one house between the tap and my house, and if I saw correctly, the tap had six connectors plugged in.
Here are some more things we discussed:
Here's what I plan to do:
Also something to note:
@EG What do you think? Does the packet loss difference between the tap and the house (which I estimate to be about 100ft of cable length) seem normal to you? Thanks 🙂
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realyc
Contributor
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24 Messages
4 years ago
How do I message a Comcast customer support again? The newly updated forum site does not let me reply to any of the previous ComcastXXX contacts anymore...
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realyc
Contributor
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24 Messages
4 years ago
This is a followup from my post from more than a month ago. https://forums.xfinity.com/t5/Your-Home-Network/Mysterious-amp-Frustrating-Intermittent-Packet-Loss-amp/td-p/3417384 I'm reposting, because I cannot figure out any way to message a Comcast employee anymore ever since the forum upgrade.
I have done all I could on my part. I've made a new coax outlet right in the garage, right behind the cable TV box, using new coax cables, and wired my house using Ethernet cables all the way. The issue went away for a week or two, but now, unfortunately, I'm once again getting an unacceptable amount of packet loss, and they again correlate with "MDD message timeout" errors on the modem. This has become intolerable for me. Please, could I ask Comcast to replace the drop (I'm assuming that means the coax cable from the tap to my house)?
THANK YOU!!
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realyc
Contributor
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24 Messages
4 years ago
Ugh ok so that comment got merged. So @ComcastKaitlan, I know there's the new chat icon but when I send a response, it just tells me that the other person is not accepting private messages. Could one of you actually just send me a direct message so I can reply? Thank you...
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EG
Expert
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110K Messages
4 years ago
Send a message to "Xfinitysupport" so that any available team member gets it.
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user_2d6d13
Visitor
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5 Messages
4 years ago
@realyc Having a similar issue, posted here: https://forums.xfinity.com/conversations/your-home-network/docsis30-works-31-no-longer-functional/605f901fc264e93e85820b74
Lines all tested by rep, all "green". Been using an SB8200 for a year with no issues until 2 weeks ago when I started getting dropped packets every 10 seconds. Ping tests show my connection dropping consistently 9 to 10 successful ping replies then 3 to 5 timeouts, rinse, repeat. When I can sneak in a speed test I could get great speed for 10 to 15 seconds then nothing, then it comes back. Rep said Xfinity pushed an update to the SB8200's in my area and killed my modem, that they saw 5 others on the same day with the exact same issue. Reverted back to my old SB6183 @ DOCSIS3.0, everything works fine but slower - 500Mbps down, 5Mbps up. On the Reps advice I bought an MB8611 - SAME ISSUE as my SB8200!! Connection is up and down every 5 to 15 seconds - no way this is a modem issue, can't be my lines inside or out (again, worked for the past year, and recently tested as all green), and my old modem still works fine.
Possible my account is messed up with Xfinity? Possible they've killed off personally-owned DOCIS3.1 modems in my area? The call rep did tell me they "just recently upgraded their systems" and that they have heard a lot of people are now having problems with self-owned modems.
Please advise!
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realyc
Contributor
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24 Messages
4 years ago
@user_2d6d13 That's interesting... I do suspect that their firmware for the SB8200 is somewhat suboptimal. The problem for me is, there are really no issues with the DOCSIS 3.0 channels, but the DOCSIS 3.1 OFDM channel is horrible. What my modem appears to want to do is to go up to 3.1 (light turns blue), and then realizes "oh it doesn't work well" and falls back to 3.0 (light turns green). It's when it falls back to 3.0 that I get a spike of packet losses. But sometimes during the day it would just keep doing that over and over.
Image: March 27 11pm PingPlotter graph - this is *after* Comcast said they replaced an "aerial line". I don't know what that means but I didn't even see a Xfinity vehicle in the neighborhood. Working with customer support is a real pain and.. you know, I'm trying my best to stay calm and stick to reason. Top graph is ping to my router; middle graph is ping to first comcast node, and bottom plot is ping to google.
What I wish my modem would do is just forever give up on the DOCSIS 3.1 channel and just stick with DOCSIS 3.0. It's able to get 1000Mbps down and 35 up, so why bother. And Comcast denies there's any problem with the DOCSIS 3.1 channel because its diagnostics just ignores that channel and says everything's fine. That's why it's really irritating.
The tech that last visited did notice a significant packet loss at my house's incoming line vs at the tap though (my house is 5 losses/min? at the tap is zero). The cable between them is what I'm trying to get Comcast to replace, and getting them to do this is... hard.
Do you know of any DOCSIS 3.0 modem that can get 35Mbps upload on Xfinity? That's mostly the reason I upgraded to Gigabit. I don't want 500 down 5 up :( Sad.
(edited)
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