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I've been having this issue for almost two months at this point: modem signal levels and power output look acceptable as well as all DOCSIS 3.0 channels seem to be healthy. However, the OFDM channel on 159 is having a lot of correctable errors and the modem fluctuates between a state multiple times a day where I get my full 1GBits to a very slow 32Mbits when the issues on the OFDM channel are observed. First off however, some data to show the status of the modem (Netgear CM2000):
Over time the signal levels are pretty steady too (I record them with a tool I wrote to visualize the levels in Grafana):
Upstream levels are steady too:
Correctable and uncorrectable errors over time (OFDM only):
Logs
Note how the issue usually starts with an Event Type code 3 on the OFDM channel, then followed by events 20 and 22 alternating.
Thu Jan 28 08:01:19 2021 (Notice (6)) CM-STATUS message sent. Event Type Code: 22; Chan ID: 159; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=08:36:00:00:00:00;CMTS-MAC=00:38:df:08:73:25;CM-QOS=1.1;CM-VER=3.1; Thu Jan 28 08:00:54 2021 (Notice (6)) CM-STATUS message sent. Event Type Code: 20; Chan ID: 159; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=08:36:00:00:00:00;CMTS-MAC=00:38:df:08:73:25;CM-QOS=1.1;CM-VER=3.1; Thu Jan 28 07:56:19 2021 (Notice (6)) CM-STATUS message sent. Event Type Code: 22; Chan ID: 159; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=08:36:00:00:00:00;CMTS-MAC=00:38:df:08:73:25;CM-QOS=1.1;CM-VER=3.1; Thu Jan 28 07:55:54 2021 (Notice (6)) CM-STATUS message sent. Event Type Code: 20; Chan ID: 159; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=08:36:00:00:00:00;CMTS-MAC=00:38:df:08:73:25;CM-QOS=1.1;CM-VER=3.1; Thu Jan 28 07:54:15 2021 (Notice (6)) CM-STATUS message sent. Event Type Code: 3; Chan ID: N/A; DSID: �; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=08:36::00:00:00:00;CMTS-MAC=00:38:df:08:73:25;CM-QOS=1.1;CM-VER=3.1;
Observations:
- Other modems show the same behavior. In these two months I also tried an Arris S33 and a Motorola MB8600 modem, both of which show the exact same behavior and log entries when the issue occurs.
- Restarting the modem usually recovers speed for a few minutes to a few hours. At some point the modem falls back into this "slow mode" on 32MBits max.
- All speed measurements were done directly connected to the modem without wifi or a router in between
- I had three techs come out. The first tuned the power levels a bit, but they were ok to begin with.
- The second tech didn't know what to do because the signal levels look fine, however was seeing that Channel 159 for OFDM was having "some sort of issue".
- Third tech came out and tried questioning my networking equipment, nonwithstanding the fact that the issue occurs directly at the modem and is observable when directly connected. He reset the modem, it came back to full speed for 10m and he called it done at that point. He did however enable "monitoring" on the upstream power outputs of the modem, but when I checked in a week later he claimed that no issues were observed.
The log entries are pretty telling in the sense that the modem is unhappy with the OFDM channel. Unfortunately, at this point I don't know how to proceed further and get Comcast's attention at the right level. It sure seems like my local head end (CMTS) is having an issue somewhere with Docsis 3.1.
