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Dave9's avatar
Contributor III

For tech nerds - reading cable modem logs

This post is mainly for tech nerds like me who want to be able to do some troubleshooting on their own. Let me start by saying that I've been very happy with Cox service overall. I'm working through an issue with them right now relating to several problems that had been building up for a while. This was causing some speed issues, packet loss, and latency spikes. Speed is good now when I'm not dropping packets. I get 800/30 on Gigablast which is good enough for me. Every now and then I'll even hit the full 940 down. So I can't complain about that. But when the packet loss and latency starts, I go looking in the logs to try to figure out what's causing the remaining issues.

I know a little bit about reading cable modem logs for things like T3 and T4 timeouts, but I'd ignored the "CM-STATUS" log entries because they show up as level 6 and I thought that since lower is worse, level 6 could be ignored. In fact it looks like these entries can contain a lot of useful information.

The Event Type Code tells what type of status change has occurred, for example in this message:

06/04/2020 00:57 74010100 6 "CM-STATUS message sent. Event Type Code: 16; Chan ID: 159; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: 0 1 2 3.;CM-MAC=(redacted);CMTS-MAC=(redacted);CM-QOS=1.1;CM-VER=3.1;"

The Event Type Code of 16 means "DS OFDM profile failure - FEC errors were over limit on one of the assigned downstream OFDM profiles of a channel". (DS = downstream, FEC = forward error correction). This seems to mean that there were too many correctable errors on the DOCSIS 3.1 downstream channel. Maybe that's not a huge issue, I'm not sure. But it seems like something to take into account.

Another one I've been seeing a lot is 20 which means "NCP profile failure - FEC errors were over limit on NCP". If you haven't fallen asleep reading this and have some knowledge of networking, you can google DOCSIS NCP and you'll be able to understand what this means. Maybe not the biggest problem in the world, but it seems like it shouldn't be happening dozens of times per day.

The rest of the codes can be found in the DOCSIS 3.1 MAC and Upper Layer Protocols Interface Specification which you can download from the link below. In version I20, it's on page 465. It will be on different pages in different versions. I couldn't find any easy way to just paste that table here.

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