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Botairtx's avatar
Botairtx
New Contributor
4 years ago

Weird issue with OFDM channel causing disconnects

Hi, last week a storm blew through and a fallen tree took out some cables, resulting in loss of connection. It has been since fixed, but ever since I have been experiencing intermittent internet issues.

Symptoms:

My internet connection would suddenly drop briefly, any streaming would be interrupted, all VoIP would disconnect and online games will disconnect immediately.

The connection will be restored immediately (usually within 30s), but this can happen in intervals of 20 minutes to as low as every other minute.

I receive intermittent packet loss when doing a packet loss test.

Troubleshooting so far:

Ever since the initial outage so far, I have replaced my modem 3 times. First was a refurb CM1000, then a new CM1000, now a Arris Surfboard SB200 (I have 500Mbps)

I consistently (every ~5 minutes) see the following message in my modem logs, across all 3 modems (MAC address redacted):

CM-STATUS message sent. Event Type Code: 16; Chan ID: 159; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: 2.;CM-MAC=XXX;CMTS-MAC=XXX;CM-QOS=1.1;CM-VER=3.1;"

It would sometimes alternate between Event Type Code 16 and event type code 24. The Chan 159 corresponds to the OFDM channel, and is throwing a lot of correctable codewords (around 10% of total codewords). I will have posted the signal levels and the modem logs as an image to this post

I called tech support and after being on the phone with them for 1 hour, they said they detected intermittent packet loss and could see an OFDM error code on their end

I have requested a tech to come out, but he said that the signal levels are perfect and that the problem is the modem. I am almost 99% sure its not my modem at this point since I have tried 3 of them and all have the same issue. 
https://imgur.com/a/nu4jwlEhttps://imgur.com/a/BWTkSKKhttps://imgur.com/GemgqoI

Any help would be greatly appreciated.

ARRIS SB8200
cm1000
internet
Internet Connection
ofdm
packet loss

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  • Dave9's avatar
    Dave9
    Contributor III
    4 years ago

    It's almost never the modem. Support techs always blame the modem and I think that costs Cox and their support a lot of credibility. I wish they would change that policy but I'm just a customer so what can I do?

    I'm still learning about OFDM, but what I can tell you is that Event Type Code 16 means "Downstream OFDM profile failure" which is caused by "FEC errors were over the limit on one of the assigned downstream OFDM profiles of a channel" and Event Type Code 24 means "OFDM profile recovery" which is caused by "FEC recovery on OFDM profile". FEC means "forward error correction" and that refers to correctable codewords.

    From what I understand, you want to see a lot of correctable errors on the OFDM channel because that means it's running at its maximum possible speed. When you start seeing uncorrectable errors, that's when you should worry. I think the messages you're seeing are just the modem shuffling around OFDM profiles for maximum performance.

    Your downstream signals and error rates look excellent so I would suggest looking for other errors in your modem log related to upstream connections such as T3 timeouts or Dynamic Range Window violations. The OFDM CM-STATUS messages might make it hard to find those errors but keep refreshing the page to see what you can catch.

    • Botairtx's avatar
      Botairtx
      New Contributor
      4 years ago

      Now that you mention it, I did see some dynamic range window violations upon startup of the modem. However I have never seen them pop up when my internet drops. However the CM-Status messages always coincide with my interruptions so I thought that might be the issue. Here is a screenshot of a modem boot I had.https://imgur.com/a/eLsCkbH

      • Dave9's avatar
        Dave9
        Contributor III
        4 years ago

        Maybe there is some type of disruption associated with changing OFDM profiles. I don't know enough about how it works to say for sure. Hopefully someone who knows more than me can answer that. It does seem like it's reassigning your OFDM profiles a lot. I only get this type of message anywhere from every few hours to every few days.

        I think the DRW violations at startup are normal. The only thing I've noticed there is that when my connection is working, the DRW messages are the ones about "6dB below" whereas when I'm having problems I see the "exceeds value" messages.