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Thread: For Cable modem users: let's compare DOCSIS signal levels

  1. #1
    Moderator anon's Avatar
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    For Cable modem users: let's compare DOCSIS signal levels

    First, a quick explanation.

    • Upstream power: should be between 40 and 50 dBmV. Whenever this value gets out of range it's usually by going above 50, which means the modem is pumping too much power attempting to compensate for a bad connection.
    • Downstream power: should be between -10 and 10 dBmV, and ideally 0, which would mean the signal is received with the same exact strength it is transmitted by your ISP. If nonzero, it's better for it to be positive rather than negative.
    • SNR a.k.a. signal-to-noise ratio: should be 35 dB at least. Since this is a ratio, it has no upper limit, and the higher the better.

    (Note that these hints assume DOCSIS 3.x and the modulations displayed in the screenshot)

    • Unerrored codewords: data that was received without problems.
    • Corrected codewords: data that got corrupted in transit, but was fixed by Forward Error Correction. Less severe than the ones below, but you'll still want them to be as close to zero as possible.
    • Uncorrectable codewords: same as above, but couldn't be fixed. You want these to be as close to zero as possible. If you get thousands of these on one or more channels, you have interference or bad wiring along the way.



    As you can see, my levels are pretty good, but I've heard of people getting perfect zeros on their downstream power, or SNRs in the low 50s, so I thought it'd be fun to compare.
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  2. Who Said Thanks:

    Instab (23.10.20) , cloud99 (23.10.20)

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    Advanced User yoco's Avatar
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    I have cable internet! How do I see these results?
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    Moderator anon's Avatar
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    Enter your modem's configuration panel and you'll find it somewhere. Sometimes you don't even need to log in to see it, since this data is not confidential and very useful for troubleshooting.
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    Advanced User yoco's Avatar
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    I know how to get into router, but how do I get into modem? I think it's Ubee or something like that!
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    Moderator anon's Avatar
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    Try 192.168.100.1
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    Advanced User yoco's Avatar
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    Is it this?

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    Moderator anon's Avatar
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    Yes Your signal levels are on par with mine and very good. The high amount of uncorrectable codewords is a bit worrying, but it's okay as long as you don't have any noticeable problems with your service. As you can see I have all zeros there... perhaps the quality of my building's cable setup is exceptionally great, making the contrast high, but I doubt that.
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    yoco (26.10.20)

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    Moderator anon's Avatar
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    For comparison purposes, here's a modem with bad signal levels. Note the high amount of uncorrectables and how a few channels are effectively disabled.

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    Moderator anon's Avatar
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    Here's another, less severe example of bad signal levels.

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