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How does Comcast manage unused channels when a DOCSIS 3.0 modem supports more downstream channels than the data plan requires?
I have a DOCSIS modem that supports 24 downstream channels. My plan (300Mbps) only requires 16 downstream channels, so 8 of the modem channels (17-24) appear to go unused.
My modem reports receiving noise or data on those channels. This data is not valid, and it generates Corrected and Uncorrected errors across the 8 channels. In contrast, no errors are seen on the 16 channels used by my plan.
Rebooting the modem can change the frequency spectrum it uses - sometimes 400MHz-550MHz, sometimes 525MHz-675MHz - and this results in more or fewer errors occurring on the 8 unused channels, depending on the frequency used, I'm guessing. When there is little to no data/errors on these unused channels, I get the full 300 Mbps downstream performance. When it's at its worst, this data can bring the modem to a grinding halt, and performance can drop below 10 Mbps, just because it's spending so much effort handling these errors.
Regardless of the spectrum used, channels 1-16 always remain error-free; the errors only occur in channels 17-24. And upstream traffic is never impacted by errors or performance degradation.
Does anyone have any idea where this data might be coming from?
How does Comcast manage the "unused" channels of a modem connected to their service? Could other customers' data be "leaking" into channels adjacent to those used by my internet plan, and they are delivered to my modem as a result?
Should Comcast be configuring my modem to "shut off" channels 17-24? Or is there a way I can do it myself?
EG
Expert
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110.2K Messages
3 years ago
It doesn't work that way. All bonded channels that are available / locked on to are utilized and load-balanced equally regardless of one's subscribed to speed tier. It's one combined pipe. They can't just "shut off" some channels and neither can you
Something else is going. It's not unusual that higher channel frequencies are noisier than lower channels.
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user_b8948b
Visitor
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3 Messages
3 years ago
Thanks for the quick response.
if that’s the case, then something very odd is going on.
In the cases where I see sufficient downstream performance and no packet loss when pinging the upstream node from the modem, channels 17-24 show no signal - 0dB SNR. Any time I see any positive SNR on those channels, it is accompanied by high error rates on them, and significant packet loss.
Conversely, I see a solid 40dB SNR on channels 1-16, with virtual no errors at all - single figures over days, if any.
Power across all channels is 6-7dBmV.
So it seems like Comcast are only sending me valid data over channels 1 to 16, even if all channels are active. Any data sent over channel 17 to 24 seems to be spurious or invalid - and not my data, I suspect.
What do you think might be going on? Is my modem faulty, and the connection is working - when it works - by a fluke?
(edited)
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EG
Expert
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110.2K Messages
3 years ago
You should get a tech out to check the line(s) for high frequency ingress.
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user_b8948b
Visitor
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3 Messages
3 years ago
Thanks for your help and time, EG. I appreciate it!
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