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Forum Discussion
24 channel from 32? / day 8/ unstable xfinity internet / power 14.3 dBmV / SNR 36.5db
xfinity 400Mb service
CM1000 V7.01.01
last 8 days internet drops for 10min to 1-2 hours 5-8 times a day.
one coax line straight to my cm.
Line levels checked at home show the bottom 3-4 channels slowly going below 35db snr/mer to 33 and below. The signal was around 2dbmv before maintenance. It looks like an xfinity tech cranked up the power on the amplifier and now my power levels are 14dbmv and the signal snr ratio is higher by about .5-.8 but still the bottom 3-4 channels fall below 33db.
CM1000 with latest xfinity/netgear approved firmware V7.01.01
I live 9-12 miles of cable run through a few small towns to their nearest snmp/ noc point.
I am wondering if I should go to a 24 channel docis 3.0 modem to avoid the last 3-4 channels. Would this be more reliable for me? Perhaps this line will not be good enough to enable 3.1 & 32 channels without constant fettling.
24 channel modem might be my next stop.
they want to add a -6db filter. when it is added the snr level drops by ~1db or so. clearly that doesn't help.
Any advice?
16 Replies
I think you have a misunderstanding of your levels for upstream/downstream.
Your downstream (signal coming in) levels are high. That can cause errors. Thats the cmts sending the power to your device. This can by high if you're to close or the amplifier is turned up to high. It should be between -7.5dbmv to 7.5dbmv.
The upstream is the power that your own modem is sending to the cmts. It should be roughly 35-49dbmv. If its lower, it tends to indicate you're close to the hub and the modem doesn't need as much power to communicate with it. When you're further away, you need more power.
Them adding an attenuator can help based off your current numbers. It'd cut your downstream power down and your upstream would increase because of the attenuator
yes the upstreams are fine currently
It is a few downstreams that fall below 33db(SNR/MER) during the day and the internet becomes unstable and requires the cm to reboot.
lowering the power(dBmV) does not help with the SNR levels currently.
The tech says the LTE cell tower is interfering with channels on the modem for downstream and making the SNR values go down and thus the service fails eventually or part of it.
I do not know enough about 32 bonded channels, but it sure seems like a bad design if 2 or more channels cause the entire service to fail. If that is my symptom, measured so far, that it makes me want to avoid those channels some how. forcing to 24 channels and not using that with a 24 channel modem came to mind.
the tech says the cable plant is not to use 733.75MHz & 739.75MHz anymore but they turn it on occasionally. so much of this is out of our control. i would sacrifice speed for stability, that's a inclination to go to move to 16 or 24 channels and find a way to lock channels with good signal metrics. doesn't seem likely though with xfinity or the way cm's are designed.
perhaps some AI in the modem to select better channels over time?
I really do not want to spend my life monitoring xfinity....
jms23 wrote:yes the upstreams are fine currently
It is a few downstreams that fall below 33db(SNR/MER) during the day and the internet becomes unstable and requires the cm to reboot.---If your snr is dropping that much, the signal is getting intererence from somewhere. It should be fairly consistent and not change a whole lot.
lowering the power(dBmV) does not help with the SNR levels currently.----Its not about the SNR. Its about the errors you have in your line. You have a bunch of them. You can have the best SNR possible but if you have errors, then theres a problem with the line, the cmts, or the modem
The tech says the LTE cell tower is interfering with channels on the modem for downstream and making the SNR values go down and thus the service fails eventually or part of it.----this is a load of BS.
I do not know enough about 32 bonded channels, but it sure seems like a bad design if 2 or more channels cause the entire service to fail. ----its not the channels, its the interference thats causing it. If that is my symptom, measured so far, that it makes me want to avoid those channels some how. forcing to 24 channels and not using that with a 24 channel modem came to mind. ----doesn't work like that. You can feel free to try though.
the tech says the cable plant is not to use 733.75MHz & 739.75MHz anymore but they turn it on occasionally. so much of this is out of our control. i would sacrifice speed for stability, that's a inclination to go to move to 16 or 24 channels and find a way to lock channels with good signal metrics. doesn't seem likely though with xfinity or the way cm's are designed.
perhaps some AI in the modem to select better channels over time?----the cmts from your ISP is the one that sets the channels to use. Not netgear or the modem.
I really do not want to spend my life monitoring xfinity....
You also have a lot of errors in the line. Check for bad/old/corroded connectors. Kinks or damage to the line.
going to a 24 channel modem isn't going to help and will potentially hurt as the docsis 3.1 modems enable extended upstream power levels and the ofdma channels can transmit more data
thank you for replying
% of errors is 0% after the last cm reboot. cable from tap to house was replaced yesterday.
739.75MHz down still has a low SNR compared to peers at 42db
can netgear adjust the channels it uses or put a range of channels or frequencies not to use?
is this in the works as an enhacement somehow?
with LTE and 5G cranking up post pandemic, it seems this will become more of an issue with interference with cable?
jms23 wrote:thank you for replying
% of errors is 0% after the last cm reboot. cable from tap to house was replaced yesterday.
739.75MHz down still has a low SNR compared to peers at 42db----its at 39. anything over 33 is within spec for snr.
can netgear adjust the channels it uses or put a range of channels or frequencies not to use?---No. its set by the isp
is this in the works as an enhacement somehow?---doubtful as the isp controls it
with LTE and 5G cranking up post pandemic, it seems this will become more of an issue with interference with cable?---per the prior comment, this is a load of bs. the towers might pull some bandwidth and if thats causing the issue, your isp needs another hub for the tower seperate from consumer connections
CM1000 V7.01.01
24 hours in without internet issues.
changes by xfinity.
1)adjusted the level of their amplifier.
2)changed the spectrum of Mhz my 32 download channels attach to. The highest level I attach to is channel 32 on 685750000hz (687.5MHz). They claim they were getting interference from LTE on their line on 733.75&739.75MHz prior to this.
result on metrics from cm1000 I can see:
download streams only here:
dBmV values are 5.2 to 6.8 with a -6db filter(at comcast box on house) in line.(downstream)
SER/MER values are 40.8 to 42.0 (downstream)
uncorrectable errors are 0
correctable errors are 1,000-9,000 on 2.1billion words or correctable error rate of 0.00038%
SpeedTest with a pc via ethernet, 1Gb wired connection to R7000 which is wired to CM1000:
14.2ms 486Mb down 12Mb up on a 400Mb plan
I think this looks good for now. Not sure i buy the interference with their 700+Mhz line with LTE(red herring?) but I'll take this for now.
Appreciate the insight and assistance thus far!
Thank You All !
