Very slow downstream, seem a lot of channels are not working?

Hi, we are having very slow (2/20 Mbps on 300/20 plan.....) and have had several fruitless chat sessions and one tech visit. When the tech was here, he was able to get 250Mbps while being directly plugged in to the model, but I never got more than 150, and usually 20 or less. Upload is consistently around 20Mbp, what it should be. I finally figured out how to get the info off the modem and it looks like several channels are not connected at al, and some others seem to have many errors, but I am not sure how to read this info. So in case anyone does understand, here it is. I have another tech visit due tomorrow, but I want to be as informed as possible when they get here, so I don't need a third visit. Are there some obvious points of failure that this info gives? Thanks in advance!

Oh, it's a Netgear CM500 modem. Previously had an Arris 6183, same problem. All connected to an Airport Extreme, but the problem is the same when plugged directly into the modem.

ChannelLock StatusModulationChannel IDFrequencyPowerSNRCorrectablesUncorrectables
1LockedQAM25632705000000 Hz-5.1 dBmV39.1 dB00
2Not LockedUnknown17615000000 Hz0.0 dBmV0.0 dB00
3Not LockedUnknown18621000000 Hz0.0 dBmV0.0 dB00
4Not LockedUnknown19627000000 Hz0.0 dBmV0.0 dB00
5LockedQAM25620633000000 Hz-5.2 dBmV29.4 dB53963
6Not LockedUnknown21639000000 Hz0.0 dBmV0.0 dB00
7Not LockedUnknown22645000000 Hz0.0 dBmV0.0 dB00
8LockedQAM25623651000000 Hz-5.6 dBmV27.9 dB1517364039
9LockedQAM25624657000000 Hz-5.3 dBmV36.2 dB00
10LockedQAM25625663000000 Hz-6.1 dBmV33.3 dB00
11LockedQAM25626669000000 Hz-5.6 dBmV36.1 dB00
12Not LockedUnknown27675000000 Hz0.0 dBmV0.0 dB00
13Not LockedUnknown28681000000 Hz0.0 dBmV0.0 dB00
14LockedQAM25629687000000 Hz-6.4 dBmV30.5 dB00
15LockedQAM25630693000000 Hz-6.1 dBmV27.1 dB20785211962
16LockedQAM25631699000000 Hz-6.4 dBmV34.9 dB00
Upstream Bonded Channels
ChannelLock StatusUS Channel TypeChannel IDSymbol RateFrequencyPower
1LockedATDMA25120 Ksym/sec23300000 Hz51 dBmV
2LockedTDMA12560 Ksym/sec18500000 Hz51 dBmV
3LockedATDMA35120 Ksym/sec30600000 Hz52 dBmV
4LockedATDMA45120 Ksym/sec37000000 Hz51 dBmV

Re: Very slow downstream, seem a lot of channels are not working?

You have a MAJOR signal level and  broadcast tv ingress noise issues, looks like there's splitters in line with open ports.  Maybe badly corroded connections and chewed up outside line as well.

The modem must be directly connected to the outside grounding block if you're internet only. All connectors must be shiny and a tad more than finger tight.


If you have cable tv. the modem must be on the first 2 way splitter with any boxes split off it's other port.

Whatever the first tech didn't fix is tthe issue which is coaxial not modem related.


Proven Sharer

Re: Very slow downstream, seem a lot of channels are not working?

1)  Print a copy of these signal levels so you can show the tech when he arrives, even if Spectrum 'accidentally' resets your modem and clears all the log messages. 

2)  Point out the SEVEN missing DS channels [17, 18, 19, 21, 22, 27, & 28].  These all show SNR (Signal to Noise Ratio) of 0.0 

3)  Point out another FIVE DS channels with SNR below the minimum acceptable value of 34 dB [20, 23, 25, 29, & 30].

4)  Show the tech that all four US channels are running above 51 dBmV while the maximum power allowed is 50.

5)  Ask the tech how your modem is supposed to work with only FOUR of 16 DS channels at a proper working level.  Tell him his job today is to get them all above 34 dB SNR as measured by your modem, or provide a credit on your monthly bill until they are compliant.  FWIW, it takes four good bonded DS channels just to make a working 60 Mbps connection.


Re: Very slow downstream, seem a lot of channels are not working?

Thanks, this is helpful to know for when the tech arrives!

Re: Very slow downstream, seem a lot of channels are not working?

Thank you. I will make sure that I have all the info (including the responses to this post) handy when the tech arrives.