Observer

Only 4 downstream channels: My modem supports 8.

Recently I noticed two issues with my 100 Mbps service.  The internet works OK although occasionally speedtest results are low.

 

The first issue is that Spectrum seems to have changed the network management and where I used to get 8 QAM256 downstream channels with modulation, I now get 4.  I use my own Zoom 5431J DOCSIS 3.0 modem which supports 8 bonded channels down.   

 

The second issue is whenever the modem bonds to channel 13 it has a high uncorrected error rate.  I don't think the modem power measurements are accurate.  They vary considerably over time.  However, channel 13 tends to be outside the recommended QAM256 limits of +/- 12 dBmV .  The modem negotiates new downstream channels over time so this problem comes as goes as channel 13 is replaced by another channel.

 

The modem is only device connected to the incoming cable.  I don't have TV service.  The internet works OK but I expect the high error rate is causing packet retries.  

Only 4 of 8 channels have modulationOnly 4 of 8 channels have modulation

12 REPLIES
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Established Sharer

Re: Only 4 downstream channels: My modem supports 8.

It appears you are suffering from broadcast TV Signal ingress on RF Channel 24 (530 - 536 MHz).

 

Check all your cable connections.  Make sure they are more than finger tight.  Terminate (with 75 ohm resistors, F-Type) all unused cable outlets.  You shouldn't have any unattached cable drops in your home.

 

What market are you in? 

Observer

Re: Only 4 downstream channels: My modem supports 8.

I am in the Charlotte, NC market.  There are TV stations at RF channel 25 and 23 but none at RF channel 24.  The connector to the cable modem is tight.  I don't have cable TV service so the cable in from the street goes directly to the cable modem and no where else.

 

I have a whole home distribution amplifier for the signal from the OTA antenna.  The power supply for the cable modem and the OTA amplifier are plugged into the same power outlet.

 

Established Sharer

Re: Only 4 downstream channels: My modem supports 8.

I use the same modem up here in Ohio, and I'm quite familiar with its performance.  After reviewing your numbers, it sure looks like Spectrum is currently upgrading their cable plant in your area to enhance their internet service offerings.   Four of the assigned DS channels (9-12) are not connected to video programming but that could be corrected overnight.  It also looks like TWC may have previously placed a 'trap' filter in the coax drop from the pole, which needs to be removed with their new channel lineup.  

 

You've already found that problem channel DS13 has low signal and extremely high errors.  The real world limits are more like -8.0 to +10.0 dBmV level coupled with a minimum SNR of 34 dB.  If DS13 ever gets assigned as the 'home' #1 channel in your modem before the interference problem is repaired, you will have terrible internet performance or none at all. 

 

Perform a factory reset on your modem so that it gets a fresh network initialization from Spectrum and a new block assignment of DS channels.  Then post the signal level page again.

Observer

Re: Only 4 downstream channels: My modem supports 8.

Factory resetting the modem resolved the issues.  I now have  8 download channels and all channels are within +/- 12 dBmV.  To factory reset the Zoom 5341J, I pressed and held the reset button for 5 seconds.  Be careful to do it in one action as a short press resets the modem without a factory reset.

 

Zoom5431J_AfterFactoryReset.PNG

 

 

 

Expert

Re: Only 4 downstream channels: My modem supports 8.

That is not resolved, You need to have Spectrum out to remove the internet only trap on the line out at the pole as well as remove the extra splitters in line. It is a coaxial not a modem issue, they deleted qam tv that was in the 470-550 mHz range and now use those as modem channels.... It has been an issue in Austin texas for 3 years and then NC. The filter kills everything from 50-530 mHz and MUST BE REMOVED

You also have at least one splitter in line with open ports that must be removed.

The cable and tv channels slightly overlap one another, there may also be ingress but  the low/nonexistent levels from the trap must be corrected first... - 8 is too low on the good channels

 

Established Sharer

Re: Only 4 downstream channels: My modem supports 8.

Glad to see you are up and running with all 8 DS channels again.  Yes, the Zoom's reset button is a bit touchy but holding it in for more than 5 seconds forces the full reset.  Please mark your solution as "PROBLEM SOLVED" using the pulldown Options menu on the upper right side.

Expert

Re: Only 4 downstream channels: My modem supports 8.

Problem is masked, not solved.../. TWC needs to come out and remove the TV band trap from the pole tapoff

Next time modem resets it may end up on the filtered out channels...

This was a horrible issue in Austin and to a lesser extent in NC as there were far fewer filter traps installed.

 

Observer

Re: Only 4 downstream channels: My modem supports 8.

I don't understand your suggestion to ask Spectrum to remove the TV band trap.  Are you saying the downstream power levels should be closer to 0 dBmV?  After the factory reset, the modem is always getting 8 downstream channels and the power level is consistently around -8 dBmV.  I reset the modem and again got 8 downsteam channels.

 

From my perspective, Spectrum is providing the expected level of service.   I have 100 Mbps service and get 118 Mbps down and 11.8 Mbps up measured by both speed test and spectrum speed test.  After 64 hours,  the modem shows 0 uncorrectable errors and 84 correctable errors.  This seems reasonable and and no justification for a service tech visit.

Zoom5431J_After64Hours.PNG

 

 

 

 

Expert

Re: Only 4 downstream channels: My modem supports 8.

You  have 1 or 2 issues. DS level is too low for internet only service as well as a trap on the line killing off channels below 550 mHz.

 You have temporarily eliminated the problem, not corrected them .

 In Austin Tx they had to change out/ remove thousands of filter traps that they forgot about when they added DS channels.., In NC the same problem occured to a lesser extent.

 You also have too wide a difference between US and DS levels indicating possibly waterlogged outside line. Please post a photo of the outside tap off block on the pole and whether the line is underground or run thru trees.

Problem will only get worst.