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acellier
@cox.net

acellier

Anon

[CA] flakey Signal to Noise Ratio on high bonded channel(s)

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poor high-channel SNR
We have been having intermittent poor connectivity problems. Techs came out twice, changed connectors and internet filter. (New filter is UBR-2/78DF - anyone know the specs?)
We continue to observe (SB6120 modem) the upper channel (873 MHz, 867 MHz) SNRs drop to 30 and 29 dB from nominal 33-34 with Signal negative 4 to 6 dBV. The 855, 861 MHz channels tend to stay in the 34-36 dB SNR range with Signal negative 1 to 3 dBV. Seems like the top channels are marginal at best.
Communication interruptions have resulted in hundreds of zero-byte files ftp'd to server from our IP cameras, and we even got locked out of our server for a day because of a glitch on one of those transmissions. More than just annoying.
Does anyone have actionable suggestions? There is no competitive source of HSI in our area.
dn0
join:2005-07-05
USA

dn0

Member

You need to press Cox in CA to fix your problem. The signal level is in the good range, although that doesn't mean that it is "good". The techs that come out all have meters capable of reading the MER of each and every QAM carrier from any point from the tap port at the pole/pedestal on out . They need to look specifically at the problem channels as 29dB SNR (or truly Modulation Error Ratio) is not acceptable. Problems start with 256 QAM carriers when the MER drops below about 31-32dB. I say keep bugging them, you cannot fix the signal problem yourself without the proper tools. If the tech's meter reads a good MER at the modem connector, and your modem still shows a low MER (SNR), then it may be the modem..... (ex-maintenance tech, 18 years at Cox, Time Warner - Back handed many a lazy service tech )

acellier
@cox.net

acellier

Anon

@dn0 - Thank you for that information about their meters - it is like pulling hen's teeth to get any information - I suspect the filter was designed for 861 MHz top channel which seems to be the nominal in D-3 specs; Cox somehow has chosen to go 2 slots above that, and the filter seems to be cutting off ...
My background is communications engineering (MSEE Stanford; decades in various spaceborne systems).
dn0
join:2005-07-05
USA

dn0 to acellier

Member

to acellier
It would seem that the filter should only block channels 2 thru 78, or about 55 - 550 MHz (approx.). I am not sure what the DF stands for (maybe Data Filter?), that may be some custom configuration that your local Cox provider ordered. And the filters are rated to 1 GHz, so that shouldn't be an issue.
I personally believe you have an issue not related to the filter and you need a tech to verify that your levels and MER are per design at the tap feeding you. If they are, then problem is between the tap and modem. If they are not 100% at the tap, then they need to send plant maintenance, etc. However they do it locally. With a clean signal at your modem, even at -4/-5dB, you should see SNR's of 35dB or higher on all carriers.

edit: here's a link to explain PPC's trap a little bit for you: »www.ppc-online.com/broad ··· hp?id=67

acellier
@cox.net

acellier

Anon

Thank you very much for the link to the filter manufacturer! I can see that they are generally (video channels) bandstop devices, and otherwise pretty flat to 1000 MHz.
But it does seem odd that the upper 2 channels (in my case) are so strongly impacted, relative to the lower 2 (which are very close by, in relative terms).
Perhaps the issue is some form of interference, rather than a channel distortion. Have you ever known of problems with harmonics of video channels, or cross-modulation of adjacent channels, perhaps if a distribution amplifier is overloaded?
dn0
join:2005-07-05
USA

dn0

Member

The answers are yes and yes. As you obviously understand, when all those 6 MHz channels are jammed together, just one out of wack carrier can cause cascading issues. Even a dent in the coax somewhere can create an impedance problem then we see reflections that can snowball into bigger issues.
I don't want to under or over explain anything with you as I can understand that you are very RF aware. Maybe you should just take the RF meter from the next tech that shows up and help him out. Not joking, I don't mean to cut down on any techs, but it is the entry level spot. You have to learn some how.
BTW, we really don't see any Hum, intermod, cross-mod problems any more since the digital change over. All of those analog distortions are now kind of grouped into the MER of the Digitals. If the MER is good (about 36+ at 256QAM) and there are no Bit Errors, then the data stream is clean. Period.

I'll keep checking back acellier, I am convinced that your issue will not be resolved until you see better numbers at your modem.

acellier
@cox.net

acellier

Anon

Click for full size
We have had only a few zero byte transfers since the filter was changed out - but SNRs continue poor - today the SNR on highest channel got really low (28 dB) and errors were climbing fast.

SDKiwi
join:2002-05-27
El Cajon, CA

SDKiwi

Member

If you want someone to look into your issue, please register here and IM me or Coxtech1 with your HFC Modem MAC address.