Upstream SNR problem

Hello,

noise is due to multiple reasons.
Abnormal noise for example appeared when you have a:
- problem of transmit power levels of your modems
- cable network not correctly protected from hertzian attacks (old amplifiers, old cable, open wallets, bad connectors/connections...)
- high signal generating noise near your network (lights, GSM/LTE/DVB-c..., police freq, bus, baby phones....)
- reflected noise from modems (more you have modems on the same node more you will have noise)
...
to solve it, you have to go on site and check noise locally.
You need to correlate modems with problems (high power level, noise...) in order to find addresses where you need to control your network

good luck

In a perfect world, you will be running 6.4MHz width, 64QAM and the USSNR will be flat 36.

But as we all know, cable networks aren't perfect, and it's easy for noise to get into the system.

DOCSIS is able to handle some noise, but if there is too much you will end up with packetloss / dropouts.

The thresholds for when the loss kicks in will depend on your combination of width and QAM.

Generally if the USSNR is > 27 or so, your end users shouldn't notice any impact, even with 6.4 width and 64 QAM.

If you drop from 6.4 to 3.2, it will boost the USSNR by about 3dB, at the expense of losing around half the channel capacity. Same again if you go from 3.2 to 1.6 MHz width.

If you drop from 64QAM to 16QAM, your system will be able to run OK at lower SNR, again at the expense of channel capacity.

One handy feature is to configure the CMTS to run at high width/QAM values, and automatically drop back when SNR or uncorrectable thresholds are hit.

Also you have control over what frequencies to place the upstreams. I like to use highest possible frequencies as noise is usually worse in the lower ranges. You can hardcode these frequencies or setup a spectrum group on the CMTS to allow it to try and auto-locate the cleanest spectrum.

The CMTS has some other techniques to counter noise. Make sure you have got the "ingress-noise-cancellation" feature activated, and also assuming you don't have some really ancient modems in your plant you should turn on the "equalization-coefficient" feature.

In terms of how to fix the noise, I reckon there are a couple of main tips :

1. Make sure the node is correctly balanced. If the balance isn't right, the amps will can pump up small amounts of noise and make them very big. This includes making sure the coax amps are correct, optical links are correct, and headend/cmts padding is correct.

2. Once the node is confirmed to be balanced, it is usually a divide and conquer type approach. A few different methods available, but the coarsest one that describes the technique is to start at the node and start disconnecting legs one at a time. Keep an eye on the SNR. Once you have found the leg with the noise, you have to proceed down it and repeat the process until you find the source.

Maybe you can check which of the cm are sending uncorrectable codewords (from scm verbose command). Then you try to tackle these modems by either reseting them and see which of them when you resets improve the US MER

Hook an analyzer up in your headend, set it to look at 5-50mhz, and hook it into your return splitter that should be on the output of your return receiver. If you can hook the video out to a modulator, and put it out on an empty channel in your system, you can use a small battery-powered tv to help you track and eliminate the noise in your return. 90% of ingress and return noise will come out of a customer's house(loose connectors, bad/damaged cable etc). Once in a while you'll find a mainline crack with it, but usually you'll pick those up with your leakage meter before you chase it down on the return.

US noise and low SNR is very easy to find.
SNR is the easiest noise source to track.
Use your US spectrum, and perform isolations if allowed.