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Upstream SNR problems

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Upstream SNR problems

Can I get some recommendations on a meter that will help in pin-pointing devices or connections causing high noise levels on the upstream side. We are experiencing high noise levels intermittingly on networks of ours...sometimes as low a SNR=6. I am looking for a meter that will help us find exact location of the noise problems on the network...we use CMTS 1000's on most networks.


Upsteam SNR problems

Big challenge here. With test equipment I think you still have to isolate legs by breaking upstream connectivity (pulling pads) to see if noise falls off back at the headend.

Other solution is begin to replace older CMTS's with newer ones that support per CM modem upstream snr stats. They dont have to be "new" cmts's, just used ones that support this.

Most older CMTS's give you an upstream reading for the channel. A group of modems in a particular area (with a localized noise issue) can negatively impact this average. Per modem cm stats kept on the CMTS are based on individual modem bursts. This way you can look at the modems on a particular upstream port and see which ones have a lower value that are dragging down the channel numbers. Looking at where these modems reside based on physical address can help pinpoint legs that may have return issues with common components.

Another basic thing you can look at is the receive power level reported at the CMTS for all the modems. If they are registered, but not making it back upstream at the configured target level on the cmts they will drag doen the average as well.

Some options

I agree with what mtntrailseeker comments concerning the isolation of noise in the physical networks.

But if your are BSR1000 CMTS, you can see SNR values of each cable modem.

The following command displays these values:

show cable modem phy

This is an example:

ESC-BSR1K # show cable modem phy
MAC Address USPwr USSNR Timing
(dBmV) (tenthdB) Offset
0012.3456.7890 0 291 1846
0012.0987.6543 4 274 1894
0012.0192.3747 5 275 1933
0013.3747.0192 6 276 1934

For both USPwr values, as for the USSNR, divide among 10, to get the value in the corresponding dimensional. In the case of USPwr (Upstream Power) would be in dBmV and USSNR (Upstream SNR) dB.

The above example is interpreted as follows:

ESC-BSR1K # show cable modem phy
MAC Address USPwr USSNR Timing
(dBmV) (tenthdB) Offset
0012.3456.7890 0.0 29.1 1846
0012.0987.6543 0.4 27.4 1894
0012.0192.3747 0.5 27.5 1933
0013.3747.0192 0.6 27.6 1934

Necessary levels of SNR upon the level of digital modulation in the case of Upstream, are commonly used QPSK and 16QAM modulation. These are the recommended values:

QPSK: 12 dB minimum. 15 dB or higher recommended. (often used in upstream channels)
16 QAM: 18 dB minimum. 21 dB or higher recommended. (often used in upstream channels)
64 QAM: 24 dB minimum. 27 dB or higher recommended. (often used in downstream channels)
256 QAM: 30 dB minimum. 33 dB or higher recommended. (often used in downstream channels)

* There is no upper SNR limit, however, 40 dB is the highest most people see. Going above 40 dB is possible though.

These values were seen at the following website:

Based on the records of which are installed cablemodems, can relate the information of each SNR to locate the sector that is failing and the physical network where the problem occurs.

Nota: los valores de SNR son instantáneos y de los cablemodems que están online.

Try searching for a command equivalent to the CMTS 1000 (Arris).



First of all You should use

First of all You should use some software to have often reading snr from cmts - like every 3 to 5 seconds. Then try to disconnect return path at half of Your network see what happens. And then easily and fast in cooperation with field technicians You'll have problem solved / found. Even with old, good Arris :)

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