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Sweet spot values

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Svestenik
Sweet spot values

Hello,

What values are defined as best for :

Downstream Power (measured at the modem end)
Upstream Power (measured at the CMTS)
Signal to Noise ratio at both ends
Microreflections
FEC
and any other parameter that is vital for good instalation.

Since I am a new to the DOCSIS world, and have much to learn, it would be good if anyone would like share their practical experience in these matters.

Anonymous (not verified)
Sweet spot values

Downstream Power should be 0 dBmV at cm input
Upstream Power should be 0dBmV or your configured receive value from the CMTS
SNR the greater the better
Microreflections are meaningless on all CMTS I know except for recent releases on the BigBand Cuda. It has been redefined to something more meaningful in D 3.0 but that's not implemented out there yet.

Have a look at the d spec. It has some assumptions for cable networks defined.

Anonymous (not verified)
Sweet spot values

Rudy,
You are bang in with your response..... but I'd like to add the following:

You should try to engineer your plant so that modems receive as close to zero as possible - thus giving you +/- 15dBmV of wiggle room in case the customer adds splitters, or the inevitable plant variables emerge.
The modem's transmit level should be sufficient to overcome plant attenuation, arriving at the upstream receiver with a SNR of "at least" 20dB. My experience shows that a modem transmitting in the 40's, can get to a CMTS with a receive power of 0 (zero) with a reasonable SNR.
Microreflections do count - to a degree - as they indicate potential fibre breaks or Coax cracks. The higher the microreflection count, the more likely that ingress is going to leak in and impair the channel SNR

Hope this helps !

Svestenik
Sweet spot values

jimatarris wrote:Rudy,
You are bang in with your response..... but I'd like to add the following:

You should try to engineer your plant so that modems receive as close to zero as possible - thus giving you +/- 15dBmV of wiggle room in case the customer adds splitters, or the inevitable plant variables emerge.
The modem's transmit level should be sufficient to overcome plant attenuation, arriving at the upstream receiver with a SNR of "at least" 20dB. My experience shows that a modem transmitting in the 40's, can get to a CMTS with a receive power of 0 (zero) with a reasonable SNR.
Microreflections do count - to a degree - as they indicate potential fibre breaks or Coax cracks. The higher the microreflection count, the more likely that ingress is going to leak in and impair the channel SNR

Hope this helps !

My Motorola modems at the end of the line register via SNMP a microreflections count of 28-29 dBc. Is that something to start worrying about ?

frnkblk
Sweet spot values

I have lots of CMs in that range...most are 20-30, many are at 70-80 dBc.

Frank