[MUD_Wizard]

Originally Posted by Kushan

Possibly because the SNR targets have shifted over the years as the network has migrated from DOCSIS1 to DOCSIS2 to DOCSIS3.

That's not it at all. The SNR targets/thresholds haven't shifted for 256 QAM over the years and 256 QAM has been used for a long time.

SNR thresholds are based on modulation, not whether it's Docsis 1/2/3 as such. Docsis 1 did introduce support for QPSK and 16 QAM modulations and Docsis 2 added 8-QAM, 32-QAM and 64-QAM.

256 QAM lower threshold ranges from 28 to 30 dB.
Add 3 dB headroom for local conditions and you get 33 dB.
VM will often quote one or the other of those figures, but they haven't moved.

Data corruption, visible in the form of post-rs errors, usually starts around 34 dB, but the noise errors need to come in clumps in order to trigger post-rs errors. If they're spread out enough then you'll just get lots of pre-rs errors and no or few post-rs errors.

However, to make things more complex different Superhubs display slightly different RxMER figures on the same circuit, which you have to take into account when giving advice.


Btw, the next modulation down is 64 QAM, the lower threshold for which is 22 to 24 dB. So they won't be quoting that.

Originally Posted by Kushan

I know when I was there, there was still a distinction between ex-NTL and ex-TW areas in terms of acceptable power levels and SNR (Likely to do with the CMTS/UBR systems used by each franchise). Not all of the agents paid particular attention to the distinctions, though.

There are slight differences in power level ranges due to area, but the current range of -6 to +10 dBmV (downstream) generally holds, with some fine tuning by area. RxMER / SNR has an impact on how far outside that range you can go before experiencing problems.

CMTS manufacturers is a whole other conversation.