[Kushan]

Originally Posted by MUD_Wizard

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.

You need to define "A long time". I'm talking about ~5 years ago now and I don't know what the OP's timeframe is, nor if there are agents that still use the "old" guidelines (Wouldn't surprise me).

Originally Posted by MUD_Wizard

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.

I'm not disagreeing with anything you're saying, however all I can tell you is what VM staff were actually trained to look at. I'm not saying it's correct, just that's what the training told agents to look for. Modulation never came into it (it was very briefly glossed over).

Originally Posted by MUD_Wizard

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.

Again, agents were taught to look at it slightly differently. They largely ignored post-rs errors for individual customers and instead only looked at the errors on a specific line/cable. They have (or had) a tool that gave a lovely colourful view on the errors in that area and a lovely arbitrary %. If the % was above a certain number, it was raised as an outage. If not, it was left to it (Ironically, the threshold was something like 30% but when a line was completely down, the tool only ever reported a static 25% - NFF!). For individual customers, it was T3 and T4 timeouts that mattered in this case (As well as power/SNR).

Originally Posted by MUD_Wizard

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.

No indeed, they quoted 20dB in some areas as being acceptable (ex-NTL if I recall).

Originally Posted by MUD_Wizard

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.

The power levels these days are definitely more consistent and it was aligned internally with the Docsis 3 rollout, however prior to that the range was much larger (I want to say -10 to +12 but I'm not 100%) and again, did differ by area on occasion.

If I had to guess, the guidelines being different for ex-TW and ex-NTL possibly stem from the merging of the two. I would not be surprised if both companies simply had different guidelines for their agents and Virgin simply decided to keep treating them separately until they got their ducks in a row.