Quote:
Originally Posted by General Maximus
Interesting that they have decided to go down the 64qam route rather than bonding a 3rd upstream channel
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Adding a 3rd channel lowers the maximum transmit power using 16QAM to 52dBmV, so you lose both 2dB of SNR and modems that were marginal are now at maximum transmit.
Running at 64QAM over 2 channels maximum transmit power is 54dBmV, and in addition VM can use dynamic upstream modulation, so despite 64QAM requiring a better SNR than 16QAM (27dB vs 21dB after error correction, etc) if the cable is too noisy it can drop down to 32QAM or 16QAM as required.
Can't make allowances for modems that are too attenuated, and indeed a way to raise SNR on the upstream is to apply attenuation. When ntl moved to 16QAM part of the process in at least some cases was to apply 3dB additional attenuation to upstreams, making modems transmit 3dB higher to hit the target power and increasing SNR by 3dB in a stroke.
Once the networks have been properly realigned so no modems are in the danger zone 3rd and 4th channels will appear, possibly together in some cases as the power limits for 4 upstreams are exactly the same as for 3.