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Getting 4 upstreams at 64QAM requires either
DFB lasers in the nodes or digitised return paths. In some areas nodes and amplifiers need replacing.
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I've haven't worked on the network for a while, so could be wrong but I believe the digital returns for some reason can only go up to 65mhz, so VM are trying to move away from them. They are only used in segmentable nodes anyway. Although it's been a good 18 months so the situation may have changed!