I'm not saying that it can't be done, you can run 20/2 on DOCSIS 2 but... it's a waste of bandwidth on the cable. All it takes is someone to saturate their connection for a long period of time and that data channel is as good as gone for the rest of the people using the same data channel.
That is why DOCSIS 3 is so good, you can connect on multiple data channels and spread the data across multiple data channels thus reducing the demand on one channel and potentially slowing down other customers on the same node.
Again, it all comes down to the fact that DOCSIS 3 allows for more efficient use of bandwidth on the cable.--
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