[kwikbreaks]

There's obviously more to it than just this but my way of looking at it (which may be wrong) is...

The small street cabinets are just dumb splitters which join all the individual modem coax cables to one cable which goes on to other cabs and eventually back to what's called the optical node which is another big street box and serves a shedload of modems. That's where the cable RF tech gets converted to (I think) tcp/ip which goes by fibre back to another concentration point housing loads of fibre routers and then on again to VM's major infrastructure connecting to the internet backbone.

In the RF side of things there are RF amps and attenuator banks which are used to make sure the modems get signal levels they can handle and the optical node sees the correct level from the modems without them busting a gut to deliver it but none of that is relevant to congestion.

CMTS/UBR/Optical node are different names for essentially the same thing - the kit that interfaces coax to fibre.

DOCSIS1 and DOCSIS3 are different RF level technologies. Both can be carried on the same coax along with the TV channels. Either or both can be oversubscribed and as I said it sounds like your old docsis 1 was oversubscribed but the docsis3 isn't (yet).