[Sephiroth]

My Sticky Post in the VM Forums (Power Levels & SNR) explains a lot of this.

It works like this (but can vary in different parts of the country according to who was the original cable supplier):

1/
A small street cabinet connects either 16 or 32 customers. A larger street cabinet connects 48. It is passive equipment.

2/
The cable from a cabinet passes along to another cabinet configured as above.

3/
5 or 6 cabinets go to an optical node (with 240V labelled) and it's fibre from there. This cabinet is active.
or 96
4/
In high density population areas, you might get 20 coax cabinets feeding the optical node.

5/
The optical node originally would have supported only 2 upstream channels and these might have been split between two nodes at the VM end line card. So that's a heck of a lot of users who think they're getting, say 5 meg upstream sharing just 40 meg upstream capacity.

6/
A downstream bonding group currently has 4 downstream channels with c. 200 meg capacity to share across one or two optical nodes. VM intend that this should rise to 8 downstream channels. VM are also introducing bonded upstream channels.

That explains how over utilisation can occur.

The current infrastructure upgrade programme should double all the capacities I've mentioned. For high contention areas, VM should be adding more optical nodes and line cards and CMTS devices at their end to cope with the additional channels allocated. That's what they say they're doing.

7/
The optical node connects via fibre to a line card on a UBR/CMTS at the local hub. Many optical nodes terminate on a single line card. An individual's downstream and upstream (both shared with other users) terminate on the same line card and (AFAIK) in the same Service Group.