L3 cabinets are mini nodes, generally feeding up to 500 homes each. They have a direct feed to the larger VHUB unit (serving a node area of up to 3000). They are fed on multi core blown fibre along the red/green larger blown fibre ducts. Each L3 cabinet will feed up to 10 L4 distribution cabinet (48 connections per cabinet) in a direct trunk feed from its parent L3 cabinet. In turn each L4 cabinet then has up to 50 direct blown fibre feeds to properties that terminate inside the toby (termination box).
The developers guide has some information of the high level blown fibre topology if that's any use?
www.virginmedia.com/developer.
HTH