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Re: Virgins Infrastructure
I've read everyone's posts but I'm not sure you've actually answered the OP's question.
In the diagram he has posted, the 'Distribution node's' would be the CMTS's (or UBR's whatever you want to call them) they will have either fibre going out to the 'optical nodes'.
Distance wise for these you are talking roughly 80 miles max from there out to an 'optical node' which is a cab or small hubsite (You can have MUCH longer fibre runs than this, but they cost a lot more and aren't suited to this kind of application).
You then have Coax going either from them direct to the houses, or sometimes out to smaller RF cab's - These distances are pretty limited as noise is caused on every termination point along the route, without rebuilding the signal and then amplifying it (which is almost never done, it's easier to just run more fibre)
Going back from the CMTS's to the outside world, you'll usually have a link to the core network (really big routers, which handle local routing), and then a link form there to the backbone (REALLY big routers, which shove HUGE amounts of traffic between each other) - this is pretty standard network design
The backbone then has peering links out to other networks outside of the one you are talking about. All connections between Access, core and backbone are likely to be 1 gig or 10 gig fibre links, and there's usually quiet a few of them, depending on required capacity - these can be hundreds of miles apart from each other .
Phew - I hope that answers your question. That's pretty much how all ISP networks are designed they just use different types of access links (EG BT use the pre-existing copper lines instead of installing Coax)
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