"Putting all your eggs in one basket" is a phrase that comes to mind, or so it seems.
The way networks used to be run is that different services used different infrastructure and worked in different ways. Indeed, many services would inherently allow re-routing around problems either automatically or manually. If one service failed another continued as it did not use the same infrastructure.
Apparently, 21st Century Networks are different. It seems that with 21CN you can have lots of single points of failure, like a router in Slough perhaps, that can simultaneously take out lots of services and somehow not allow them to be re-routed! You can have Ethernet links and broadband services go down at the same time even though they are not actually in that area, just because they are routed via Slough.
Of course, in the good old days, a failure would mean using spare equipment or mending a fibre, and it would be done within a 4 hour window at most. But no, with 21CN you can have huge routers that don't apparently have hot, or even vaguely warm, spares, and so the idea of things fixed in 4 hours is out of the window.
Yes, this is a lot of guess work about the network of a major UK telco, but it is guess work based on the fact that 10% of our broadband and Ethernet lines have been down for NINE HOURS so far, and all that has happened during that time is some more lines going off in another area...
I guess we now know what 21CN means though. Thanks for showing us the way.