Overbooking used to be called the "contention ratio" back in my day when I was working with Nortel CVX 1800 modem racks (and before that too with older, crappie

Overbooking used to be called the "contention ratio" back in my day when I was working with Nortel CVX 1800 modem racks (and before that too with older, crappier, less dense racks running off E1s or even ISDN). A full rack of 1000 modems at 56kbps means 7Mbps bandwidth needed to feed the box. We had ~20 racks, so we'd need (in theory), 140Mbps up to the internet to service them all at a 1:1 contention ratio. In truth, we had ~35Mbps - a very generous 4:1 contention ratio. Why does it work? Well, when you're browsing the web, you load a page (and use bandwidth), and then you read it and don't use bandwidth. For every second of download, you probably have 30+ seconds of idle bandwidth while you're reading those emails or web pages or whatever - and yes, one manager was pushing us to get to 30:1 contention, which was industry norm at the time - so this actually works fine. One colleague who had worked at a major competitor told us they regularly pushed to 100:1 and no customer seemed to care. We actually benchmarked them (and a bunch of other ISPs), and yeah, was a little slow at peak, but actually not awful most of the time. The problems started to come when people wanted to max out their connection more because of an evolution of applications, technology and expectations (in short DSL, the rise of P2P services and people "being online" more was a triply whammy), and so more customers wanted to use every bit of bandwidth every second, all day, every day. In the UK, even the BBC was being asked to do something because the launch of iPlayer absolutely crucified a lot of ISPs. I was out of the game by then, but approaches seemed to include citing breach of acceptable use policies (very unpopular), blocking certain services (one of the reasons why the major ISPs block TPB in the UK), or - in at least one case I heard about - traffic shaping those "greedy" customers until they went away. If you throttle someone who is paying for 10Mbps down to 1Mbps, because they've been taking all 10Mbps 24x7 for the last three months, they'll get frustrated and leave your service with the only real cost being a bad online review you can blame on backhaul network/bad copper/bad modem, and which is drowned out by other "normal" users claiming you're a great ISP. And yes, I do know of at least one ISP who did this regularly - a former staffer told me a few years after he left.