i didn't realize fiber could be overbooked similar to cable. I had a 3gbit plan but downgraded to 500mbit because barely any transfers would go over 300mbit during the day. I kept thinking the servers I download from are overloaded, but it makes more sense my provider overbooks than the entirety of the internet being slow... time to put in some complaint calls during peak hours!
Many residential fiber deployments are PON with some amount of overbooking on the last mile. If the fiber is 2.5g down/1.25g up and split 4 ways, not everyone can have the full 1g/1g service.
But even if you have point to point fiber, that probably has an upstream connection that's less than the aggregate bandwidth of the end user connections it manages. And so on until you get through your providers network to internet transit/peering. And once you get through to the hosting network too.
I ran hundreds of 1g/10g boxes at a facility with 80g aggregated to the world.
Those boxes would be 40 or so to a rack, with 2x10g to the switch (800gbps aggregate) and probably 4x10g to the upstream switch. Maybe 4x40g to the upstream.
