> - Currently, the circuit where the user connects is arbitrarily decided by the demo user. In a real system with thousands of circuits, it'd be very difficult to properly assess which circuit the customer might connect to. When adding a new customer to a service, how does the operator decide, based on customer's location, which circuit to provide the service to ?

I'm not exactly sure what you're asking, but port allocation is, depending on the ISP's deployment model, either going to be fixed at the time the infrastructure was built, or whoever is doing the last metre install will choose a random available port on the switch. The subscriber will be assigned to that port in the RADIUS or equivalent database, and the BNG will query the subscriber based on DHCP Option 82 port information added by the switch. You could also map the subscriber based on MAC address, but this doesn't really work unless you don't support customer provided equipment on their end.