|While I've never played with this type of technology it will be software that either runs on a server, or on it's own separate device. |
As to how it works, again I can't say, I would suspect they monitor the MAC address of each connection or possibly the socket each client connects to, and keep a running total of bandwidth used per session.
So once you connect your usage would be zero. Later when you quit and go to pay they only have to check the "bandwidth used" field for your connection and the rest is simple math.
(total used - free amount) * fee per MB = $ you owe them
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.