It's impossible to enforce traffic limiting *during* a users session. So if a user is a tiny bit below their limit and logs in again, they can go over their limit. The server will only catch & enforce their limit on the next login.
It is possible, but that depends on your NAS equipment.
Chillispot will use the radius reply attribute ChilliSpot-Max-Total-Octets to specify how much octets the user is allowed to transfer. Once the user passes the limit he is deauthenticated and his session ends.
BTW. Chillispot (free software) also supports ChilliSpot-Max-Input-Octets and ChilliSpot-Max-Output-Octets atributes, if you want to separatelly limit the traffic. All the radius attributes Chillispot supports are documented here: http://www.chillispot.org/features.html#mozTocId36714 -- damjan | дамјан This is my jabber ID --> damjan@bagra.net.mk <-- not my mail address!!!