expiration or session-timeot

[Bozhan Boiadzhiev]

Hi,
here is something i can't understand.
If i set some user Expiration attribute for example 23.08.2008,
and this user is connected to my NAS, how NAS will stop that user.
Better explanation.
I have setup mikrotik hotspot with radius authorization.
Authorization works. User have access with given username and password,
but i want to give user access to service for example for 7 days.
Expiration attribute give me ability to set date when account expires.
What happen on that give date. How radius will "tell" NAS to "unsubscribe"(cancel) 
access of that user.

Thanks

ps.
sorry for bad english :(