Logging active user sessions within domain

I am setting up a wireless internet service provider network as described in
"Deploying Wireless Provisioning Services (WPS) Technology" MS Server 2003
whitepaper.

As part of the authentication process by IAS I need to check whether a new
user connection exceeds a max concurrent session parameter defined in the
user contract (eg. private user only one session at a time, enterprise user
no more than x concurrent sessions).

When a user authenticates thru IAS I can see logon info to the IAS server
(that is also the domain controller) in the log. An instant later the user
logs off from IAS but until disconnect he is active in the domain and can
access the WAN thru the gateway.

What I am looking for is a way to track the active session or to know that
the session has been terminated by a disconnect.

Thanks in advance