Recent firmware images (3.2.x) include an option called 'Sync Accounting Records' which can be enabled in wlan RADIUS configuration:
Q. What does this option do?
A. When enabled, the accounting data for a particular users session is passed among APs as the client roams. So say a client authenticates on AP1 then later roams to AP2 where its session ends due to either expiry of session time or the client logging off the network. In this case AP1 would have sent the RADIUS Accounting Start when the session started, however AP2 wil send the RADIUS Accounting Stop, with data details of both AP1 and AP2 included in that message.
Without this option each AP maintains and sends its own accounting data. So on roam from AP1 to AP2, AP1 would ahve sent a RADIUS accounting stop message, and AP2 would first send a RADIUS Accounting Start, then at the end of the users session a RADIUS Accounting Stop.
Q. When is this feature useful?
A. If the RADIUS server is not able to consolidate accounting messages and sessions across different APs, this feature can help by having the AP create the consolidated session instead. There is more inter-AP messaging for this feature to work so enable it only if the deployment topology (RADIUS server and AAA post-processing) require it.