Refresh interval for end users devices

Hi, I have a two devices, one phone and a windows 7 machine. I can connect to my my both devices with wireless and get authenticated with freeradius. (this is great) But I am noticing that every time devices refresh it actually re-authenticate which I can see int the radius logs. For example, my phone got connected - authenticated, then i locked the phone, after a while I open the phone, when it re-scan the network it actually sends the authentication request- gets re-authenticated. Is there a way, it only authenticates once, if you are in same wireless access point ? I meant, when device re-scans the network and if you are with the same Wireless access point not to send the auth request to the server over and and over ? Thanks, R