RADIUS
Remote Authentication Dial-In User Service (RADIUS) is defined in [RFC2865] (with friends), and was primarily used by ISPs who authenticated username and password before the user got authorized to use the ISP’s network. 802.1X does not specify what kind of back-end authentication server must be present, but RADIUS is the “de-facto” back-end authentication server used in 802.1X.
Roaming
Roaming is the movement of a client from one AP to another while still transmitting. Roaming can be done across different mobility groups, but must remain inside the same mobility domain. There are 2 types of roaming:
A client roaming from AP1 to AP2. These two APs are in the same mobility group and mobility domain