RADIUS is a client/ server protocol that runs in the application layer. The Network access server, the gateway contains a RADIUS client component that communicates with the RADUIS server and contains a AAA protocol that manages the network access in two step process. AAA stands for authentication, authorization and accounting.  The RADIUS server then returns one of three responses to the Network Access Server: 1) Access Reject, 2) Access Challenge, or 3) Access Accept.