Typically, a user proves to the system who they are by entering a valid user name and a valid password. Authorization is the process of identifying what a user can do. For example, after logging in to a system, a user may try to issue commands. The authorization function determines whether the user is permitted to issue those commands. In some systems, authorization and authentication are merged into a single process.
Accounting is the process of measuring the resources a user has consumed.
Typically, accounting measures the amount of system time a user has used, or the amount of data a user has sent and received. The three functions defined above are of paramount importance to effective network management and security. They therefore should be provided at the point of network entry. Although the term 'NAS' is used throughout this documentation, you may find it referred to elsewhere as a Communication Server, Remote Access Server, or Terminal Server. The NAS is a device which accepts dial-in access from telephone or ISDN lines through modems or ISDN terminal adapters.