Abstract: A method and apparatus for performing SIM-based authentication and authorization in a WLAN Internet Service Provider (WISP) network supporting the universal access method (UAM) of authentication and authorization enabling roaming for customers of mobile service providers onto said networks. In addition, the invention provides a secure way of authenticating the customer's client device to the mobile service provider's network by employing temporary credentials for authentication that provide privacy of the user's identity and prevent replay attacks. Finally, if the WISP network supports the ‘pass-through’ facility, the authentication can be done more securely and quickly.