I was pretty interested to find this article by Mae Kowalke on SIP vs. Radius titled Top Five Reasons SIP is Better Than RADIUS. Radius stands for Remote Authentication Dial In User Service and is an an authentication, authorization and accounting protocol for controlling access to network resources. However, in more and more cases, a switch is being made to the newer Session Initiation Protocol.
This article contains an interview with Barry Sher of IVR Technologies and Sher does a good job of explaining the differences. As you may recall I have written about Sher and IVR Technologies before. Enjoy.