Radius is an acronym that stands for Remote Authentication Dial In User Service is a protocol used for applications such as IP mobility and network access “both for local access and roaming access. It is a protocol for Authorization, Authentication and Accounting. RADIUS was originally developed by Livingston Enterprises. But now there exist many commercial and open source RADIUS servers. RADIUS server is responsible for the verification of the login information (Username & Password) that is passed over the RADIUS protocol. It uses any of the authentication protocols like PAP, CHAP or EAP to authenticate this information and then provided the desired level of access if things were right. The login information is passed on to a Network Access Server (NAS) over Point-to-Point Protocol (PPP). RADIUS also provides billing information and the servers are notified when a session starts and stops so that the user is billed accordingly. The billing information is also kept as a log in the servers so that they can be used accordingly in case of any analysis is required. RADIUS servers can also read and write information from various data sources like text files and database etc. Simple Network Management Protocol (SNMP) is used for the remote monitoring of RADIUS servers. RADIUS is extensible, and many of the commercial products have their own hardware and software implementations for their own dialects. RADIUS has a limitation when coming to the mobile communications involving 3G devices. They require highly sophisticated billing functions as they provide more services. Hence, a replacement for RADUIS, DIAMETER is also under consideration. The DIAMETER protocol provides full backward compatibility with RADIUS protocol and is expected to solve the current issues related to RADIUS.