Supposed you are the Chief Network Engineer of a certain Telco. Your company is planning for a big business expansion and it requires that your network authenticate users connecting using analog modem

TACACS+ is designed primarily for device administration AAA - authenticating IT staff managing routers and switches - and is not suited for authenticating end-user remote access sessions over modems, DSL, or VPN.

DIAMETER is the next-generation successor to RADIUS used in modern mobile core networks such as LTE and 5G, and was not the established standard for the mixed legacy remote-access environment described.

Kerberos is a ticket-based authentication protocol designed for LAN environments using a trusted Key Distribution Center, and it does not natively support remote access scenarios such as analog modem or DSL connections.

RADIUS (Remote Authentication Dial-In User Service, RFC 2865) was originally designed for dial-up modem authentication and has since been extended to support DSL, 802.1X wireless, and VPN access through a wide range of Network Access Servers. Its lightweight UDP-based design and broad NAS vendor support make it the industry standard AAA protocol for authenticating diverse remote access technologies in a single unified framework.