Remote Access Services

The term was originally coined by Microsoft when referring to their built-in Windows remote access tools. (RAS) Remote Access Service is a service provided by Windows NT which allows most of the services which would be available on a network to be accessed over network link. The service includes support for dialup and logon, presents the same network interface as the normal network drivers but somewhat slower. RAS works with several major network protocols, including TCP/IP, IPX, and NBF. It is not necessary to run Windows NT on the client - there are client versions for other Windows operating systems. RAS enables users to log into an NT-based LAN using a modem, X.25 connection or WAN link.
Starting in the mid-1990s, several manufacturers such as U.S. Robotics produced "modem terminal servers". Instead of having RS-232 ports, these would directly incorporate an analog modem and these devices were commonly used by Internet service providers to allow consumer dial-up. Modern versions interface to an ISDN PRI instead of having analog modem ports.

(IAS) Internet Authentication Service in Microsoft Windows Server 2003, Standard Edition; Windows Server 2003, Enterprise Edition; and Windows Server 2003, Datacenter Edition is the Microsoft implementation of a (RADIUS) Remote Authentication Dial-in User Service server and proxy. As a RADIUS server, IAS performs centralized connection authentication, authorization, and accounting for many types of network access including wireless, authenticating switch, and remote access dial-up and virtual private network (VPN) connections. As a RADIUS proxy, IAS forwards authentication and accounting messages to other RADIUS servers. RADIUS is an (IETF) Internet Engineering Task Force standard.