For details on RAS please see RAS Buyer's Guide and the RAS product Cyclades PR4000
A Remote Access Server (RAS) is a network box that connects remote dialup clients to a Local Area Network (LAN. RAS are used by Internet Service Providers (ISP) and Corporate Network Managers to allow home users, small offices, and telecommuters to connect to the Internet or the corporate LAN from remote sites using analog phone lines or ISDN lines.
Since the mid-90's, with the popularization of the Internet, the demand for dialup remote access has been growing exponentially. Until faster options are broadly available, analog phone lines and ISDN lines are the more common media to remotely connect personal computers or home offices to the LAN or to the Internet.
A RAS is a box that attaches to the LAN at the central site and provides connection to dialup lines. It allows remote clients to access the LAN (and possibly the Internet) by using modems or ISDN terminal adapters.
In the past, RS-232 Terminal Servers were used to provide remote access. A server-based or stand-alone box would attach to the LAN and provide multiple RS-232 serial interfaces. Those serial ports were connected to a modem bank, which on its turn, connected to the analog phone lines coming into the building.
With the crescent demand, the number of required phone lines started to grow to a point where it was impractical to increase their number at the central site. The phone company, instead of delivering individual phone lines to you, bundle 23 or 30 channels in one T1 or E1 ISDN-PRI line.
Typically, a RAS will have one or more Ethernet ports for connection to the LAN and one or more T1/E1/ISDN-PRI ports for the incoming calls (a T1/ISDN-PRI line is equivalent to 23 phone lines, while an E1/ISDN-PRI line is equivalent to 30 phone lines).
A RAS will typically have internal digital modems and will be able to terminate both analog and digital calls and connect to remote clients using Point-to-Point (PPP) protocol.
Traditionally, RAS are stand-alone, integrated and self-contained boxes running an Internetworking Operating System on a proprietary hardware specifically designed for Remote Access. They connect to the LAN and to the income dialup lines and all you need to do is to configure them properly to get them working.
But there is also the possibility of building a RAS using standard server hardware (usually a PC) with a Networking Operating System (typically, Linux or Windows). The user installs PCI RAS adapter (with digital modems and ISDN-PRI interfaces) in a PC server, install the OS and the proper device drivers and configure the system to work as a Remote Access Server.
So, maybe the first question you face when selecting a RAS solution is this: Stand-alone or server-based?
In general, stand-alone solutions are better for the average user because of the following factors:
Integration: The RAS solution is complete and there is no need for software or hardware integration. That eliminates the possibility of hardware and software incompatibilities and installation problems and gives you a single point of contact for technical support and problem solving.
Reliability: Stand-alone solutions run on hardware and software that was specifically designed for remote access. They are more integrated, more compact, dissipate less power, and, because of that, are more reliable than a solution based on general-purpose components.
Cost: Usually, commodity PC hardware and open source software tend to drive costs down, making server-based solutions attractive for technical users who already have the PC and are willing to face the potential problems of integrating the solution from standard components. But, the PCI adapters for RAS available in the market today still carry a price tag that makes it difficult to justify a server-based solution.