Access Server

NAS is also the abbreviation for network-attached storage.
----------------
Remote Access Server
A server that is dedicated to handling users that are not on a LAN but need remote access to it. The remote access server allows users to gain access to files and print services on the LAN from a remote location. For example, a user who dials into a network from home using an analog modem or an ISDN connection will dial into a remote access server. Once the user is authenticated he can access shared drives and printers as if he were physically connected to the office LAN.
See the Server Types page in the Quick Reference section of Webopedia for a comparison of server types.
 http://searchnetworking.techtarget.com/definition/network-access-server
A network access server (NAS) is a computer server that enables an independent service provider (ISP) to provide connected customers with Internet access. A network access server has interfaces to both the local telecommunication service provider such as the phone company and to the Internet backbone.
The server authenticates users requesting login. It receives a dial-up call from each user host (such as your computer) that wants to access the Internet, performs the necessary steps to authenticate and authorize each user, usually by verifying a user name and password, and then allows requests to begin to flow between the user host and hosts (computers) elsewhere on the Internet.
The term network access server may refer to a server devoted entirely to managing network access or to a server that also performs other functions as well. A network access server can be configured to provide a host of services such as VoIP, fax-over-IP, and voicemail-over-IP as well.
One of the most well-known network access servers, the AS5800, is made by Cisco Systems. It is a workhorse product that is referred to as a carrier-class universal access server.

Getting started with NAS

To explore how NAS is used in the enterprise, here are some additional resources:

Moving from DAS to NAS:In this tip, Rick Cook discusses how SMBs can benefit from adding storage to their existing networks in the form of NAS.

Small-midsized business NAS product evaluation: This article compares and contrasts four very different NAS systems: the Buffalo TeraStation Pro II Pro, the Reldata 9240, the Synology RS407, and the Western Digital MyBook World Edition WDG1NC5000N.

NAS FAQ: In this FAQ, Ashish Nadkarni answers today's most common NAS questions.

Cisco Lab How To Configure An Access Server
http://openvpn.net/index.php/access-server/download-openvpn-AS.html