Network access server

A Network Access Server (NAS) is a single point of access to a remote resource.

Overview

The NAS is meant to act as a gateway to guard access to a protected resource. This can be anything from a telephone network, to printers, to the Internet.

The client connects to the NAS. The NAS then connects to another resource asking whether the client's supplied credentials are valid. Based on that answer the NAS then allows or disallows access to the protected resource.

Examples

The above translates into different implementations for different uses. Here are some examples.

• An Internet Service Provider which provides network access via common modem or modem-like devices (be it PSTN, DSL, cable or GPRS/UMTS) can have one or more NAS (network access server) devices which accept PPP, PPPoE or PPTP connections, checking credentials and recording accounting data via back-end RADIUS servers, and allowing users access through that connection.

• A sample use is the captive portal mechanism used by many WiFi providers: a user wants to access to the Internet. He opens his browser. The NAS detects that the user is not currently authorized to have access to the Internet, so the NAS prompts the user for their username and password. The user supplies them and sends them back to the NAS. The NAS then uses the RADIUS protocol to connect to an AAA server and passes off the username and password. The RADIUS server searches through its resources and finds that the credentials are valid and notifies the NAS that it should grant the access. The NAS then grants the user access to the internet.

• Another use of a NAS would be in VoIP. However, instead of using a username and password, many times a phone number or IP Address are used. If the phone number is a valid customer then the call can be completed. Other uses might be if the phone number has long distance access or if a telephone card and has minutes left.

Associated Protocols

Although not required, NAS are almost exclusively used with AAA servers. Of the AAA protocols available, RADIUS tends to be the most widely used. The Diameter base protocol extends RADIUS services by providing error handling and inter-domain communications. This protocol is used in networks like IP Multimedia Subsystem (IMS).

Access Servers In Practice

Access servers are specialized routers handling voice, data, fax, and video traffic, that emerged principally beginning with the internet. A number of companies have developed access server product lines, including Cisco, USR/3Com, Lucent, Avaya, Patton, Juniper, and others. The explosive development of internet based networking fostered the IP versions of many forms of communications. For example, voice telephone calls, which traditionally were handled with dedicated telecommunications cabling, increasingly were converted to voice over IP traffic (VOIP) and run over the internet. Access server applications continue to proliferate; while the began with dialup internet and developed into VOIP phone calling, they are now entering into video streaming, and companies such as Netflix, Youtube, and a variety of other companies are presently developing content designed to be streamed over the internet. Use this source for a review of the development and characteristics of access servers.