Asterisk is an open source software implementation of a telephone private branch exchange (PBX). Like any PBX, it allows a number of attached telephones to make calls to one another, and to connect to other telephone services including the PSTN.

Asterisk is free software, released under the GNU General Public License (GPL). Mark Spencer of Digium originally created Asterisk and remains its primary maintainer; dozens of other programmers have contributed features and functionality. Originally designed for the Linux operating system, Asterisk now also runs on OpenBSD, FreeBSD, Mac OS X, Solaris, and Microsoft Windows, although as the "native" platform, Linux is the best-supported of these.

The basic Asterisk software includes many features previously only available in expensive proprietary PBX systems -- voice mail, conference calling, interactive voice response (phone menus), and automatic call distribution. Users can create new functionality by writing dial plan scripts in Asterisk's own language, or by adding custom modules written in C.

To attach ordinary telephones to a Linux server running Asterisk, or to connect to PSTN trunk lines, the server must be fitted with special hardware. (An ordinary modem will not suffice.) Digium and a number of other firms sell PCI cards to attach telephones, telephone lines, T1 and E1 lines, and other analog and digital phone services to a server.

Perhaps of more interest to many deployers today, Asterisk also supports a wide range of Voice over IP protocols, including SIP and H.323. Asterisk can interoperate with most SIP telephones, acting both as registrar and as a gateway between IP phones and the PSTN. Asterisk developers have also designed a new protocol, IAX, for efficient trunking of calls among Asterisk PBXes.

By supporting a mix of traditional and VoIP telephony services, Asterisk allows deployers to build new telephone systems efficiently, or gradually migrate existing systems to new technologies. Some sites are using Asterisk servers to replace aging proprietary PBXes; others to provide additional features (such as voice mail or phone menus) or to cut costs by carrying long-distance calls over the Internet (toll bypass ).

VoIP Telephone Companies have begun to support Asterisk PBX's. A recent article in the Geek Gazette, explains how to configure an Asterisk system to work with Broadvoice.

External link

• Asterisk home page
• Asterisk Wiki
• Russian Asterisk Community