Asterisk is an open source telephony platform for Mac, Linux, FreeBSD and Unix. It is used as a telephone exchange, as an IVR (Interactive Voice Response system) or as a connection between telephony based on different techniques and protocols. Asterisk is responsible for the conversion from one protocol to another and the connection between connections based on different protocols.
The name comes from the asterisk symbol (*). This symbol is usually used as a wildcard in the computer world. Asterisk is extremely flexible and is therefore also called the Swiss army knife of telephony.
License and availability
Asterisk is free software released under version 2 of the GPL. Mark Spencer of Digium is the founder of the Asterisk Project and is still a member of the core developer team. Many other programmers and volunteers have already added features and reported bugs. Asterisk was originally written for Linux, but it also works under OpenBSD, FreeBSD, Mac OS X. Since Linux is the development platform, Asterisk is best supported under Linux.
Opportunities
Asterisk has many options including voicemail, conference calling, interactive voice response and automatic call distribution. It is easy to add new possibilities. This can be done with dial-plan scripts, modules or Asterisk Gateway Interface scripts in Perl or other languages.
Hardware
Special hardware is required to attach analog telephones to a Linux server with Asterisk or to connect the exchange to a PSTN. Digium and some other companies sell PCI cards to connect phones, PSTN, T1 and E1 lines and other analog or digital telephony services to a server.
Calling with Asterisk can be done with IP telephones or with an ATA (Analog Telephone Adapter). An ATA connects analog telephones to a digital Voice over IP network.
Protocols
Asterisk supports a wide range of Voice over IP protocols, such as SIP and H.323. This allows it to work with most SIP phones, or serve as a gateway between the phone and the PSTN. Asterisk also has its own protocol, IAX (Inter Asterisk Exchange), which allows to connect different Asterisk exchanges.
By supporting both traditional and VoIP telephony services, Asterisk allows to efficiently develop new telephony networks, or to gradually replace existing and legacy systems. Others use it to add features (such as voicemail or phone menus) or to cut costs by sending international calls over the Internet.