Asterisk

Asterisk
Name	Asterisk
Developer	Digium
Initial release	1999
Operating system	Linux, BSD, macOS
License	GPL

Contents

Overview
History
Uses and Applications
Variants and Forms
Technical Characteristics
Cultural and Linguistic Significance
See also

Asterisk

Asterisk is an open-source telephony software toolkit that provides voice over IP (VoIP), private branch exchange (PBX), interactive voice response (IVR), and conferencing functions. Developed originally by Mark Spencer and maintained by Digium and the Asterisk Community, it integrates with telephony hardware and protocols to serve enterprises, call centers, and service providers. Asterisk interoperates with standards and products from companies such as Cisco, Avaya, Ericsson, Microsoft, and Juniper.

Overview

Asterisk implements switching and media handling for endpoints using protocols like SIP, IAX, H.323, MGCP, and RTP while interfacing with hardware from vendors such as Sangoma, Digium, Cisco, Avaya, and Polycom. It supports codecs including G.711, G.722, G.729, Opus, and GSM, and works with platforms like Linux distributions, FreeBSD, macOS, and virtualization stacks from VMware and KVM. Administrators often integrate it with databases like MySQL, PostgreSQL, and LDAP directories such as OpenLDAP and Microsoft Active Directory, and with orchestration tools from Red Hat, Canonical, and SUSE.

History

Asterisk was created in 1999 by Mark Spencer at Digium as a software PBX to run on commodity hardware, influenced by earlier telephony systems from Nortel, Alcatel, and Ericsson. Early adoption grew among projects and companies such as Fonality, FreePBX, Elastix, and Sangoma, leading to ecosystem growth with contributions from developers associated with Red Hat, Debian, and Ubuntu. Standardization and interoperability work involved the IETF, ETSI, ITU, and collaborations touching implementations by Cisco Systems, Microsoft, and Avaya. Over successive releases Asterisk added features integrating with OpenSSL, FFmpeg, and PJPROJECT, while industry events like AstriCon and conferences hosted by O’Reilly and the Linux Foundation showcased developments.

Uses and Applications

Asterisk is deployed by small businesses, large enterprises, Internet telephony service providers (ITSPs), call centers, and academic institutions. Operators combine Asterisk with business applications from Salesforce, SAP, Oracle, and Microsoft Dynamics for contact center workflows, and with analytics platforms such as Splunk and ELK Stack for monitoring. Emergency services, universities, hotels, and government agencies have used Asterisk-based systems alongside equipment from Panasonic, NEC, Siemens, and Ericsson to provide IVR, voicemail, conferencing, and unified communications. Integration with cloud providers like Amazon Web Services, Google Cloud Platform, and Microsoft Azure enables scalable hosted PBX and SIP trunking solutions.

Variants and Forms

Asterisk has spawned distributions and front-ends including FreePBX, Elastix, Issabel, and PBX in a Flash, and commercial products from Sangoma, Digium, and Schmooze. Forks and integrations incorporate projects such as Kamailio, OpenSIPS, Freeswitch, and Janus for SIP routing, session border control, and media bridging. Hardware appliances and gateways combining Asterisk with boards from Digium, Sangoma, and Rhino enable PSTN connectivity using ISDN, analog, and T1/E1 interfaces, while virtual appliance images target platforms like VMware, Hyper-V, and Proxmox.

Technical Characteristics

Asterisk’s architecture comprises a core PBX engine, channel drivers, media modules, dialplan interpreter, and APIs like AMI, AGI, and ARI for external control. It implements codecs and DSP routines, supports SRTP, TLS, and ZRTP for secure media and signaling, and integrates with libraries such as OpenSSL and libpri. Administrators script call logic in the Asterisk dialplan and control flows via external applications using AGI with languages including Python, Perl, PHP, and Node.js. Scalability employs clustering techniques with SIP proxies such as Kamailio and OpenSIPS, load balancers from HAProxy and NGINX, and database replication using Galera, Patroni, or PostgreSQL streaming.

Cultural and Linguistic Significance

The asterisk symbol has historical uses in typography, mathematics, and annotation in texts by publishers like Oxford University Press, Cambridge University Press, and Merriam-Webster, and appears in works by authors associated with Penguin Random House and HarperCollins. In computing and user interfaces from Apple, Microsoft, and Google, the symbol marks footnotes, wildcard patterns in shells like Bash and PowerShell, and censorship in media governed by bodies like the FCC and BBFC. In linguistics, the asterisk denotes reconstructed or ungrammatical forms in studies published through JSTOR and presented at linguistics conferences such as LSA and ACL.