Asterisk: Open source IP PBX

              Looking for more information on low-cost VoIP service? Here's what you need to know about Asterisk, a popular open source IP PBX provider. Deb Shinder outlines the features and benefits of this freely available program.
 

Private branch exchange (PBX) equipment traditionally has been costly, but along with IP-based voice communications came PBX software. Like other types of software, some PBX programs are released as open source products under the free GNU General Public License (GPL). The most popular open source PBX software is Asterisk from Digium, originally written for the Linux open source OS but now also available in versions for Windows, Mac OS X, and other operating systems. In addition to the GPL version, there is a commercial license that allows closed code.

In a other article on this website, we discussed the Inter-Asterisk Exchange (IAX) protocol used for communications between two Asterisk PBX units or between an Asterisk client and server. Now, let's take a look at some of the components of the Asterisk system and how they work.

Asterisk advantages

The most obvious advantage of open source software is cost; you can download Asterisk for free. The current version as of this writing is 1.2.13, and beta 3 for version 1.4.0 is also available to the public. The feature list for Asterisk is long and includes advanced features that are often associated with high end (and high cost) PBX equipment, such as conference bridging, call monitoring, call queuing, and voice mail. You can view a list of supported features at Asterisk's Web site.

In addition to its own IAX protocol, Asterisk also supports H.323, Session Initiation Protocol (SIP), Media Gateway Control Protocol (MGCP), and Skinny Client Control Protocol (SCCP). That means you can use SIP phones or Cisco Skinny clients with it. It will also interoperate with traditional (analog) telephone technologies such as Foreign Exchange Station (FXS) and Foreign Exchange Office (FXO), the interfaces that deliver and receive regular PSTN service.

IAX has advantages over SIP when there is a need to go through network access translating (NAT) devices, and it is more firewall friendly. IAX uses User Diagram Protocol (UDP) instead of Real-Time Transport Protocol (RTP) for signaling and media, which results in less overhead and lets you send more calls through the same link. It only uses one port (4569).